Download A Guide to Space - Department of Physics and Astronomy

A Guide to
Space
Project Fulcrum is supported by the National Science Foundation and the University of
Nebraska, in partnership with Lincoln Public Schools
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1. Introduction
What do you want
1.1. How to Use These Materials
students to know and be
able to do?
1.1.1. Philosophy. Project Fulcrum is based on
the strategy shown in Figure 2.10. The first
aspect of planning a lesson is deciding what it is
How will you know that
you want your students to know and/or be able
they have learned what
you wanted them to
to do. The second step is to determine what
learn?
criteria you (or the CRTs) will use to evaluate
whether they have learned the items you picked
in the first step. The final step is to pick those
What materials, activities,
discussions, etc. will help them
activities, materials, etc. that will accomplish
learn what you want them to
your goal and facilitate your evaluation of your
learn?
students. Don’t get in the habit of picking the
activity first. Activities should serve your goals Figure 1.1: Lesson design philosophy
for your students, not vice-versa.
1.1.2. Background Material. The background material in this section includes information on
the basic concepts required to understand space, plus some additional materials on nature of
science, technology and history.
1.1.3. Objectives. Each LPS objective is stated, and then the fundamental concepts that are
necessary to master the objectives are discussed, with references to the appropriate background
sections.
1.1.4. Key Concepts. Each objective has multiple smaller ideas, all of which are necessary to
understand if students are to meet the objectives. These are presented as bullets, with the goal
being to be as specific as possible.
1.1.5. Activities. The activities are not presented in a specific order. You should choose
activities via the goals they address. You may plan different activities for different sets of
students, depending on their needs and their sophistication. Do not assume that the order in
which they are presented here necessarily is the order in which you should utilize them.
1.1.6. Work in Progress. This is a work in progress and is only a draft at this point. We
welcome your input, ideas and other contributions.
1.2. Opportunities to Work with a Project Fulcrum Scientist
Project Fulcrum scientists are graduate students pursuing advanced degrees in math, science or
engineering. They will plan with you to identify hands-on activities and other resources that can
help your students master the LPS objectives for the particular unit.
Project Fulcrum scientists are not teachers: they are there to partner with you and help you
achieve the goals you have for your students. Working with a Project Fulcrum scientist has
many benefits
• An opportunity for your students to make contact with a working scientist and broaden
their image of science and scientists;
• Content expertise, including innovative ways to demonstrate and experiment with
concepts that sometimes are difficult to teach;
• Increased opportunities to use hands-on, inquiry-based experiences to help your students
learn.
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One ‘Lead Teacher’ is required for each school. Preference will be given to schools that have
teachers interested in working with the Lead Teacher on the same unit. Lead Teachers from
different schools will meet to share ideas and resources, forming a community of practice based
around a specific content unit. The Lead Teacher has the following responsibilities:
•
•
•
•
•
•
•
•
•
Attend a three-hour hands-on content workshop prior to the start of the unit.
Attend a two-hour Project Fulcrum orientation meeting to learn how Project Fulcrum
works
Attend a two-hour planning meeting prior to starting the unit
Complete a weekly journal during the time you are working on the unit
Attend a two-hour meeting at the midpoint of the unit
Attend a two-hour end-of-unit meeting
Participate in pre- and post-surveys
Write a final reflective essay on how the experience has affected the way you teach
science
Communicate with other teachers at the school who are participating.
Lead teachers will be paid at the rate of $18/hr (with one hour allotted for each journal). All
payments are made at the end of the quarter in which the unit is taught.
2. Objectives
2.1. Objective 4.3.1 - The student will be able to name and describe the parts of
the solar system.
2.1.1. Key Concepts
ƒ The Solar System is a grouping of nine known planets that orbit the Sun.
ƒ The Sun is a star, just like the stars in the night sky.
2.1.2. Vocabulary
Earth is the planet we live on and is the third planet from the Sun. The Earth is the only known
planet that has an atmosphere that can support human life.
Jupiter is the fifth planet from the Sun and the largest planet in the solar system. One thousand
Earths could fit inside Jupiter if it was hollow. Jupiter has more mass than all of the other
planets combined.
Mars is the fourth planet from the Sun and is commonly called the ‘red planet’ because of its red
rocks, soil, and sky. The red color is due to the presence of iron oxide (aka: rust). Mars is the
third smallest planet and is thought to be the best candidate for harboring life of any type.
Mercury is the closest planet to the Sun and is the second smallest. The surface of Mercury is a
lot like the Moon: It has has many craters from meteor impacts.
Neptune is the eighth planet from the Sun and the fourth largest. Neptune looks blue due to the
large amount of methane in the atmosphere. If Neptune were hollow, 60 Earths would fit inside
of it.
A Planet is body in space that doesn’t give off light and only reflects light from stars. There are
nine commonly accepted planets in our solar system that revolve around the Sun.
Pluto is the furthest planet from the Sun on average and the smallest by far. Pluto is made up of
mostly rock. A mission to Pluto called New Horizons was just launched in January 2006 and
will reach Pluto in 2015. Pluto is the only one of the nine known planets we have not sent a
space mission to explore.
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Saturn is the sixth planet from the Sun and is the second largest. Saturn is known for its rings,
which are made up mostly of ice and a small amount of rock.
The Solar System includes the Sun and nine known planets orbiting around the Sun. The Solar
System also includes asteroid belts and comets.
A Star is a body in space made up of gases at very, very high temperatures that provides its own
illumination (in contrast to planets, which reflect light). Our Sun is an example of a star.
The Sun is the star about which the nine known planets of our solar system orbit. It is the major
source of the light and warmth for Earth.
Uranus is the seventh planet in the solar system and is the third largest. Like Neptune, Uranus
appears blue due to the methane in its atmosphere. Uranus has rings like those of Saturn, but the
rings are not as pronounced.
Venus is the second planet from the Sun, sixth largest, and is very close to the size of Earth.
Venus is the most-visited planet by unmanned spacecraft from Earth. Scientists believe that
Venus was once very much like Earth, but that the extreme heat from the Sun would have boiled
away any water on the surface.
Pictures – (From the closest to furthest from the Sun, photos courtesy of NASA)
The Sun as seen from the Skylab Space Station
Mercury from Mariner 10 spacecraft
Venus from Galileo spacecraft
Earth showing Africa from space
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Mars from Viking Orbiter
Jupiter from Hubble Space Telescope
Saturn from Voyager II
Uranus from Voyager II
Neptune from Voyager II
Pluto and its moon Charon from Hubble Space
Telescope
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Figure 2.1: The orbits and positions of the planets. In order to fit them on one piece of paper,
they are not drawn to scale. All the planets except Pluto orbit in the same plane. Pluto’s orbit is
canted with respect to the other planets.
