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Why is 24h (angle) equivalent to 360°? The earth does not spin 360° in 24h.

When giving astronomical coordinates, my GCSE study guide says right ascension is measured in hours where 24h right ascension = 360°. It also says that 360° of rotation is a sidereal day - 24h 56minutes. So 360° in the celestial sphere of right ascension should be 23h 56minutes, not 24h. I assumed that this was a method to correct for the rotation of Earth (and therefore the apparent coordinates of the stars). Are they separate systems? Or is this GCSE cutting corners? Could somebody please explain this to me.

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  • $\begingroup$ For the details of the usage convention among astronomers, Astronomy might be a better place to ask. $\endgroup$
    – Emilio Pisanty
    Commented Jun 22, 2017 at 14:30
  • $\begingroup$ Oops, sorry - didn't know about the astronomy section. Posted here for that reason, and that it for my GCSE Physics exam. Sorry again $\endgroup$
    – Jayemby
    Commented Jun 22, 2017 at 14:31
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    $\begingroup$ No, it's on topic here - and it's also on topic at the astro site. If you want to migrate it, you can flag for moderator attention, but it's on topic here. $\endgroup$
    – Emilio Pisanty
    Commented Jun 22, 2017 at 14:35

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On average, the Earth rotates 360° with respect to the Sun every 24 solar hours. The "on average" is important here because of the equation of time. On average, the Earth rotates 360° with respect to the Sun every 24 sidereal hours. The "on average" is not so important in this regard because all that is left out is the very long scale precession and the very tiny nutations.

Hour angle is a measure of angle, not of time, with 24 hour angles representing a full rotation (as opposed to 360 degrees representing a full rotation). The hour angle of the Sun is closely correlated with time as expressed in solar hours, minutes and seconds. The hour angle of a star is similarly closely correlated with time as expressed in sidereal hours, minutes and seconds.

Sidereal time is a different time scale than is solar time. A mean sidereal day is slightly shorter than is a mean solar day, which means that a mean sidereal hour is slightly shorter than is a mean solar hour, and so on.

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  • $\begingroup$ So 360° is exactly 24h (angle)? Then how do we correct for our rotation within the reference frame of the celestial sphere? (I may file that as a separate question) $\endgroup$
    – Jayemby
    Commented Jun 22, 2017 at 14:34
  • $\begingroup$ @Jayemby - Yes, 360° is exactly 24h (angle), by definition. Correcting for rotations is indeed a separate question. It's on topic here, but also in astronomy.stackexchange.com. $\endgroup$
    – David Hammen
    Commented Jun 22, 2017 at 15:08

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