If the Earth rotates at a constant speed why doesn't sunrise change as we orbit the Sun?

For example if viewing the Earth's orbit from above at time N let's assume the Earth is at the 3 'o' clock position and so the point on the Earth's surface at the equator perpendicular to the Sun would be in daylight (~12pm local time) let's call this point X (would be at the Earth's 9 'o' clock position).

Six months later the Earth would be at the 9 'o' clock position in its' orbit and assuming the Earth has completed exactly 182 full rotations that point X will again be at the Earth's 9 'o' clock position but would now be facing away from the sun therefore it would be dark but exactly 182 days have passed so it should be (~12pm local time).

I'm probably overlooking something obvious but hoping someone can explain why this obviously isn't what happens in reality.

Many thanks.

  • $\begingroup$ The only logical explanation is that a day isn't technically a full rotation of the Earth but slightly more or less (not sure which way around) in order to adjust for orbiting. $\endgroup$
    – RichTurner
    Oct 2, 2020 at 13:33
  • $\begingroup$ Yes - are you asking about sidereal vs. solar day? $\endgroup$ Oct 2, 2020 at 14:00
  • $\begingroup$ Also, the Earth does not complete an integral number of rotations over half a revolution. You should clarify what you mean by "six months" $\endgroup$ Oct 2, 2020 at 14:01

1 Answer 1


This shift is already included in a "day".

24 hours is already the time from sunrise to sunrise, meaning the Earth has performed a full rotation, plus a little bit more since it has moved along a degree in its orbit. That's solar time, the common usage of a "day".

The time the Earth needs to rotate exactly once,so that a star is in the same position in the sky, is a sideral day. It's slightly shorter at 23 hours 56 minutes and 4 seconds.

You can convert between them based on the fact that there's one less solar day than sideral days in a year:

$$d_{sideral} = \frac{year}{\frac{year}{d_{solar}} + 1}$$

Note that the difference between a solar and a sideral day is far from the only subtility arising from the Earth's orbit.


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