2.1.3. Some Controversy: Is Pluto a Planet and Is There a Tenth Planet?
You may have read in the newspapers that there are debates about whether Pluto is a planet and
whether there are actually only nine planets. Right now, the solar system includes nine known
planets (described above); however, this definition is in flux. Part of the problem is that
astronomers don’t really have an agreed-upon definition for what makes a planet a planet.
In 2000, the Rose Center for Earth and Science at New York City's American Museum of
Natural History (one of the most prestigious planetariums in the country) left Pluto out of its
planet exhibit. Why? Many astronomers argue that Pluto should never have been classified as a
planet. It is very small, its orbit is not in the same plane as the other planets’ orbits, and we are
discovering that there are lots of objects of comparable size in the same region.
Pluto occupies a part of the solar system called the Kuiper belt (Ky’-per belt). The Kuiper Belt
is a region in our outer solar system that contains many comets that have orbits of less than 200
years. The Kuiper belt lies beyond Neptune's orbit and may contain as many as 100 million
Kuiper-belt comets. Objects in this belt are commonly referred to as Kuiper belt objects. Not
the most glamorous name, but descriptive.
In 1999 the International Astronomical Union (IAU), which is a professional society of
astronomers, decided against making Pluto a minor planet or listing it as both a planet and a
member of the Kuiper belt.
The story gets even more complicated, however. In 2005, Scientists at Palomar Observatory
(outside San Diego, CA), announced that they had discovered what they claim is a tenth planet,
which they proposed calling ‘Xena’. Planet Xena has one moon, which the team is calling
Gabrielle. Planet Xena, whose official name is 2003 UB313, is now at its aphelion – the furthest
distance from the Sun – which is about 9 billion miles away from the Sun. This makes it about
100 times more distant than the Earth, and about three times more remote than Pluto. UB313 has
a highly elliptical orbit that is inclined about 45 degrees from the main plane of our Solar
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System. (Pluto’s orbital plane also is different than the other planets’ orbital planes.) The
distance of closest approach will be about 3.5 billion miles and the orbital period is 557-years.
For comparison, Pluto’s mean distance from the Sun is about 3.6 billion miles and it orbits in just
248.5 years. Most importantly, Xena is thought to be about one-and-a-half times larger than
Pluto.
If Pluto is a planet, surely Xena is a planet; however, there are a number of astronomers arguing
that Pluto be dropped from the ‘official’ list of planets. The controversy is not settled – keep an
eye on the newspaper for more information. This is a potentially good place to communicate to
your students that science isn’t ‘done’ – we constantly are learning new things and revising our
models of objects like the solar system.
2.1.4. Activities
Remembering the Order of the Planets: Have the entire class help create a mnemonic device
for the initials of the planets from Mercury to Pluto, m.v.e.m.j.s.u.n.p. For example, “Many
Very Excited Martians Jump Super Umbrellas Near Pluto” or “My Very Eager Mother Just
Sewed Us New Pajamas”.
2.1.5. Resources
General Information:
www.solarviews.com
www.nineplanets.org
www.nasa.gov (has an Educators and a Students section)
Pluto and Xena
ƒ http://www.nytimes.com/2001/01/22/science/22PLAN.html?ex=1138424400&en=eaf48b431
b11a4dc&ei=5070 is a New York Times article about Pluto being or not being a planet
ƒ http://www.studyworksonline.com/cda/content/article/0,,EXP666_NAV442_SAR920,00.shtml has a table that lets students compare the characteristics of other
planets with Pluto and decide for themselves whether Pluto is a planet or not.
ƒ http://www.telescopes.com/new-planet/index.php has a news story about planet Xena.
ƒ http://cfa-www.harvard.edu/cfa/ps/icq/ICQPluto.html
2.2. Objective 4.3.2 - The student will be able to describe the motion of objects in
the sky such as sun, moon, and planets.
2.2.1. Key Concepts
ƒ Planets move around the sun in a path called an ellipse.
ƒ The motion of the planets is determined by gravitational interactions. The strongest
interaction is between the Sun and the planets; however, planets also exert gravitational
attraction to each other.
ƒ Satellites stay in orbit about their planets because of the gravitational attraction of the planet.
ƒ A constellation is a grouping of distant stars.
2.2.2. Vocabulary
A Constellation is a formation of stars seen as a figure or design in the night sky; such as the
Big Dipper (Ursa Major) or Little Dipper (Ursa Minor). For more information and a
constellation list, the Hawaiian Astronomical Society has a helpful site at www.hawastsoc.org in
the “Deepsky Atlas” section.
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An Ellipse is an oval-like shape that
describes the path of the planets in the solar
system as they travel around the Sun.
Taurus
Aries
Gemini
Gravity is the force of attraction between
Cancer
Pisces
bodies in space and other objects. Planets
with larger masses have a larger attractive
force and the gravitational force gets
Earth
Leo
Aquarius
stronger as objects get closer to each other.
Sun
The Moon is a small ‘planet’ that revolves
Virgo
Capricorn
around the Earth and reflects the light of
Sagittarius
Libra
Scorpio
the Sun during the night. Neil Armstrong
became the first person to step on the
Moon on July 20th, 1969.
An Orbit is the path a body in space takes as it travels around another body in space. The Earth
is in orbit around the Sun, and the Moon orbits the Earth.
A Satellite is any body in space that orbits another body. The Moon is a satellite of the Earth.
Satellite also refers to any object launched to orbit Earth or another body in space.
2.2.3. The Stars
The stars in the night sky appear to move from night to night relative to a reference on Earth like
a tree or the top of your house. Stars rise in the east and set in the west, like the Sun and Moon.
Although the stars move relative to the Earth, they do not appear to move relative to one another.
Groups of star are called constellations.
Although you can think of constellations rising and setting, stars in different parts of the sky
move at different rates (relative to the Earth – they don’t move relative to each other). Each
hemisphere has one point that doesn’t move. If there are no stars right at this point, the stars near
the point move in a very small circle,
moving once around the circle each day.
The closer you are to a pole, the less
motion you observe. The point in the
Northern Hemisphere is approximately
located by the North Star, Polaris. The
circular motion can be observed by
mounting a camera to take time-exposure
pictures of the motion of these stars. (See
http://antwrp.gsfc.nasa.gov/apod/ap980912
.html for such a picture)
Figure 2.2: Orion as seen from the Hubble Space
Telescope. (The lines are drawn to help you see
the constellation.)
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The Celestial Sphere. The Sun (along with the Moon and planets except for Pluto) moves
across only a certain path in the sky. The stars are independent of the motions of the Sun and the
planets and they maintain the same positions relative to each other. Because the stars do not
appear to move relative to one another, it is convenient to picture them as being contained upon a
giant sphere that surrounds the Earth. If you expanded the Earth until it reached the stars, the
Earth’s equator would be the celestial equator. Earth’s North Pole would be the North Celestial
Pole and Earth’s South Pole would be the South Celestial Pole. If you stand in a flat area at
night, you will see a one-half of the celestial sphere in the sky. You can think of the celestial
sphere as the boundaries to the universe.
