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Interesting lunar / planetary orbit observation


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All of the planet's apparent movement across the night sky is due to the rotation of the earth.

Although the vast majority of the moon's movement across the night sky is also caused by the rotation of the earth, a small percentage of the moon's movement is due to the rotation of the moon around the earth.

I wondered, how much of the moon's movement across one night's sky is caused by the rotation of the moon around the earth.

When the moon and a planet happen to be close together, at some time during the night, over a period of two night's observation, we can figure it out.

Last night at about 6:30, when I looked at the moon, I could see Jupiter, next to the moon, just a bit to the north.

So tonight, when I look at the moon at 6:30, the east-west distance bertween the moon and Jupiter will be the distance the moon moved across the sky, in a 24 hour period, that was due to the rotation of the moon around the earth.

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  • 1 month later...

Last night at about 10 PM, AZ-time, the moon and Jupiter were next to each other, east / west-wise. So tonight at 10 PM, the moon should appear about 12 - 14 moon-widths, to the east of Jupiter.

So the astronomy question of today is: Since the moon rises in the east and sets in thee west, why will the moon be to the east of Jupiter, tonight?

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"Earth swung between the sun and Jupiter on Jan 5th. This was Jupiter’s yearly opposition – when it was opposite the sun – rising in the east as the sun was setting in the west, and setting in the west as the sun was rising in the east. That time has passed, and Jupiter is now visible for less of each night, but it still stays out past midnight throughout April."

http://earthsky.org/astronomy-essentials/visible-planets-tonight-mars-jupiter-venus-saturn-mercury

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As an example, on one night at 9 PM, the moon and Jupiter were next to each other, east / west-wise. Then on the following night at the same time, the moon is noticeably to the east of Jupiter. There can only be one reason for that.

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Feels like this one is done...

To us, the moon rises in the east and sets in the west. But the moon actually revolves around the earth from west to east, or counter-clockwise (looking down from the north pole).

I can't believe it took me till this point in my life to figure that out (from the info in the OP).

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