Download Lecture 3, PPT version

Outline - Jan. 21, 2010
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Seasons (pgs. 28-30, 36-40)
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Phases of the Moon (pgs. 42-43, pgs. 45-48)
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Angular Sizes (pg. 31)
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Planetary motions (pgs. 49-51)
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Stellar parallax (pg. 52)
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and click on “Class Notes”
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Hypothesis & Prediction
(science in action!)
Hypothesis: The cause of the seasons on earth is due to the changing
distance between the earth and the sun over the course of a year.
If this hypothesis were correct, what would you expect to
observe? (i.e., what predictions does the hypothesis make?)
What do we actually observe when it comes to the seasons?
Changing Earth-Sun distance is not the cause of the seasons
Over the course of the year in Boston, what will you notice about:
1. The height of the sun at noon
2. The rising and setting points of the sun (i.e., is it always due
east and due west, respectively?) **note typo correction
Day after day, the sun appears to take a slightly different
track in the sky, and the track correlates with the seasons.
Path of the Sun at Different Times of Year
From March 21-22 (spring equinox) to
September 22-23 (autumnal equinox), the
sun rises and sets north of due east/due
west, so the days are long.
From September 22-23 to March 21-22,
the sun rises and sets south of due
east/due west, so the days are short.
Equniox = equal night
The height of the sun at noon and its
rising/setting point depend upon your
latitude on earth. The poles are truly
extreme (constant day/night for months).
Midnight Sun
Near the earth’s poles, the sun never sets during mid-summer (= late June
for the North Pole, = late December for the South Pole).
You pay for all this summer sun by never seeing the sun in mid-winter!
Tilt of the Earth Relative to the Sun’s Rays
The earth is inclined at
23.5o to the plane of its orbit
around the sun. Notice how
the angle at which sunlight
hits the earth is different at
different latitudes. This is
the true cause of the
seasons.
What season is it in South
America in the picture?
What season is it in North
America in the picture?
Solar “Irradiance”
equator
mid-latitude
(similar to Boston)
polar climates
The greater is the angle between the surface of the earth and the sun’s rays, the
more power per unit area the surface of the earth receives.
Seasons and the Earth’s Orbit
You experience winter when your hemisphere is pointed away from the direction of the
sun, and summer when your hemisphere is pointed toward the direction of the sun.
Precession of the Equinoxes
Like a spinning top, the earth’s rotation axis
“precesses”, constantly changing the direction
of the North pole with respect to the sky.
This “minor motion” is very slow (takes 26,000
years to complete), but is important to
navigation by the stars!
Right now, the North Star is “Polaris” (the tail
star of the Little Dipper). Five thousand years
ago the North Star was Thuban, and in 14,000 it
will be Vega.
Phases of the Moon
Names of lunar phases, left to right:
Waning Crescent, Third Quarter, Waning Gibbous, Full, Waxing Gibbous, First
Quarter, Waxing Crescent
The amount of the moon’s face that is “lit up” increases daily from New Moon
to Waxing Crescent to Full, then decreases daily from Full Moon to Waning
Crescent to New Moon.
“Shadow of the Earth” does not cause the phases of the moon
Earth and moon, both showing the “First Quarter” phase, seen from space.
Cause of Phases of the Moon:
Changing angle between the earth and sun
The only region of the
moon that you can
observe from earth is the
hemisphere of the moon
that is on the inside of the
moon’s orbit.
At new moon, you look in
the same direction to see
both the sun and moon. At
new moon the far side of the
moon is in total daylight!
At full moon, you look on
opposite sides of the sky
to see the sun and the
moon. At full moon, the far
side of the moon is in total
darkness!
At the quarter moons, the
earth, moon, and sun make
a right-angle triangle.
When does the moon rise and set?
(make it simple - assume 12 hours from rise to set)
On the diagram, you can see anything that is
180o from you (the extent of your horizon)
Sunrise = 6am, Sunset = 6pm
The moon rises about 50 minutes
later from one day to the next.
New moon must rise and set with the
sun (6am and 6pm), respectively.
Full moon must rise when the sun is
setting (6pm), and must set at sunrise
the following day (6am).
First quarter is mid-way between new
and full, so it must rise at noon (i.e. 6
hours later than the new moon rises)
and set at midnight (i.e., 12 hours after
it rose).
Choose a spot on the earth to stand, then ride along
with the earth as it rotates counter-clockwise
Third quarter is mid-way between full
and new, so it must rise at midnight
(i.e., 3 hours later than the full moon
rises), and set at noon (i.e., 12 hours
after it rose).