The Babylonians used the stars to keep track of time and to navigate. They noticed that the sun
moved only through a particular segment of the sky, called the ecliptic. They marked the ecliptic
by using a set of stars that could be viewed as belonging to twelve constellations, most of which
represented animals. The word zodiac means ‘circle of animals and this band of twelve
constellations is called the zodiac. The Sun appears to traverse the ecliptic once per year,
spending approximately one month in each of the constellations of the zodiac. The ecliptic and
the celestial equator intersect at two points, directly opposite one another. These points
correspond to the equinoxes and when the Sun appears at these points, day and night are each
about 12 hours long at all locations on Earth.1 The Sun traces a path through the sky that is
inclined by an angle of 23.5 degrees relative to the celestial equator. The Sun appears to move
along the ecliptic at a rate of about 1° per day.
2.2.4. Planetary Motion
Five planets can be observed with the naked eye: Mercury, Venus, Mars, Jupiter, and Saturn.
Uranus, Neptune and Pluto are so far away that they require a telescope. Although planets look
like stars in the night sky, they don’t behave the same way as stars. Planets rise in the east and
set in the west (like stars), but they drift a bit to the east relative to the stars. Stars move across
the sky but maintain their positions relative to each other. The planets can have different
positions relative to the fixed background of stars. This feature (along with the fact that star
twinkle and planets don’t) is what allows planets to be identified as distinct from stars. The
Greek word ‘planetes’ means wanderer, which arose from the unusual motions of the planets
compared to the stars.
ellipse
foci
aphelion
b
a
c
foci
perihelion
Figure 2.3: The quantities describing anFigure 2.4: The aphelion and perihelion
ellipse: the semi-major axis, a, the semi-on an elliptical orbit.
minor axis, b, and the distance from one
foci to the origin, c.
1 See the applet at:
http://www.ioncmaste.ca/homepage/resources/web_resources/CSA_Astro9/files/multimedia/unit1/celestial_sphere/celestial_sphere.html
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Orbits. From the start of recorded history, people took note of where the planets were and how
their positions changed each night. This enabled them to know that different planets travel at
different speeds, because the time between Mercury appearing in the same position is much less
than the time between Saturn appearing in the same position. The period is the time is takes a
planet to make one complete circle around the Earth.
Anatomy of an Ellipse If you place two points on a line and attach a string to each point, then
draw a pencil line in all positions where the string is stretched fully, you get an ellipse. The
sums of the distances from the foci to any point on the ellipse are constant. An ellipse has a
semi-major axis (the longer of the two with the total length denoted by 2a) and a semi-minor axis
(the shorter of the two). If c is the distance from the origin to either of the foci, the ratio c/a
gives you the eccentricity, e. The quantities a and e completely define an ellipse.
The smaller e is, the more circular the ellipse is. The Earth's orbit is very close to a circle, with
e = 0.017. Mars has an eccentricity of 0.093 and Mercury has an eccentricity of 0.206. Most
other planets have an eccentricity comparable to the Earth’s. Pluto has such a large eccentricity
(0.248) that it actually becomes close to the Sun than Neptune during part of its orbit. The
eccentricities of Earth and Mars are small enough that if you saw a scale drawing of an orbit on a
sheet of paper, your eye would not be able to distinguish it from a circle. The orbit of Comet
Halley, on the other hand, has e quite close to 1.
Different positions along the orbit have been given names to make it easier to talk about them.
The perihelion is the position when the planet closest to the Sun. The aphelion is the position
when the planet is farthest from the Sun, as shown in Figure 2.4. (helios is ‘Sun’). For satellites
orbiting Earth (which also have an elliptical path), we speak of the perigee and the apogee. (geos
is Earth in Latin.)
Johannes Kepler found that the planets must move around the Sun with variable speed. A planet
close to perihelion moves quickly; when it is close to aphelion, it moves more slowly. The area
is proportional to the distance from the planet to the Sun and how far the planet travels in a
particular time.
Although the ellipse is a symmetric shape, the motion of the planet along the ellipse is not
symmetric. One can make a loose analogy with a stone thrown upwards. It starts off with some
speed, and slows as it rises. At the very top of its path, it
comes to a stop and reverses direction. It them speeds up
again. The motion of a planet about the Sun is similar.
2.2.5. Gravitational Interaction
The reason the planets stay in orbit about the Sun, and the
reason their paths are elliptical is because of the force of
Gravity. All objects with mass have a gravitational attraction
to each other. The gravitational attraction increases as the
masses of the objects increase and also increases as the
objects get closer to each other. The Sun has the largest mass
of any object in our solar system, which is why the planets
orbit about it. The gravitational law was discovered by Sir
Figure 2.5: The Moon as seen
Isaac Newton.
from Apollo 17.
2.2.6. Satellites
Satellites, such as the Moon, stay in orbit about their respective planets via the attraction of
gravity.
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2.2.7. Activities
1. Print a sky map from www.skymaps.com in the Downloads section and pick the map for the
appropriate month and year. Give these maps to the students to use with their parents at home.
The maps show the different constellations and planets that should be visible along with the
locations of visible planets in the night sky. This also can be used as a reference for the each
month.
2. Print a “Star Finder” from spaceplace.nasa.gov under projects and have the students construct
a Star Finder to experiment with at home. This website also has a lot of other science and space
related games, projects, and information geared toward students.
2.2.8. Resources
ƒ www.astro.wisc.edu/~dolan/constellations/ - This site has many links to interactive sky
charts, a constellation index, and different space photos. It is a great site if you are curious or
want more in-depth information.
ƒ skyandtelescope.com/observing/ - This site has an interactive sky chart that can be
customized to your location. The sky view can be rotated to see what is on the horizon and
what is directly overhead.
2.3. Objective 4.3.3 - The student will be able to create a scale model of the solar
system showing relative distance and size.
2.3.1. Key Concepts
ƒ Students should understand that a scale drawing applies the same reduction ratio to whatever
is being scaled.
ƒ Students should be able to create a scale drawing of the planets’ sizes.
ƒ Students should be able to create a scale drawing of the planets’ distances from the Sun.
ƒ Student should be able to compare the planets’ dimensions or distances from the Sun using
ratios.
2.3.2. Vocabulary
Ratio – The relation between to numbers expressed as 6 to 1, 6/1, or 6:1. This can be applied to
scaled-down drawings with 1,000,000 kilometers to 1 inch, for example.
Scale – A proportion used to determine the relationship of a model to what it is representing. A
scale of 1 inch on a map equals 4 miles on the Earth.
