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Astronomy question
 Posted: 08 January 2008 07:35 AM [ Ignore ]   [ # 106 ]
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Mriana - 07 January 2008 08:43 PM
retrospy - 07 January 2008 10:36 AM

Is that how you did the experiment Mriana?

Well, yes, but that would mean that moon would not be spinning on it’s axis or moving at all, so it really isn’t a very real similation.

Yes, we almost hashed out the missing link.

The difference between the pumpkin on the merry-go-round and the earth & moon is that the merry-go-round is a single solid force.  The moon orbiting the earth is affected by two forces, revolution & spin.  We know it is two forces because the spin of the earth & the moon are independent of their revolutions.  For example the earth is spinning multiple times for every one spin of the moon.

The way to represent this on the merry-go-round experiment is that both you (earth) and the pumpkin (moon) will be on mini merry-go-rounds that function like a compass (always point in one direction) on top of the main merry-go-round.  If the pumpkin had no spin it would always face north and you would see all sides (once every revolution).

The tricky part is the massive coincidence that the moon is spinning at the exact same time that it revolves around us.  If you increased the moons spin to spin twice in one revolution, we (on earth) would only observe one spin.  It is tricky to visualize, but the main distinction is the two separate forces involved, revolution (orbit) & spin (rotation).  The main merry-go-round accounts for the revolution and the separate mini “merry-go-round compasses” account for the spins.

Does that distinction make sense?

Edit: I like the example Fotobits made while I was writing this.  The shuffeling of feet is a good indicator that spin (rotation) is happening at the same time as revolution (orbit).

[ Edited: 08 January 2008 07:41 AM by retrospy ]
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 Posted: 09 January 2008 01:59 AM [ Ignore ]   [ # 107 ]
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To phrase Dennett: the moon really seems to wobble. The animated gif is perfectly ok.

The moon would not seem to wobble if:

1. it would make a perfect circle around the earth
2. it would move in exactly the same plane as the earth’s equator

Explanation:
1. Because the spinning around its axis is regular, but the moon moves in an ellipse around the earth (which does not make a regular movement: at the farthest point it is moving slower than at the closest point), we sometimes see a little more of its right, then of its left side. And also, the moon seems smaller (when it is at the farthest point of its orbit), and then bigger (when it is closest to us in its orbit).

2. Sometimes we look a little at the nort pole of the moon (when the moon is a little bit south of the equator), then a little of the south pole (when the moon is a little bit north of the equator).

These 2 effects together are the reason that we can see about 59% of the moon’s surface.

So the moon does not really wobble, but from the earth, it really seems to do.

It is a great gif!

GdB

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 Posted: 09 January 2008 08:42 AM [ Ignore ]   [ # 108 ]
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Mriana - 06 January 2008 10:19 AM
fotobits - 06 January 2008 10:15 AM

The Moon revolves on its axis in the same amount of time it orbits the Earth, thus the Moon always keeps the same side facing Earth and we can only see that side from Earth.

Three words- Illogical!  Illogical!  Illogical!  We have to see the other side eventually.  It’s just not logical otherwise.  You can keep saying that to me, but it makes no sense.

In general, when we observe a rotating object, it is logical to expect to see the other side eventually no matter how slow or fast it rotates. When the rotating object also revolves around us, this also holds EXCEPT for the case when it is in synchronous rotation.

It is natural to be very skeptical of any “explanation” unless one can clearly see why it should be so.

Let’s assume the moon is NOT rotating as it orbits the earth. Look at the diagram below:

Start tracking the moon from B with the lunar peak facing the earth orbiting the earth anti-clockwise. When it reaches A, the lunar peak will be facing away from the earth because the moon is NOT rotating. This means that we can see the other side of the moon at A. Why? The moon appears to have turned clockwise by 180 degrees when it is at A even though it is NOT rotating.

This is how an orbiting body presents itself even if it does NOT rotate.

However, the moon in the diagram at A shows the lunar peak facing the earth. This is because the moon has rotated 180 degrees anti-clockwise. Therefore, by rotating anti-clockwise at the same rate it is orbiting the earth anti-clockwise, the moon always shows one hemisphere to any observer on earth.

QED

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