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* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
A Guide to Space Project Fulcrum is supported by the National Science Foundation and the University of Nebraska, in partnership with Lincoln Public Schools Version 1.0 1/26/06 Project Fulcrum 1 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. Version 1.0 1/26/06 Project Fulcrum 2 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 The Solar System is a grouping of nine known planets that orbit the Sun. 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. Version 1.0 1/26/06 Project Fulcrum 3 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 Version 1.0 1/26/06 Project Fulcrum 4 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 Version 1.0 1/26/06 Project Fulcrum 5 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 Version 1.0 1/26/06 Project Fulcrum 6 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 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 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. http://www.telescopes.com/new-planet/index.php has a news story about planet Xena. 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 Planets move around the sun in a path called an ellipse. 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. Satellites stay in orbit about their planets because of the gravitational attraction of the planet. 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. Version 1.0 1/26/06 Project Fulcrum 7 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.) Version 1.0 1/26/06 Project Fulcrum 8 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 Version 1.0 1/26/06 Project Fulcrum 9 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. Version 1.0 1/26/06 Project Fulcrum 10 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 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. 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 Students should understand that a scale drawing applies the same reduction ratio to whatever is being scaled. Students should be able to create a scale drawing of the planets’ sizes. Students should be able to create a scale drawing of the planets’ distances from the Sun. 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) Version 1.0 1/26/06 Project Fulcrum 11 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 Version 1.0 1/26/06 Project Fulcrum 12 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 Understand how the days on Earth change because of rotation around a central axis Understand how it takes an entire revolution around the Sun for one year to pass on Earth. Understand how the angle of the Earth’s axis and the revolution of the Earth around the Sun affect the seasons. 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. Version 1.0 1/26/06 Project Fulcrum 13 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. Version 1.0 1/26/06 Project Fulcrum 14 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. Version 1.0 1/26/06 Project Fulcrum 15 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. Version 1.0 1/26/06 Project Fulcrum 16 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 Version 1.0 1/26/06 Project Fulcrum 17 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. Version 1.0 1/26/06 Project Fulcrum 18 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 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. 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. 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 Understand where asteroids and the asteroid belts are located. Be able to differentiate between comets, meteors, meteorites, meteoroids, and craters that can be formed when an impact occurs. Know that our galaxy is the Milky Way Understand the importance of the North Star Version 1.0 1/26/06 Project Fulcrum 19 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 Version 1.0 1/26/06 A comet traveling through space Project Fulcrum 20 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 http://www.solarviews.com/eng/tercrate.htm is a gallery of crater pictures. Version 1.0 1/26/06 Project Fulcrum 21 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 Version 1.0 1/26/06 Project Fulcrum 22 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. Version 1.0 1/26/06 Project Fulcrum 23 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: www.cnn.com - The ‘Science and Space’ section has updates as well as an archive of older space related headlines. www.planetquest.com – There are interactive space maps in the ‘Planets’ section that describe planet history, composition, myths, and different features about the planets. www.nasakids.com – Many interactive activities, as well as a ‘Teacher’s Corner” with more information about space education. Version 1.0 1/26/06 Project Fulcrum 24 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. 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. 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. 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. Version 1.0 1/26/06 Project Fulcrum 25 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/ Version 1.0 1/26/06 Project Fulcrum 26 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. Version 1.0 1/26/06 Project Fulcrum 27 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/ Version 1.0 1/26/06 Project Fulcrum 28