2.3.3. Activities
1. Toilet Paper Solar System – Using the ratio of one sheet of standard toilet paper to 10 million
miles, a scale model of the solar system can be created in the hallway. Using the following chart
below, unroll your solar system and have a student stand at the location for each planet. (From
Dr. Tim Slater, Montana State University, solar.physics.montana.edu/tslater)
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Number of Sheets
from Sun
# of Tissues from
previous object
# of Units
(feet or yards)
Sun
0.0
0.0
0.0
Mercury
3.6
3.6
1.2
Venus
6.7
3.1
2.2
Earth
9.3
2.6
3.1
Mars
14.1
4.8
4.7
Jupiter
48.4
34.3
16.1
Saturn
88.7
40.3
29.6
Uranus
178.6
90
59.5
Neptune
280.0
101.0
93.3
Pluto (avg. orbit)
366.4
86.4
122.1
Celestial Object
This could also be done with a piece of string or rope a little over 122 units long, with the units
being either feet or yards depending on the space (see chart). Place a piece of tape in the planet’s
location so it can be wound up and reused next year.
2. Solar System Scale Model – Using a long field or sidewalk, some stakes, and different sized
balls to create a scale model of the solar system. Start with a Sun ball 11cm or about 4 inches in
diameter at your starting point. Moving away from the Sun, mark and label the location of the
planets. Place a ball of the correct size at each location as well. The table below shows the
planet, distances from the Sun, and ball diameters. Set this activity up before hand and the walk
the students from the Sun to Pluto.
Planet
Distance from Sun
Approx. Planet
(yds)
Diameter (mm)
Sun
0
110
Mercury
5
0.4
Venus
9
0.4
Earth
13
1
Mars
20
0.5
Jupiter
67
11
Saturn
122
9
Uranus
246
4
Neptune
386
4
Pluto
507
.2
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2.4. Objective 4.3.4 – Students will be able to understand how the seasons and
phases of moon are affected by the motion of the Earth and moon the moon are
affected by the motion of the Earth and moon
2.4.1. Key Concepts
ƒ Understand how the days on Earth change because of rotation around a central axis
ƒ Understand how it takes an entire revolution around the Sun for one year to pass on Earth.
ƒ Understand how the angle of the Earth’s axis and the revolution of the Earth around the Sun
affect the seasons.
ƒ Understand how the revolution of the Moon around the Earth changes the phases of the
Moon.
2.4.2. Vocabulary
Axis – A straight line about which a body rotates or seems to rotate. The Earth’s axis is slightly
off from the North and South Poles and is tilted about 23.5 degrees. This accounts for the
change in seasons as the Earth revolves around the Sun.
Day - The length of time it takes a planet to rotate once about its axis. There are 24 hours in a
day on Earth, but a day on Jupiter is less than 10 hours.
Revolution – The orbital path taken by the planets around the Sun. The Earth completes one
revolution around the Sun in 1 year.
Rotation – The turning of a body around a central axis. The Earth completes one rotation
around its axis in 24 hours.
Year – The length of time that it takes a planet to travel around the Sun. A year on Earth is 365
days, 5 hours, and 49 minutes; which is why an extra day is added every fourth calendar year. A
year on Pluto is over 248 Earth years and a year on Mercury is only about 88 Earth days.
2.4.3. The Motion of the Sun.
Figure 2.6: The Sun early in the morning (top), at noon
(middle) and
E in the afternoon (bottom).
The Day.
Observations of the
behavior of the Sun over a long
N
S
period of time show that some things
W
do not change, even over the course
of many years. For example:
E
• The Sun always rises from
N
S
roughly the same direction (east)
and sets in the opposite direction
W
(west)
E
• In between rising and setting,
N
S
the Sun follows a long arc. The
Sun is furthest from the horizon
W
halfway between rising and
setting. We call this position
noon. We can use this periodic (regularly repeating) motion to define the day as the time
from one noon to the next.
The position of the Sun is different at different times of the day. A vertical pole placed into the
ground casts a shadow. The shadow will be long in the morning and afternoon, and shortest
when the shadow cast by the pole points south (or north) – which happens at noon.
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Consider three different positions of the Sun, as shown in Figure 2.7. Remembering that shadow
always points away from the Sun, we find that:
• In the morning, the Sun is in the east and the shadow points to the west.
• The shadow falls due north when the Sun it at its highest point.
• In the evening, the Sun is in the west, and the shadow is toward the east.
The Week. The fact that there are seven days to the week is a result of there being seven
stars/planets (The Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn) visible to the naked
eye. The days were named according to how long each took to move across the sky. Saturn’s
day became Saturday, Sun’s day became Sunday, Moon’s day became Monday. The remaining
days are named after French, Italian, or German words for the remaining planets. The number
seven acquired some of its mystique from there being seven known planets.
The Seasons in Terms of the Position of
the Sun as Seen from Earth. If you track
Path in December
Path in July
the position of the Sun carefully for a year
E
and use trees or telephone poles as
references, you would notice that the Sun
doesn’t follow the same path every day.
The path and the position of the Sun,
change depending on the time of year.
S
N
Figure 2.7 shows the different paths of the
Sun at different times of the year.2
The direction of the shadow when the Sun
W
is directly overhead does NOT change day
to day, but the directions of shadows at
other times of the day do change. The Sun Figure 2.7: How the path the Sun takes through
rises exactly in the east and sets exactly in the sky changes with the seasons.
the west only twice each year. These special days are called equinoxes because the length of the
day and the length of the night are approximately equal. The autumnal equinox in is the fall and
the vernal equinox is in the spring.
The positions of the sunrise and sunset move south as fall changes into winter. The steepness of
the curve traced by the Sun does not change, nor does the rate at which the Sun moves along the
path; however, the length of the curve becomes shorter. The Sun takes less time to travel the
shorter path, which decreases the number of hours of sunlight during the winter months. The
winter solstice, when the Sun takes the shortest path of the year through the sky, is around
December 21, which is halfway between the equinox dates (typically September 23 and
March 21). The winter solstice is the day with the fewest daylight hours.
After the winter solstice, sunrise and sunset positions move northward, and days get longer as the
path of the Sun gets longer. The summer solstice is the day when the Sun crosses the horizon at
its most northerly position (usually around June 21). The summer solstice is the day of the year
with the longest number of hours of sunlight. The process repeats every year.
2 You can see the path of the Sun as a function of day and month using the applet at http://solar.anu.edu.au/Sun/SunPath/ or at
http://www.jgiesen.de/SunView/index.htm.
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The Seasons as Seen from Space. The year is divided into four parts by the two solstices and
the two equinoxes. The solstices are the longest day and longest night, and the equinoxes are
when night and day are equal. These define the starts of the seasons. The time between the
spring equinox in 2003 (March 21) and the fall equinox in 2003 (September 22) is 184 days;
however, the time between the fall equinox and the spring 2004 equinox (March 20) is 181 days.
The equinox positions correspond to the Earth being on exactly opposite sides of its orbit. Why
are there three days fewer in summer than there are in winter?
