<p>This is called &quot;parallax&quot;. The observed position of a star (as measured by RA and Dec) does vary ever so slightly.</p>
<p>For example, Proxima centauri, the closest star has a parallax of 0.77 arcseconds.  For context, there are 3600 arcseconds in one degree. The moon is about 1800 arcseconds across.</p>
<p>The parallax can only be measured by very careful telescopic observations.</p>
<p>Other stars are further away, and have even smaller parallax.</p>
<p>The underlying RA and Dec grid stays fixed, but stars (in particular nearby stars) move relative to it.</p>

CC BY-SA 4.0
2023-05-26T20:11:12.713Z
2023-05-26T20:11:12.713Z
James K
<p>I think the RA and Dec of a celestial object should change (ever so slightly) with the Earth's position in its orbit. So do we ignore this in general?</p>

CC BY-SA 4.0
2023-05-26T06:16:18.803Z
What do you mean "the celestial sphere revolves"  The motion of the "celestial sphere" is entirely the result of the motions of the Earth.  Of course individual stars might move relative to the sphere, but these motions are uncoordinated.
James K
CC BY-SA 4.0
2023-05-26T06:40:35.363Z
RA & Dec are zero at the equinox, but the equinox direction changes slowly, see https://en.wikipedia.org/wiki/Equatorial_coordinate_system#Primary_direction Is that what you're asking about?
PM 2Ring
CC BY-SA 4.0
2023-05-26T06:47:48.967Z
@PM 2Ring Another way of putting my question is, Is the position of the earth w.r.t the sun gonna affect the RA and Dec?
Ambica Govind
CC BY-SA 4.0
2023-05-26T07:11:54.997Z
@AmbicaGovind Are you thinking of [parallax](https://en.wikipedia.org/wiki/Stellar_parallax)?
Sten
CC BY-SA 4.0
2023-05-26T07:12:05.913Z
The RA/Dec system is based on the centre of the Earth, the location of the Sun relative to that centre is irrelevant.
PM 2Ring
CC BY-SA 4.0
2023-05-26T07:30:42.377Z
@Sten Yes! So does parallax have nothing to do with RA/Dec? Why not?
Ambica Govind
CC BY-SA 4.0
2023-05-26T07:38:48.657Z
@AmbicaGovind Parallax does change an object's sky coordinates (very slightly)! I think people (including myself) just didn't understand the question at first.
Sten
CC BY-SA 4.0
2023-05-26T14:41:45.587Z
The answer is "of course!".  The apparent effects of the Earth's position on nearby stars is very subtle, but measurable, and is one way missions like GAIA and HIPPARCOS measured the distance to the stars.  But the effects on nearby planets is much greater.  And a person's position on the Earth can affect the position of the Moon by about 1 degree (twice the diameter of the moon).
Greg Miller
<observational-astronomy><declination><right-ascension>
Does RA/Dec vary with the Earth's orbit around the Sun?
Ambica Govind
# Does RA/Dec vary with the Earth's orbit around the Sun?

(Asked by: username_0 on 2023-05-26)

I think the RA and Dec of a celestial object should change (ever so slightly) with the Earth's position in its orbit. So do we ignore this in general?

**Tags:** observational-astronomy, declination, right-ascension
> **Comments:**
> **username_1**: What do you mean "the celestial sphere revolves"  The motion of the "celestial sphere" is entirely the result of the motions of the Earth.  Of course individual stars might move relative to the sphere, but these motions are uncoordinated.  
> **username_2**: RA & Dec are zero at the equinox, but the equinox direction changes slowly, see https://en.wikipedia.org/wiki/Equatorial_coordinate_system#Primary_direction Is that what you're asking about?  
> **username_0**: @PM 2Ring Another way of putting my question is, Is the position of the earth w.r.t the sun gonna affect the RA and Dec?  
> **username_3**: @AmbicaGovind Are you thinking of [parallax](https://en.wikipedia.org/wiki/Stellar_parallax)?  
> **username_2**: The RA/Dec system is based on the centre of the Earth, the location of the Sun relative to that centre is irrelevant.  
> **username_0**: @Sten Yes! So does parallax have nothing to do with RA/Dec? Why not?  
> **username_3**: @AmbicaGovind Parallax does change an object's sky coordinates (very slightly)! I think people (including myself) just didn't understand the question at first.  
> **username_4**: The answer is "of course!".  The apparent effects of the Earth's position on nearby stars is very subtle, but measurable, and is one way missions like GAIA and HIPPARCOS measured the distance to the stars.  But the effects on nearby planets is much greater.  And a person's position on the Earth can affect the position of the Moon by about 1 degree (twice the diameter of the moon).  

---

## 1 answers

### Answer by username_1 on 2023-05-26. Score: 5 | Accepted answer

This is called "parallax". The observed position of a star (as measured by RA and Dec) does vary ever so slightly.

For example, Proxima centauri, the closest star has a parallax of 0.77 arcseconds. For context, there are 3600 arcseconds in one degree. The moon is about 1800 arcseconds across.

The parallax can only be measured by very careful telescopic observations.

Other stars are further away, and have even smaller parallax.

The underlying RA and Dec grid stays fixed, but stars (in particular nearby stars) move relative to it.


---

