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Foundations of Astronomy Part 3
• Questions on iClicker Registration
• Questions on Essay
Discuss Einstein Essay
Clicker Question:
If Earth rotated twice as fast as it
currently does, but its motion around the
sun stayed the same, then which of the
following is true:
A: the night would be twice as long
B: the night would be half as long
C: the year would be half as long
D: the year would be twice as long
E: the length of a day would be unchanged
Pre-Tutorial Question
Imagine you are standing in the northern hemisphere.
Looking directly north, you see a star just above the
horizon. A little later you notice that it has shifted position
slightly. Which way did it move?
a) to the right, (east)
b) to the left, (west)
c) up, (rising)
d) down, (setting)
Lecture Tutorial in Introductory Astronomy
• Motion – Pages 1-6
POST-Tutorial Question
Imagine you are standing in the northern hemisphere.
Looking directly north, you see a star just above the
horizon. A little later you notice that it has shifted position
slightly. Which way did it move?
a) to the right, (east)
b) to the left, (west)
c) up, (rising)
d) down, (setting)
POST-Tutorial Question
You are looking toward the north and see the Big Dipper
to the right of Polaris. Fifteen minutes later, the Big
Dipper will appear to have moved in roughly what
direction?
a) east (to your right)
a) west (to your left)
c) up (away from the horizon)
c) down (closer to the horizon)
POST-Tutorial Question
How much time is there between when a star rises and
when it sets?
a) less than twelve hours
a) about twelve hours
a) more than twelve hours
d) it depends on the star
The Motion of the Moon
The Moon has a cycle of "phases", which lasts about 29 days.
Half of the Moon's surface is lit by the Sun.
During this cycle, we see different fractions of the sunlit side.
Which way is the Sun here?
See Tutorial on
book’s website
for animation.
Cycle of phases slightly longer than time it takes Moon to do a complete
orbit around Earth.
Cycle of phases or
"synodic month"
Orbit time or
"sidereal month"
29.5 days
27.3 days
Eclipses
Lunar Eclipse
When the Earth passes directly between the Sun and the Moon.
Sun
Earth
Moon
Solar Eclipse
When the Moon passes directly between the Sun and the Earth.
Sun
Moon
Earth
Solar Eclipses
Diamond ring effect - just before or
after total
Total
Partial
Annular - why do these occur?
Lunar Eclipse
Why don't we get
eclipses every
month?
Moon's orbit tilted compared to Earth-Sun orbital plane:
Sun
Moon
Earth
5.2o
Side view
Moon's orbit slightly elliptical:
Moon
Distance varies by ~12%
Earth
Top view, exaggerated ellipse
Types of Solar Eclipses Explained
Certain seasons are more likely to have eclipses. Solar “eclipse
season” lasts about 38 days. Likely to get at least a partial eclipse
somewhere. Animation
It's worse than this! The plane of the Moon's orbit precesses, so
that the eclipse season occurs about 19 days earlier each year.
Recent and upcoming total and annular solar eclipses
Eratosthenes Determines the Size of the Earth in about 200 B.C.
Sun's rays
Syene
Alexandria
N
7.2o
S
Earth
He knows the distance between the two cities is 5000 "stadia".
From geometry then,
7.2o
=
360o
5000 stadia
Earth's circumference
=> circumference is 250,000 stadia, or 40,000 km.
So radius is:
40,000 km
2p
=
6366 km
(very close to modern value, 6378 km!)