The Earth moves a little faster in winter. The Earth is closest to the Sun (at perihelion) around
January 4 – and moving at its fastest speed. The half of the ellipse closest to the Sun is shorter as
well, which gives rise to a difference of 3 days. Note that Earth is closest to the Sun at the winter
solstice and furthest from the Sun at the summer solstice.
The Earth rotates on an axis that is inclined 23° from the vertical toward the Sun, as shown
schematically in Figure 2.8. The tilt of the Earth explains why days and nights vary in length,
why seasons change and why climates vary with latitude. As the Earth orbits about the Sun, the
Northern Hemisphere is oriented so that it is tilted toward the Sun in summer and away from the
Sun in winter. Figure 2.9 shows the position of the Earth at different times of the year.
axis
23°
Figure 2.8: The Earth rotates on its axis.
The beginning of a season is recognized from the length of the daylight period, the altitude of the
Sun in the sky at noon, and the length of the shadow of a vertical stick at noon. On June 22nd and
Dec 22nd, the Sun reaches its highest and lowest noon altitudes. In the summer, the North Pole is
pointed toward the Sun, so there are more hours of daylight. The noon Sun is at its highest
position of the year on the June 22nd, and the shadow of a pole will be the shortest it will be all
year. You can see from this picture why very northerly countries have periods during which the
Sun does not set. Conversely, in the winter, the North Pole is pointed away from the Sun, so the
hours of daylight are shorter, and on December 22nd, the noon Sun is the lowest in the sky it will
be all year.
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Autumnal Equinox
Winter Solstice
Summer Solstice
23°
Vernal Equinox
Figure 2.9: The seasons (drawing is not to scale). Remember that the Earth actually is closer to
the Sun in the winter than it is in the Summer.
Because of the elliptical orbit, Earth is about 2.5 million kilometers closer to the Sun in January
than its average orbit and about the same distance further away from the Sun in July. The Earth
as a whole gets 6% more solar energy in January than in July. Clearly, the distance from the Sun
is not as important as the effects of the tilted axis.
2.4.4. Hours and Minutes. The division of the day into 24 hours, the hour into 60 minutes and
the minute into 60 seconds has a number of explanations. One possibility is that it is a result of
the Babylonians using a base 60 number systems. 12 (the approximate number of daylight
hours) would be 60/5. The Babylonians had a 360 (6*60) day year, which likely was a
compromise between the 365 day solar year and the 354-day lunar year.
2.5. The Moon
2.5.1. Phases of the Moon: Like the Sun, the Moon rises in the east and sets in the west.
Unlike the Sun, the Moon takes on difference appearances – different phases – at different times
of the month. Figure 2.10 shows the different phases of the Moon as seen from Earth. There are
two crescent, gibbous, and half phases each month, but these phases are reflections of each other
different. The amount of lighted area increases from the new Moon to the full Moon and
decreases from the full Moon
to the new Moon. The Moon
is said to be waxing when
changing from New Moon to
Full Moon and waning when
changing the Full Moon to
New
Waxing
First
Waxing
Full
Waning
Last
Waning
Moon
Crescent Quarter Gibbous
Moon
Gibbous Quarter Crescent
New Moon. The phases are
mirror images of each other.
This chart is only good for the Figure 2.10: Phases of the Moon.
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Northern Hemisphere – the effect is the opposite in the Southern Hemisphere.
2.5.2. The Month: The cycle of phases repeats itself about once every 29.5 days and this is how
the month originally was defined. Many calendars were based on the phases of the Moon. The
Metonic calendar (developed by an ancient Greek astronomer named Meton) is one of the mostused lunar calendars. Unfortunately, the lunar calendar is not commensurate with the solar
calendar. This means that the number of complete cycles of the Moon does not evenly divide
into the length of the year. The Metonic calendar is corrected – seven months must be added
every 19 years to keep the calendar in synchronization with the seasons. The year has a length of
12 + 7/19 months, which turns out to be nearly 365 days. The Hebrew calendar is based on the
Metonic calendar, with each month beginning at or near the new Moon. The Moslem and
Persian calendars are true lunar calendars and depend strictly on observation of the new Moon to
begin a new month. This means that one year, summer might be in the equivalent of July, while
15 years later, summer would be in the
equivalent of December.
Figure 2.11 shows the phases in a different
way.
View
the
applet
at
http://www.scienceu.com/observatory/articles/
phases/phases.html to see an animated version
of this explanation.
2.5.3. Eclipses
During an eclipse, a celestial object (like the
Sun or the Moon) is blocked (as in the right
picture of Figure 2.12). The blockage can be
total, as shown in Figure 2.12 or partial. Solar
eclipses are when the Sun is blocked and lunar
eclipses are when the Moon is blocked. Total
eclipses happen extremely rarely. A movie of Figure 2.11: Phases of the Moon. The Sun is
a solar eclipse (a view of the Sun as seen from to the left.3
Earth)
can
be
viewed
at
http://burro.astr.cwru.edu/denise/Spring03/Jan28/Freds_dundlod_movie.mpeg.
The eclipses we see from Earth are the lunar eclipse, in which the Earth passes directly between
the Sun and the full Moon such that the Earth’s shadow falls on the Moon, and the solar eclipse,
Lunar Eclipse: the Earth passes directly between the Sun
and the Moon
Sun
Earth
Moon
Solar Eclipse: the Moon passes directly between the Sun
and the Earth.
Moon
Sun
Figure 2.13:
(Not to scale)
3
Earth
Lunar and solar eclipses.
Figure 2.12: A total eclipse of the Sun.
http://www.windows.ucar.edu/tour/link=/the_universe/uts/phases_gif.html
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in which the Moon passes directly between the Sun and the Earth, as shown in Figure 2.13. It
takes about eight minutes for a solar eclipse to be completed and about 100 minutes for a lunar
eclipse to be completed. In both, a shadow is cast that partially or fully obscures the Sun or
Moon.
2.6. Precession
In the second century BC, the Greek
astronomer Hipparchus measured stars’
brightness and positions. The star catalogue
he compiled was used for centuries. When he
Polar diameter
compared his observations with those made
12714 km
by astronomers over a century earlier, he
found a systematic shift in the positions of the
stars.
The Earth is not a perfect sphere – it is
Equatorial diameter
flattened, as if someone had squeezed the
12756 km
poles together (See Figure 2.14.)
The
equatorial diameter is 21 kilometers greater Figure 2.14: The ‘equatorial bulge’ of the
than the polar diameter. The squeezing Earth. The degree of the bulge is exaggerated.
wasn’t perfectly symmetric either, so Earth
actually is a lopsided spherical oblate. This lopsided shape is due to the rotation of the Earth
about its axis, much like a piece of pizza dough flattens out when you spin it in the air.
