<p>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.</p>
<p>Similarily, Earth rotates and the sky (apparently) rotates once 360° in 24 hours or 15° per hour.</p>
<p>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.</p>
<p>Things get more tricky if you want to take into account ellipticity of the orbit, etc.</p>

CC BY-SA 4.0
2022-09-11T13:22:00.353Z
Given that there are only 12 months in a year, that should be 30° per month, and two hours earlier each month.
notovny
CC BY-SA 4.0
2022-09-11T13:30:23.537Z
@notovny yes! Thanks for the pointer. I wondered... but without tea I didn't see my mistake.
planetmaker
CC BY-SA 4.0
2022-09-11T12:23:28.837Z
2022-09-11T13:29:46.287Z
2022-09-11T13:29:46.287Z
planetmaker
<p>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?</p>

CC BY-SA 4.0
2022-09-11T15:44:48.187Z
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
CC BY-SA 4.0
2022-09-13T00:06:34.050Z
2 hours earlier per month?
uhoh
<constellations>
Knowing what time to observe a constellation
James
# Knowing what time to observe a constellation

(Asked by: username_0 on 2022-09-11)

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?

**Tags:** constellations
> **Comments:**
> **username_1**: 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  
> **username_2**: 2 hours earlier per month?  

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## 1 answers

### Answer by username_3 on 2022-09-11. Score: 3

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.

> **Comments:**
> **username_4**: Given that there are only 12 months in a year, that should be 30° per month, and two hours earlier each month.  
> **username_3**: @notovny yes! Thanks for the pointer. I wondered... but without tea I didn't see my mistake.  

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