This slide intentionally left blank
Solar and Lunar Eclipses
Total lunar eclipse
time sequence
Total solar
eclipse time
sequence
“Angular” Size
Physically, the sun and moon have very different sizes (diameter of sun = 1.39x106 km,
diameter of moon = 3,476 km) but they appear to be about the same size in the sky
The angular size of an astronomical object (in units of
radians) is given by:
A = D/d
where D is the diameter of the object and d is the distance
to the object.
Very important: the units of D and d must be identical!!
The farther an object is from us, the smaller it will appear in the sky
Images of the planet Mars
Angular diameter of the sun
(as seen from the earth)
diameter of the sun = 1.39x106 km = Dsun
average distance to the sun = 1 AU = 1.5x108 km = dsun
angular diameter = Asun = Dsun/dsun = 1.39x106 km/1.5x108 km = 9.27x10-3 radian
Radians are usually much too large a unit of measurement for astronomical angles.
Most angles are measured in degrees, arcminutes, or arcseconds.
1 radian = 180 /  degrees = 57.3 degrees
1 degree = 60 arcminutes = 3600 arcseconds
1 arcminute = 60 arcseconds
Asun = 9.27x10-3 radian = 9.27x10-3 radian x 57.3 degrees/radian = 0.53 degree
Asun = 0.53 degree = 0.53 degree x 60 arcminutes/degree = 32 arcminutes
Total vs. Annular Solar Eclipse
Total solar eclipse
Annular (“ring”) solar eclipse
For a total solar eclipse to occur, angular size of the moon must be equal to or
greater than the angular size of the sun.
You will explore conditions for an annular eclipse on HW #1
Why don’t we get a solar eclipse with every new moon
and a lunar eclipse with every full moon?
Moon’s orbit is inclined at 5o to Earth’s orbit. It is only when the moon is
precisely in the same plane as the earth’s orbit that eclipses can occur.
How to tell a planet from a star without a telescope
Five planets are visible
without a telescope. They
look like points of light (like
stars), with one important
difference.
“Planets” comes from the
Greek word “planetes”
meaning “wanderers”.
Planets move with respect to the (fixed) stars
Locations of Venus and Jupiter over the course
of 6 nights. All photos taken when the star
“Spica” was in the same location in the sky.
Notice how Venus has moved much farther
than Jupiter over this period of time!
The primary motion of the planets is from
WEST to EAST
Sometimes the planets reverse their motion
and travel from EAST to WEST, a phenomenon
known as “retrograde motion”
Example: Retrograde Motion of Mars
East
West
From Jan. 4, 1995 to March 25, 1995, Mars is moved “backward” in the sky (east to west)
On astronomical maps, east is on the LEFT and west is on the RIGHT. Why????
From what perspective are you viewing this picture?
West
East
Astronomers look UP at the sky!
Actual image of Mars undergoing “retrograde motion”
Do you notice anything different about Mars when it is undergoing
retrograde motion? (Assume all the individual exposures were the
same length of time.) Retrograde motion is on the TOP part of the
“loop” in the sky.
It’s all about perspective!
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We see the planets move with respect to the fixed stars
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Space is really 3-d (planets are much closer than the stars), but space
looks 2-d (when you look at a picture of the sky you don’t have a
sense of depth that tells you the planets are closer than the stars)
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The planetary motion you see is due to combination of: (1) earth’s
motion about the sun and (2) the planet’s motion about the sun
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Retrograde motion happens when an inner planet (e.g., the earth)
catches up to and “laps” an outer planet (e.g., Mars)
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All planets orbit in the same direction about the sun continuously, they
only appear to go “backward”
Retrograde Motion of Mars
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Happens about once every 2 years
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Only occurs when Mars and Earth are near their distance of closest
approach (called “opposition” because Mars and the Sun are on
opposite sides of the sky, as seen from earth)
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Is currently going on! Mars will be at opposition next Friday (Jan. 29)
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For every 1 orbit of the Earth around the sun, Mars makes about 2
orbits around the sun
http://www.youtube.com/watch?v=72FrZz_zJFU
Does the earth really orbit the sun?
What proof do we have?
Once again, it’s all a matter of
perspective!
As the earth orbits the sun, nearby
stars appear to shift their location in
the sky compared to extremely
distant (“background”) stars.
The farther is a star, the smaller is
its parallax.
If p is measured in arcseconds, the
distance to the star is
d = 1/p
The closest star to us (other than the sun) has a
parallax of p = 0.7687 arcsec = 2.1x10-4 degree.
where d is in units of “parsecs”
This is much too small to see without a telescope!!
1 parsec = 1 pc = 3.26 ly
Minute Paper
A few sentences on one of the following:
* something you found particularly interesting today
* something you found particularly confusing today
* questions on things from today that you would like to know more about
Be sure to PRINT your name legibly