The flattened shape, the rotation of the Earth about its axis, and the orbiting of the Earth about
the Sun combine to cause the Earth to precess – that is, the axis about which the Earth rotates
also moves, as shown in Figure 2.15. Precession is much like the motion of a top - a top doesn’t
rotate with the axis standing straight up – the axis actually moves around. A nice applet can be
found at: http://www.jgiesen.de/astro/precession/.
Earth’s' axis meets the celestial sphere at the North and South Celestial Poles. As the axis
precesses, the locations of the poles change. The Earth’s axis describes a cone. The axis will
travel around this cone once every 25,800 years.
The North Celestial Pole (NCP) is about 1 degree from
Polaris (current North Star)
Polaris. In 2100, the NCP will be the closest to Polaris at Vega (future North Star)
amount 27 arc-minutes. Two thousand years ago, Polaris
was 12° from the NCP. (There is no bright star near the SCP
at present) In 6,000 years the Earth's axis will point towards
the star Alderamin in Cepheus, and in 12,000 years it will be
near Vega in Lyra.
NASA scientists studying the Indonesian earthquake of Dec.
26, 2004, have calculated that it slightly changed our planet's
shape, shaved almost 3 microseconds from the length of the
day, and shifted the North Pole. According to the latest
calculations, the Dec. 26th earthquake shifted Earth's "mean
North Pole" by about 2.5 centimeters in the direction of 145
degrees east longitude, more or less toward Guam in the
Pacific Ocean. This shift is continuing a long-term seismic
Figure 2.15: The precession of
trend identified in previous studies.
the Earth. Distances are not to
scale.
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2.6.1. Activities
1. Moon Phases - Have the entire class sit in a circle around a single student who is holding a
ball (the Moon) above his/her head. Give each student a piece of paper with a 6 inch circle in the
center and the word ‘Moon’ at the top. Turn the lights down (or off) and shine a flashlight (the
Sun) at the ball in the center of the circle and stay in this position. Now have each student color
in the dark part of the ball as they see it so there is a light part and a dark part of the appropriate
size. Once everyone has finished their drawings, turn the lights on and have everyone turn the
drawings toward the inside of the circle with the word Moon at the top. There should be a
drawing that is all white directly in front of the person holding the flashlight with the moons
getting more filled in you go around the circle to the halfway point where it will be all black.
The white portions should get larger and larger until your back at the beginning. Make sure to
also walk around the outside of the circle with the flashlight on the ball so each student can see
the moon change phases. Note: Make sure that the flashlight isn’t pointed directly into a
student’s eyes.
2. Earth’s Seasons – Mark an equator around a foam ball and then place a stick or pencil into the
ball’s South Pole. Have a student hold the ball at a slight angle (about 20 degrees) and walk
around it with a flashlight. There are times when the upper half of the ball is getting more light
and times when the lower half is getting more light, these are summer and winter, respectively,
with spring and fall in between them. This could also be done with a globe tilted to about 20
degrees. Make sure to note that the Earth is really what rotates around the Sun. To make this
more evident, one student could hold the Earth always tilted towards the front of the class and
walk around another student with a flashlight always pointing at the Earth. It could also be
pointed out that when it is summer in the Northern Hemisphere it is winter in the Southern
Hemisphere and vice versa.
2.6.2. Resources
ƒ Go to www.astro.wisc.edu/~dolan/constellations/ and click on “Demonstration of Moon
Phases”. When the applet loads, select ‘Both’ from the pull down menus, and click
‘Animate’. This may not be suitable for the students, but will give you a good idea of what is
happening. Notes: The dark side of the Earth is the nighttime side with the point furthest
from the Sun being Midnight; the closest point to the Sun is noon. This demo requires that
Java be installed on the computer, if it is not already installed. This can be downloaded from
java.sun.com for free.
ƒ Also, the ‘Moon Phase Activity’ in the ‘In-Class Activites’ section of
www.learner.org/teacherslab/pup/ has another moon phase activity that uses the students
heads as Earth and a styrofoam ball as the moon. There are also other space related activities
available on this website.
ƒ An applet at: http://www.jgiesen.de/moonyear/index.htm shows a complete lunar calendar.
2.7. Objective 4.3.5 - The student will be able to develop an understanding of
asteroids, meteoroids, and comets in our solar system as well as stars beyond
our solar system.
2.7.1. Key Concepts
ƒ Understand where asteroids and the asteroid belts are located.
ƒ Be able to differentiate between comets, meteors, meteorites, meteoroids, and craters that can
be formed when an impact occurs.
ƒ Know that our galaxy is the Milky Way
ƒ Understand the importance of the North Star
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ƒ Understand that a light year is a unit of distance and appreciate how large a distance it is.
2.7.2. Vocabulary
Asteroid – Any of the numerous small bodies in space that also revolve around the Sun. Most
are located between Mars and Jupiter, and can be a few to several hundred kilometers across.
Asteroid Belt – The region between Mars and Jupiter where most asteroids are located. (See
Figure 2.1.)
Comet – A body in space that is only observed in the part of its orbit that is relatively close to the
Sun. They consist of a head followed by an elongated tail made of mostly vapor.
Crater – A bowl shaped depression typically formed during the impact of a meteoroid with a
planet or moon.
Galaxy – Any of the large number of groupings consisting of stars, gas, and dust that make up
the universe.
Light-year – The distance light travels in space in one Earth year; roughly 5.88 trillion miles.
Meteor – A bright tail or streak that is seen when a meteoroid burns up in the atmosphere, also
called a shooting star.
Meteorite – A stony or metallic mass that has fallen to the Earth’s surface from space.
Meteoroid – A solid body moving in space that is smaller than an asteroid but larger than a speck
of dust. A meteoroid becomes a meteor if it burns up in the atmosphere or a meteorite if it falls
to the Earth’s surface.
Milky Way – The galaxy that contains the solar system, it can be seen at night as a wide band of
faint light in the sky.
North Star – The northern axis of the Earth points towards it in the night sky. It can be found as
the end of the handle on the Little Dipper constellation.
Universe – Everything that is contained in space; including all the planets, the stars, and the
galaxies.
Asteroids in an asteroid belt
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Meteor or shooting star burning up in the
Meteorite found on Earth
Earth’s atmosphere
Crater created by a meteorite hitting the
Earth. This crater is 20 miles west of The Milky Way
Winslow, AZ.
2.7.3. Resources
ƒ http://www.solarviews.com/eng/tercrate.htm is a gallery of crater pictures.
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3. Nature and History
of Science
3.1. Astrology
3.1.1.
Astrology vs. Astronomy.
Astrology is the belief that events on
Earth are influenced by the motions
of the planets. Astrology started
4000 years ago in Babylonia and
became part of the Greek culture
when they conquered that part of the
world. Eventually, people came to
believe that the positions of the Sun,
Moon, and planets at a person's birth
were especially significant. This was
one of the driving forces for
developing models that could predict
the positions of the planets and the
stars.
