<p>the sidereal time is the time measured by the angle between the sight line of an observer and the direct ray of a fixed star to the observer, whereas the solar time is the time measured by looking at the position of the sun in the sky like you can say it's around noon when the sun is overhead... they  are just different versions of the same time... like metre and mile are of length 1 mile is not equal to 1 metre... similarly sidereal and solar time are different units of the same time... it's often said that the solar day is 4 mins less than a sidereal day</p>

CC BY-SA 4.0
2020-01-09T15:36:45.027Z
@barrycarter but sun is also a star right??
Dustin Kylasovia
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2020-01-10T14:13:31.253Z
@barrycarter ok so did i... my mistake
Dustin Kylasovia
CC BY-SA 4.0
2020-01-09T13:14:01.727Z
2020-01-10T14:13:05.987Z
2020-01-10T14:13:05.987Z
Dustin Kylasovia
<p>The 3<sup>m</sup>56<sup>s</sup> mean difference between solar and sidereal days is due to the Earth's orbital motion around the Sun.
This adds up to 1 day per year; for every 365&frac14; solar days there are 366&frac14; sidereal days.</p>

<p>Sidereal time equals the right ascension of what's on the celestial meridian at that time.
The Sun is on the meridian at 12:00 <em>apparent</em> solar time.
These align at the September equinox, when the Sun is at RA 12<sup>h</sup>.
<em>Mean</em> solar time is ~7.5 minutes <a href="https://en.wikipedia.org/wiki/Equation_of_time" rel="noreferrer">behind</a> apparent solar time on that date and aligns with sidereal time ~1.9 days earlier.</p>

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2020-01-09T17:24:00.857Z
@barrycarter The equation of time is the difference between mean and apparent solar time.
Mike G
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2020-01-09T16:59:57.220Z
2020-01-09T16:59:57.220Z
Mike G
<p>I'm trying to wrap my head around the different definitions of time. Since mean solar time depends on the Sun, and sidereal time depends on the stars, and since the position of the Sun relative to the stars changes over the course of the year, does this mean that the difference between the two times increases and decreases over the course of the year?</p>

<p>Once a year, mean solar time and sidereal time will be the same. Is that right?</p>

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2020-01-09T11:22:56.373Z
Since one is about 0.3% faster then the other, I assume the answer is "yes, and yes" but I'm not confident enough about that to post as an answer.
uhoh
CC BY-SA 4.0
2020-01-10T08:05:28.993Z
@barrycarter But the definition of a second is the same for both times. Only the number of seconds in a day changes. So both times can't line up once a day.
usernumber
CC BY-SA 4.0
2020-01-10T12:50:06.247Z
@usernumber - Regarding your last comment, *"But the definition of a second is the same for both times. Only the number of seconds in a day changes."* That's incorrect. A mean solar day comprises 86400 mean solar seconds, while a mean sidereal day comprises 86400 mean sidereal seconds. That means there are  about 86636.555 mean sidereal seconds in a mean solar day, and  about 86164.09 mean solar seconds in a mean sidereal day.
David Hammen
<time><standards>
Does mean solar time and sidereal time sometimes indicate the same time?
usernumber
# Does mean solar time and sidereal time sometimes indicate the same time?

(Asked by: username_0 on 2020-01-09)

I'm trying to wrap my head around the different definitions of time. Since mean solar time depends on the Sun, and sidereal time depends on the stars, and since the position of the Sun relative to the stars changes over the course of the year, does this mean that the difference between the two times increases and decreases over the course of the year?

Once a year, mean solar time and sidereal time will be the same. Is that right?

**Tags:** time, standards

> **Comments:**
> **username_1**: Since one is about 0.3% faster then the other, I assume the answer is "yes, and yes" but I'm not confident enough about that to post as an answer.  
> **username_0**: @barrycarter But the definition of a second is the same for both times. Only the number of seconds in a day changes. So both times can't line up once a day.  
> **username_2**: @username_0 - Regarding your last comment, *"But the definition of a second is the same for both times. Only the number of seconds in a day changes."* That's incorrect. A mean solar day comprises 86400 mean solar seconds, while a mean sidereal day comprises 86400 mean sidereal seconds. That means there are  about 86636.555 mean sidereal seconds in a mean solar day, and  about 86164.09 mean solar seconds in a mean sidereal day.  

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## 2 answers

### Answer by username_4 on 2020-01-09. Score: 6 | Accepted answer

The 3m56s mean difference between solar and sidereal days is due to the Earth's orbital motion around the Sun.
This adds up to 1 day per year; for every 365¼ solar days there are 366¼ sidereal days.

Sidereal time equals the right ascension of what's on the celestial meridian at that time.
The Sun is on the meridian at 12:00 *apparent* solar time.
These align at the September equinox, when the Sun is at RA 12h.
*Mean* solar time is ~7.5 minutes [behind](https://en.wikipedia.org/wiki/Equation_of_time) apparent solar time on that date and aligns with sidereal time ~1.9 days earlier.

> **Comments:**
> **username_4**: @barrycarter The equation of time is the difference between mean and apparent solar time.  

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### Answer by username_3 on 2020-01-09. Score: -3

the sidereal time is the time measured by the angle between the sight line of an observer and the direct ray of a fixed star to the observer, whereas the solar time is the time measured by looking at the position of the sun in the sky like you can say it's around noon when the sun is overhead... they are just different versions of the same time... like metre and mile are of length 1 mile is not equal to 1 metre... similarly sidereal and solar time are different units of the same time... it's often said that the solar day is 4 mins less than a sidereal day

> **Comments:**
> **username_3**: @barrycarter but sun is also a star right??  
> **username_3**: @barrycarter ok so did i... my mistake  

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