Lecture
The Earth and Moon
Lecture |
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The Earth and Moon |
Moon animation created by Ed Stephan stephan@cc.wwu.edu. The "Virtual Reality Moon Phases" above, were created from ray-traced images of the Moon. A Clementine spacecraft mosaic of the lunar surface was mapped onto a sphere, and scenes were rendered as a virtual Sun "orbited" the Moon. The depiction of lunar surface features suffers geometric distortion but the terminator is correct with respect to the spherical Moon.
Take the Quiz == page down to the bottom of these lecture notes.
Reasons for the Seasons
Why does the Earth have seasons? That is, why are the days shorter in the winter in the northern hemisphere and longer in the summer?
Why is it hotter here in the summer than in the winter?
Phases of the Moon
Why do we observe a cycle of phases of the Moon?
Where is the "Dark Side of the Moon" anyway?
Why does the Moon keep the same face towards Earth?
What happens during eclipses of the Sun and Moon? Why don't we see eclipses more often?
The Moon, Sun, and Tides
Why are there high, low, and in-between tides?
How does the Moon produce tides?
How many high tides are there in a day?
Observations
Think about how high the Sun gets in the northern summer vs the winter. Does it ever get directly overhead in Seattle?
About how long is the Sun up in the summer? How long in the winter?
Travels
Have you traveled anywhere where the Sun passed directly overhead (zenith)? Where on Earth was that?
Where would you have to go to have the Sun directly overhead twice during a year?
What do the Tropics of Cancer and Capricorn represent?
How long is the night during the northern winter at the North Pole?
Rotation and Revolution of the Moon: The Dark Side of the Moon
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The question of whether or not the Moon rotates is one of the hardest for beginning astronomy students to grasp. We know it pretty much keeps the same face towards us always (although we end up seeing slightly more than 50% of its surface due to something called lunation), and so it seems logical that it does not rotate. When we state whether or not something rotates, we need to also state "relative to what?" We must have some comparison reference frame. This also applies when we say that something is moving--it is moving relative to something else. For the rotation of the Moon, the reference frame is the stars, although our star, the Sun, would be sufficient. An initial illustration will help convince you that the Moon does, in fact, rotate. Let's say we have a new Porsche, and we take it out for a few spins around the race track. We should all agree that as the Porsche speeds around the track, the driver's side is always to the inside. The driver is on the"near side" of the Porsche, and the passenger would be on the "far side" of the Porsche. Wouldn't you agree at this step that this is identical to the case of the Moon keeping one side always facing the Earth? The Earth is represented here by the central flag. An astronaut living in the crater Copernicus would be on the "driver's side" of the Moon.
Now, let's decrease the size of the track, making the radius smaller and smaller until the Porsche is speeding around the track so tightly that the driver is hanging onto the central flag.
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The dark side of the Moon is the hemisphere that is facing away from the SUN and thus not getting any light. Since the Moon does not have an atmosphere, the dark side of the Moon is very, very, very dark! When we are viewing a new Moon, we are looking at the dark side (which is usually slightly lit by light reflected from the Earth). During a new Moon, the dark side is the same as the near side. When we are viewing a full Moon, the dark side is opposite from us--the dark side is the same as the far side here. In between the new Moon and the full Moon (and back to new again), we are seeing various fractions of the Moon lit by the Sun, and the remaining fractions being the dark side. So, the near side is that side always facing the Earth, the far side is the side always facing away from the Earth. The dark side is the side facing away from the Sun, and the bright side is the side facing towards the Sun.
Tides on Earth are caused by the differential gravitational pull on it by the Moon. Because the oceans are fluid and not solid, they respond more dramatically to the tidal forces. The phrase, differential gravitational pull, simply means that the side closest to the Moon feels a much greater force than the area in the middle, and the opposite side. The Sun also affects the Earth's oceans. However, even though it is much more massive than the Moon, the tidal effects of the Sun are less because the Sun is so far away.
The Moon acts upon the Earth's oceans and atmosphere, causing two bulges to form. The bulge on the side of Earth that faces the moon is caused by the proximity of the moon and its relatively stronger gravitational pull on that side. The bulge on the opposite side of Earth results from that side being attracted toward the moon less strongly than is the central part of Earth. Earth's crust is also affected to a small degree. Other factors, including Earth's rotation and surface roughness, complicate the tidal effect. On planets or satellites without oceans, the same forces apply, but they cause slight deformations in the body rather than oceanic tides. This mechanical stress can translate into heat as in the case of Jupiter's volcanic moon Io. (From Basics of Space Flight, Ch. 3).
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Now, let's get rid of the central
flag altogether and have the Porsche "orbit" around its center. Aren't you
seeing all parts of the car in exactly the same way as when it was driving
around the original track? Wouldn't you agree in the small image shown to
the left that the Porsche is rotating? Since the motion of the Porsche
alone, with respect to the stars, is identical between these two frames,
then the Porsche must be rotating. When it was driving around the track,
its rotation period was equal to its period of revolution.