<p>This shift is already included in a &quot;day&quot;.</p>
<p>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 <a href="https://en.wikipedia.org/wiki/Solar_time" rel="nofollow noreferrer">solar time</a>, the common usage of a &quot;day&quot;.</p>
<p>The time the Earth needs to rotate exactly once,so that a star is in the same position in the sky, is a <a href="https://en.wikipedia.org/wiki/Sidereal_time" rel="nofollow noreferrer">sideral day</a>. It's slightly shorter at 23 hours 56 minutes and 4 seconds.</p>
<p>You can convert between them based on the fact that there's one less solar day than sideral days in a year:</p>
<p><span class="math-container">$$d_{sideral} = \frac{year}{\frac{year}{d_{solar}} + 1}$$</span></p>
<p>Note that the difference between a solar and a sideral day is <a href="https://en.wikipedia.org/wiki/Year#Astronomical_years" rel="nofollow noreferrer">far from the only subtility</a> arising from the Earth's orbit.</p>

CC BY-SA 4.0
2020-10-03T06:45:12.750Z
2020-10-03T06:45:12.750Z
SE - stop firing the good guys
<p>If the Earth rotates at a constant speed why doesn't sunrise change as we orbit the Sun?</p>
<p>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).</p>
<p>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).</p>
<p>I'm probably overlooking something obvious but hoping someone can explain why this obviously isn't what happens in reality.</p>
<p>Many thanks.</p>

CC BY-SA 4.0
2020-10-02T13:33:49.757Z
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.
RichTurner
CC BY-SA 4.0
2020-10-02T14:00:55.007Z
Yes - are you asking about sidereal vs. solar day?
Carl Witthoft
CC BY-SA 4.0
2020-10-02T14:01:54.903Z
Also,  the Earth does **not** complete an integral number of rotations over half a revolution.  You should clarify what you mean by "six months"
Carl Witthoft
<orbital-mechanics>
Why doesn't sunrise shift as we revolve around the sun (ignoring seasonal variations - so at the equator)
RichTurner
# Why doesn't sunrise shift as we revolve around the sun (ignoring seasonal variations - so at the equator)

(Asked by: username_0 on 2020-10-02)

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.

**Tags:** orbital-mechanics
> **Comments:**
> **username_0**: 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.  
> **username_1**: Yes - are you asking about sidereal vs. solar day?  
> **username_1**: Also,  the Earth does **not** complete an integral number of rotations over half a revolution.  You should clarify what you mean by "six months"  

---

## 1 answers

### Answer by username_2 on 2020-10-03. Score: 3

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](https://en.wikipedia.org/wiki/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](https://en.wikipedia.org/wiki/Sidereal_time). 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](https://en.wikipedia.org/wiki/Year#Astronomical_years) arising from the Earth's orbit.


---

