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Suppose you observe a particular constellation at 11 PM on a particular day at what time will you need to observe 2 months later to find the constellation in the same position as in the sky chart?

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The constellations will (apparently) rotate once through the sky in a whole year. Thus every month it changes by 30°, if you keep observation time fixed.

Similarily, Earth rotates and the sky (apparently) rotates once 360° in 24 hours or 15° per hour.

Thus for every month in the future from now, you will have to observe two hours earlier to get a nearly identical view on the sky.

Things get more tricky if you want to take into account ellipticity of the orbit, etc.

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    $\begingroup$ Given that there are only 12 months in a year, that should be 30° per month, and two hours earlier each month. $\endgroup$
    – notovny
    Sep 11, 2022 at 13:22
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    $\begingroup$ @notovny yes! Thanks for the pointer. I wondered... but without tea I didn't see my mistake. $\endgroup$ Sep 11, 2022 at 13:30

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