Astrologers focused on predicting the
future of human events and
astronomers focused on predicting
the motion of the planets, Sun, and
Moon. Many early astronomers,
Figure 3.1: The position of the Sun relative to the
however, felt that being able to
zodiac at two different times of the year.
predict the motions of the planets
would allow them to more accurately predict people’s futures. A number of famous astronomers
in the past also were astrologers – casting horoscopes was one way they supported themselves
financially. Patrons were much more willing to pay for advice about the future than they were
for scientific discovery. Tycho’s interest in making more accurate measurements was initiated in
part because of problems with his astrological calculations due to inaccurate observational tables.
Although many ancient astronomers also were astrologers, modern astronomers do not believe
that the motions of the planets affect the future.
3.1.2. Is Astrology a Science? Astrology assigns you a ‘sign’, according to the zodiac
constellation the Sun was in at your birth. Figure 3.1 shows you one problem with this, which is
that the Sun spends more time in large constellations like Scorpio and Virgo than in small
constellations like Libra and Cancer. The signs, however, all cover 30 or 31 days. Astronomers
like to point out that there actually is a 13th constellation. The Sun spends about 10 days in the
constellation of Scorpius, and then 20 days in Ophiuchus (the serpent holder). This constellation
isn’t included in astrology.
Because of the Earth’s precession, the Sun was probably in the constellation before your official
‘sign’ because the spring equinox moves westward one degree every 72 years. Three thousand
years ago, the Sun entered the ‘house’ or constellation, of Virgo in August. Astrological
forecasts today still assume that this is where the Sun is – but it actually is in the house of Leo in
August now.
A horoscope includes the position of each planet relative to the zodiac and with respect to the
person at the time of his/her birth. There are some standard rules for creating a horoscope,
although many have not changed for thousands of years despite the dramatic improvements in
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our knowledge of how the planets and stars move. There also is a strong subjective component
in how much emphasis an astrologer will give to each rule in developing the horoscope. Two
astrologers can cast different horoscopes for the same person – how do you decide which to
believe?
Activity: For one week, consult four different horoscopes (you may have to find them on the
web—make sure they are from different people and not just copies of syndicated horoscopes).
Compare the horoscopes to each other, and compare them to what happens to you each of those
days.
The question of the mechanism by which the planets influence people is unknown. The only
forces that exist between planets and people are electromagnetic and gravitational. We will
calculate in the next unit that the gravitational force due to a doctor delivering a baby is greater
than the force of gravity due to any of the planets.
What types of tests could be done to check whether astrology has a scientific basis? For
example, one might expect that leaders would share some astrological characteristics, but studies
of the birthdates of presidents or governors, etc. show that they are randomly distributed between
the signs.
An episode of NOVA (on PBS) showed a researcher working with a group of college students all
professing a belief in astrology. The researcher gave each person their own individual
horoscope. Each person found some event in their day that fit their horoscope. The researcher
then asked each person to give their horoscope to the person behind them. The students
discovered that these new horoscopes also described some event in their day.
The French researcher Michel Gaugelin sent a horoscope of a mass murderer to 150 people but
told each one that the horoscope was prepared just for him or her. Over ninety percent of them
said they could see themselves in that horoscope. The Australian researcher Geoffrey Dean
substituted phrases in the horoscopes of 22 people that were opposite of the original phrases in
the horoscopes. Ninety-five percent of time they said the horoscope readings applied to them just
as well as to the people to whom the original phrases were given.
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3.2. Nature of Science: What vs. Why
Stonehenge (see Figure 3.2), which started
being built in 2800 B.C. suggests that, even
prior to developing a written language, people
created tools to keep time. On the solstices, the
Sun lines up directly with gaps in the stones,
although we do not know for sure that this was
the intent of the original builders.
The civilizations of the Euphrates River valleys
(Babylonians and Chaldeans) and the Nile
River valleys (Egyptians) made huge
contributions to early astronomy.
The
Babylonians maintained observatories, kept
records, compiled star catalogs and were able
to predict eclipses based on those observations.
Their observations allowed them to make
calendars, which in turn told them when to
plant crops and observe religious events.
The evidence presented in this chapter shows
that the universe is a regular, predictable place. Figure 3.2: A schematic map of Stonehenge.
We can calculate and predict the behavior of the Sun, Moon, stars, and planets. We can predict
eclipses. We will show in future chapters that we can make detailed models that explain
virtually all observations. Is this science?
Systematic observation is a critical part of science. Observations prompt the development of
models and theories, and serve as their ultimate test. The Babylonians did not have any sense of
causality in their concept of nature. They believed that individual gods created and controlled
different parts of nature. Any explanation other than one originating from religious beliefs was
not within their realm of thought. It is one thing to get a series of observations and, from that, be
able to predict future behavior, but science wants to know why as well as what.
The belief that it is possible for us to understand the world and the reasons why things happen
has not always been part of human culture. Philosophy, from which physics (formerly known as
‘natural philosophy’) evolved, played an important role in man’s quest to know and understand
the world.
4. General Resources
4.1. Internet Resources
There are several good sites to get updates on NASA and other space agency missions into
space, as well as other space related news. They include:
ƒ www.cnn.com - The ‘Science and Space’ section has updates as well as an archive of older
space related headlines.
ƒ www.planetquest.com – There are interactive space maps in the ‘Planets’ section that
describe planet history, composition, myths, and different features about the planets.
ƒ www.nasakids.com – Many interactive activities, as well as a ‘Teacher’s Corner” with more
information about space education.
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ƒ
www.spaceplace.nasa.gov – Similar to nasakids.com with interactive games and student
level information about space and NASA. ‘Teacher’s Corner’ has different activities from
those at nasakids.com.
The Department of Physics and Astronomy has two observatories that have public viewing
nights that are free to the public.
ƒ
The Student Observatory is located on top of the parking garage next to Memorial Stadium.
Their website http://www.physics.unl.edu/directory/gaskell/stdobs.html gives times and
dates. Note that the telescope building is not heated and dress accordingly.
ƒ The Behlen Observatory in Mead (38 miles NE of Lincoln) (http://astro.unl.edu/observatory/)
also has public viewing nights. The same warnings about dressing for the weather apply.
ƒ The Prairie Astronomy Club meets at Hyde Observatory in Holmes Lake Park
http://www.prairieastronomyclub.org/). In addition to viewing nights, there are nights where
people are invited to bring their own telescopes and have club members help them learn to
use the telescopes more fully.
http://www.shatters.net/celestia/ - Fly through the galaxy
http://www.fourmilab.ch/yoursky/ - Gives you a sky map when you enter a longitude and
latitude.
4.2. Planetary Information
The table on the next page collects some of the relevant data for all of the planets.
