Moon orientation across sky

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John M. Haskew
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Moon orientation across sky

Post by John M. Haskew » Sat Nov 28, 2015 8:41 pm

Correct or incorrect:

The moon rises in a fixed orientation, and near the top of its ascent, it appears to rotate ninety degrees, and after rotating, it descends in a fixed orientation. It does not continuously appear to rotate from rise to set.

https://www.youtube.com/watch?v=0ARs3JCxYEo

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Chris Peterson
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Re: Moon orientation across sky

Post by Chris Peterson » Sat Nov 28, 2015 9:35 pm

John M. Haskew wrote:Correct or incorrect:

The moon rises in a fixed orientation, and near the top of its ascent, it appears to rotate ninety degrees, and after rotating, it descends in a fixed orientation. It does not continuously appear to rotate from rise to set.
False. To a first approximation, you can consider the Moon's axis of rotation to be parallel to Earth's. So the Moon's north pole is always pointing more or less north, which is why its orientation with respect to the horizon changes.

I say "with respect to the horizon" to make clear that we have to have some reference. As you turn to follow the Moon's progress, its orientation is changing with respect to you. If you track it across the sky in a video, its orientation will depend on details of your tracking system.

It would be remarkable indeed if we could observe the Moon do an instantaneous 90° change in orientation as it crosses the meridian!
Chris

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BMAONE23
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Re: Moon orientation across sky

Post by BMAONE23 » Sat Nov 28, 2015 9:48 pm

this is similar to the orientation of Orion. At my latitude, 38deg N, Orion rises on his back like waking up after sleeping, and sets on his feet.
In the eastern sky, the Pleiades rises first then Taurus then Orion awakes.
In the Western sky, Orion's feet set first followed by Taurus then the Pleiades

John M. Haskew
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Re: Moon orientation across sky

Post by John M. Haskew » Sun Nov 29, 2015 12:21 pm

Wait a minute: Clearly that moon in the video does not appear to rotate as it rises. And the same with the naked eye.

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Re: Moon orientation across sky

Post by John M. Haskew » Sun Nov 29, 2015 1:58 pm

P.S. I'm not looking for an explanation of the phenomenon, but rather a verification of the phenomenon. I am not yet able to download Stellarium, because I do not have a graphics card in my computer. I need someone to verify that the moon does, when observed with the naked eye, appear to rise fixed, and begin to turn near the meridian, and fall fixed.

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Chris Peterson
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Re: Moon orientation across sky

Post by Chris Peterson » Sun Nov 29, 2015 3:07 pm

John M. Haskew wrote:Wait a minute: Clearly that moon in the video does not appear to rotate as it rises. And the same with the naked eye.
The Moon spans a half a degree. You're looking at only a few degrees of motion across the sky, and you have to consider that there are projection issues (you can't map a sphere- the sky- to a plane- the camera sensor- without distortion).

As the Moon is moving against the sky, the angle of its polar axis is continuously changing with respect to the altaz (horizon) coordinate system. That is, it appears to rotate with respect to the horizon.
Chris

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John M. Haskew
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Re: Moon orientation across sky

Post by John M. Haskew » Sun Nov 29, 2015 3:31 pm

All I need is Stellarium. I need to see nine pictures: three during rise, three during change, three during set. In each pic I simply use the grid to check orientation.

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Re: Moon orientation across sky

Post by BMAONE23 » Sun Nov 29, 2015 5:57 pm

When you face East, the point that represents North is located NW of E. When you face West, the point that represents North is located NE of W. This difference in the respective perceived North causes the apparent shift in the Lunar North. The rotation is gradual as at any given point in the Lunar path across the sky, the moon will appear to be fixed.

North on the moon appears to rotate relative to your perceived location of Polaris (North)

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Re: Moon orientation across sky

Post by Nitpicker » Mon Nov 30, 2015 5:11 am

For most of us, relative to the horizon, the face of the Moon rotates faster as it culminates, than it does when it rises and sets. But it never stops rotating.