<p>The number -5.5 km/s in this context means that the relative line of sight velocity between Sirius and the Sun is 5.5 km/s in the sense that Sirius and the Sun are getting closer to each other.</p>

<p>Note that the line of sight velocity of Sirius as observed from the Earth will vary throughout the year, since the Earth moves with an orbital speed of 30 km/s around the Sun. Astronomical radial velocities that are quoted in catalogues will almost certainly be corrected for this and will be referred to as "heliocentric radial velocities" if corrected to the Sun;s reference frame, or "barycentric radial velocities" if corrected to the centre of mass of the solar system (which is almost the same to within 10 m/s or so).</p>

CC BY-SA 4.0
2018-08-18T16:39:50.690Z
2018-08-18T16:39:50.690Z
ProfRob
<p>If you are referring to the Earth...</p>

<p>If the number is positive, the star is moving away from us. When the number is negative, the star is moving towards us.</p>

<p>Hope that helps!</p>

<p>Source with more info: <a href="https://cseligman.com/text/stars/radialvelocity.htm" rel="nofollow noreferrer">https://cseligman.com/text/stars/radialvelocity.htm</a></p>

CC BY-SA 4.0
2018-08-19T06:34:54.873Z
What do you mean by "us"? If you mean the Earth, then the answer is incorrect
ProfRob
CC BY-SA 4.0
2018-08-19T11:48:53.570Z
I used the word us in the original question so I guess he was following suit.  Your point below is noted.
MiscellaneousUser
CC BY-SA 4.0
2018-08-19T15:03:54.760Z
Hmm... Makes sense. Thanks for pointing it out!
MystaryPi
CC BY-SA 4.0
2018-08-18T15:35:09.930Z
2018-08-19T15:04:18.667Z
2018-08-19T15:04:18.667Z
MystaryPi
<p>According to Google, radial velocity is "the velocity of a star or other body along the line of sight of an observer".</p>

<p>The radial velocity of Sirius according to Google is    −5.50 km/s. </p>

<p>If the number is positive, does that mean it's moving away or towards us?</p>

<radial-velocity>
Radial Velocity Clarification Sought
MiscellaneousUser
# Radial Velocity Clarification Sought

(Asked by: username_0 on 2018-08-18)

According to Google, radial velocity is "the velocity of a star or other body along the line of sight of an observer".

The radial velocity of Sirius according to Google is −5.50 km/s.

If the number is positive, does that mean it's moving away or towards us?

**Tags:** radial-velocity


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## 2 answers

### Answer by username_1 on 2018-08-18. Score: 2 | Accepted answer

The number -5.5 km/s in this context means that the relative line of sight velocity between Sirius and the Sun is 5.5 km/s in the sense that Sirius and the Sun are getting closer to each other.

Note that the line of sight velocity of Sirius as observed from the Earth will vary throughout the year, since the Earth moves with an orbital speed of 30 km/s around the Sun. Astronomical radial velocities that are quoted in catalogues will almost certainly be corrected for this and will be referred to as "heliocentric radial velocities" if corrected to the Sun;s reference frame, or "barycentric radial velocities" if corrected to the centre of mass of the solar system (which is almost the same to within 10 m/s or so).


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### Answer by username_2 on 2018-08-18. Score: 4

If you are referring to the Earth...

If the number is positive, the star is moving away from us. When the number is negative, the star is moving towards us.

Hope that helps!

Source with more info: <https://cseligman.com/text/stars/radialvelocity.htm>

> **Comments:**
> **username_1**: What do you mean by "us"? If you mean the Earth, then the answer is incorrect  
> **username_0**: I used the word us in the original question so I guess he was following suit.  Your point below is noted.  
> **username_2**: Hmm... Makes sense. Thanks for pointing it out!  

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