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MERCURY
VENUS
EARTH
MOON
MARS
JUPITER
SATURN
URANUS
NEPTUNE
PLUTO
Mass (1024kg)
0.330
4.87
5.97
0.073
0.642
1899
568
86.8
102
0.0125
Diameter (km)
4879
12,104
12,756
3475
6794
142,984
120,536
51,118
49,528
2390
Density (kg/m3)
5427
5243
5515
3340
3933
1326
687
1270
1638
1750
Gravity (m/s2)
3.7
8.9
9.8
1.6
3.7
23.1
9.0
8.7
11.0
0.6
Rotation Period (hours)
1407.6
-5832.5
23.9
655.7
24.6
9.9
10.7
-17.2
16.1
-153.3
Length of Day (hours)
4222.6
2802.0
24.0
708.7
24.7
9.9
10.7
17.2
16.1
153.3
Distance from Sun (106
km)
57.9
108.2
149.6
0.384*
227.9
778.6
1433.5
2872.5
4495.1
5870.0
Perihelion (106 km)
46.0
107.5
147.1
0.363*
206.6
740.5
1352.6
2741.3
4444.5
4435.0
Aphelion (106 km)
69.8
108.9
152.1
0.406*
249.2
816.6
1514.5
3003.6
4545.7
7304.3
Orbital Period (days)
88.0
224.7
365.2
27.3
687.0
4331
10,747
30,589
59,800
90,588
Axial Tilt (degrees)
0.01
177.4
23.5
6.7
25.2
3.1
26.7
97.8
28.3
122.5
Mean Temperature (C)
167
464
15
-20
-65
-110
-140
-195
-200
-225
Surface Pressure (bars)
0
92
1
0
0.01
Unknown*
0
Number of Moons
0
0
1
0
2
63
47
27
13
1
Ring System?
No
No
No
No
No
Yes
Yes
Yes
Yes
No
Global Magnetic Field?
Yes
No
Yes
No
No
Yes
Yes
Yes
Yes
Unknown
MERCURY
VENUS
EARTH
MOON
MARS
JUPITER
SATURN
URANUS
NEPTUNE
PLUTO
Unknown* Unknown* Unknown*
Data from: http://nssdc.gsfc.nasa.gov/planetary/factsheet/
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Key to the data on the previous page
Mass (1024kg or 1021tons) - This is the mass of the planet in septillion (1 followed by 24 zeros)
kilograms or sextillion (1 followed by 21 zeros) tons. Strictly speaking tons are measures of
weight, not mass, but are used here to represent the mass of one ton of material under Earth
gravity.
Diameter (km or miles) - The diameter of the planet at the equator, the distance through the
center of the planet from one point on the equator to the opposite side, in kilometers or miles.
Density (kg/m3 or lbs/ft3) - The average density (mass divided by volume) of the whole planet
(not including the atmosphere for the terrestrial planets) in kilograms per cubic meter or pounds
per cubic foot.
Gravity (m/s2 or ft/s2) - The gravitational acceleration on the surface at the equator in meters per
second squared or feet per second squared, including the effects of rotation. For the gas giant
planets the gravity is given at the 1 bar pressure level in the atmosphere. The gravity on Earth is
designated as 1 "G", so the Earth ratio fact sheets gives the gravity of the other planets in G's.
Escape Velocity (km/s) - Initial velocity, in kilometers per second or miles per second, needed at
the surface (at the 1 bar pressure level for the gas giants) to escape the body's gravitational pull,
ignoring atmospheric drag.
Rotation Period (hours) - This is the time it takes for the planet to complete one rotation relative
to the fixed background stars (not relative to the Sun) in hours. Negative numbers indicate
retrograde (backwards relative to the Earth) rotation.
Length of Day (hours) - The average time in hours for the Sun to move from the noon position
in the sky at a point on the equator back to the same position.
Distance from Sun (106 km or 106 miles) - This is the average distance from the planet to the
Sun in millions of kilometers or millions of miles, also known as the semi-major axis. All planets
have orbits which are elliptical, not perfectly circular, so there is a point in the orbit at which the
planet is closest to the Sun, the perihelion, and a point furthest from the Sun, the aphelion. The
average distance from the Sun is midway between these two values. The average distance from
the Earth to the Sun is defined as 1 Astronomical Unit (AU), so the ratio table gives this distance
in
AU.
* For the Moon, the average distance from the Earth is given.
Perihelion, Aphelion (106 km or 106 miles) - The closest and furthest points in a planet's orbit
about
the
Sun,
see
"Distance
from
Sun"
above.
* For the Moon, the closest and furthest points to Earth are given, known as the "Perigee" and
"Apogee" respectively.
Orbital Period (days) - This is the time in Earth days for a planet to orbit the Sun from one
vernal equinox to the next. Also known as the tropical orbit period, this is equal to a year on
Earth.
* For the Moon, the sidereal orbit period, the time to orbit once relative to the fixed background
stars, is given. The time from full Moon to full Moon, or synodic period, is 29.53 days.
Axial Tilt (degrees) - The angle in degrees the axis of a planet (the imaginary line running
through the center of the planet from the north to south poles) is tilted relative to a line
perpendicular
to
the
planet's
orbit
around
the
Sun.
*Venus rotates in a retrograde direction, opposite the other planets, so the tilt is almost 180
degrees, it is considered to be spinning with its "top", or north pole pointing "downward"
(southward). Uranus rotates almost on its side relative to the orbit, Pluto is pointing slightly
"down". The ratios with Earth refer to the axis without reference to north or south.
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Mean Temperature (C or F) - This is the average temperature over the whole planet's surface
(or for the gas giants at the one bar level) in degrees C (Celsius or Centigrade) or degrees F
(Fahrenheit). For Mercury and the Moon, for example, this is an average over the sunlit (very
hot) and dark (very cold) hemispheres and so is not representative of any given region on the
planet, and most of the surface is quite different from this average value. As with the Earth, there
will tend to be variations in temperature from the equator to the poles, from the day to night
sides, and seasonal changes on most of the planets.
Surface Pressure (bars or atmospheres) - This is the atmospheric pressure (the weight of the
atmosphere per unit area) at the surface of the planet in bars or atmospheres.
*The surfaces of Jupiter, Saturn, Uranus, and Neptune are deep in the atmosphere and the
location and pressures are not known.
Number of Moons - This gives the number of IAU officially confirmed moons orbiting the
planet. New moons are still being discovered.
Ring System? - This tells whether a planet has a set of rings around it, Saturn being the most
obvious example.
Global Magnetic Field? - This tells whether the planet has a measurable large-scale magnetic
field. Mars and the Moon have localized regional magnetic fields but no global field.
The term "terrestrial planets" refers to Mercury, Venus, Earth, Moon, Mars, and Pluto.
The term "gas giants" refers to Jupiter, Saturn, Uranus, and Neptune.
This data is from NASA: http://nssdc.gsfc.nasa.gov/planetary/factsheet/
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