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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?

GrapefruitIsAwesome
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James
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  • You might find this rise and set graph useful. It shows the rise and set time of various objects, including constellations: https://www.celestialprogramming.com/riseandsetgraph/index.html – Greg Miller Sep 11 '22 at 15:44
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    2 hours earlier per month? – uhoh Sep 13 '22 at 00:06

1 Answers1

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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.

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