<p>The heliocentric velocity $V_\mathrm{H}$ of an object is its velocity wrt. the Sun. When you measure an object's velocity, you measure it in the reference frame of Earth, which revolves around the Sun with ~30 km/s (varying a bit from aphelion to perihelion), so convert to $V_\mathrm{H}$ you need to know the time of the year of the observation (unless the line of sight toward your object is exactly perpendicular to the ecliptic plane), as well as the angle between the line of sight, and the line of sight toward the Sun. This involves a number of sines and cosines that you can find in e.g. <a href="https://books.google.dk/books?id=JhU6V1yDRCgC&amp;pg=PA153&amp;lpg=PA153&amp;dq=geocentric%20to%20heliocentric%20velocity&amp;source=bl&amp;ots=NBLRwDXsOW&amp;sig=NyR5Kz4lechA7DqrWrnFaBRsydY&amp;hl=en&amp;sa=X&amp;ved=0CB4Q6AEwADgKahUKEwiw8PiOlPLIAhVLcRQKHVtsAkE#v=onepage&amp;q=geocentric%20to%20heliocentric%20velocity&amp;f=false" rel="nofollow">Barbieri (2006)</a>.</p>

<p>If you further want to convert from $V_\mathrm{H}$ to the reference frame in which the Milky Way's center is at rest, the Local Group is at rest, or the Cosmic Microwave Background is isotropic (the "cosmic" frame), <em>then</em> you use the formula you link to, adding a similar term as described above, but instead using the velocity (i.e. speed and direction) of the Sun wrt. to the given frame.</p>

CC BY-SA 3.0
2016-06-03T08:26:18.847Z
I know it's been a loooong time, thanks a lot!
RaynDrop
CC BY-SA 3.0
2015-11-02T16:31:38.710Z
2015-11-02T16:31:38.710Z
pela
<p>My task is to calculate the heliocentric velocity of an object to correct for wavelength in a spectrum. I have the RA, Dec and observation date, and I know that the  NED velocity correction calculator(<a href="https://ned.ipac.caltech.edu/forms/vel_correction.html" rel="nofollow">https://ned.ipac.caltech.edu/forms/vel_correction.html</a>) will do the job, and I found out the equation here (<a href="https://ned.ipac.caltech.edu/help/velc_help.html#notes" rel="nofollow">https://ned.ipac.caltech.edu/help/velc_help.html#notes</a>), but honestly I don't have a clue where these parameters come from. Could some one give me some hints on the reasoning behind it? Thanks!</p>

CC BY-SA 3.0
2015-09-27T21:56:28.803Z
This sounds like a homework question, which is generally considered a poor question for Astronomy Stack Exchange. It may be closed. Could you at least show what you've tried so far?
HDE 226868
CC BY-SA 3.0
2015-11-02T07:36:51.237Z
I have updated the question and included the links to the calculator and the equation.
RaynDrop
CC BY-SA 3.0
2015-11-02T07:37:00.187Z
@HDE 226868: I assure you this is not a homework related question and I have the equation now but I just want to know how to get it. Would you mind giving some hints? Thanks.
RaynDrop
<velocity>
How to calculate the heliocentric velocity of an object?
RaynDrop
# How to calculate the heliocentric velocity of an object?

(Asked by: username_0 on 2015-09-27)

My task is to calculate the heliocentric velocity of an object to correct for wavelength in a spectrum. I have the RA, Dec and observation date, and I know that the NED velocity correction calculator(<https://ned.ipac.caltech.edu/forms/vel_correction.html>) will do the job, and I found out the equation here (<https://ned.ipac.caltech.edu/help/velc_help.html#notes>), but honestly I don't have a clue where these parameters come from. Could some one give me some hints on the reasoning behind it? Thanks!

**Tags:** velocity
> **Comments:**
> **username_1**: This sounds like a homework question, which is generally considered a poor question for Astronomy Stack Exchange. It may be closed. Could you at least show what you've tried so far?  
> **username_0**: I have updated the question and included the links to the calculator and the equation.  
> **username_0**: @HDE 226868: I assure you this is not a homework related question and I have the equation now but I just want to know how to get it. Would you mind giving some hints? Thanks.  

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## 1 answers

### Answer by username_2 on 2015-11-02. Score: 1 | Accepted answer

The heliocentric velocity $V\_\mathrm{H}$ of an object is its velocity wrt. the Sun. When you measure an object's velocity, you measure it in the reference frame of Earth, which revolves around the Sun with ~30 km/s (varying a bit from aphelion to perihelion), so convert to $V\_\mathrm{H}$ you need to know the time of the year of the observation (unless the line of sight toward your object is exactly perpendicular to the ecliptic plane), as well as the angle between the line of sight, and the line of sight toward the Sun. This involves a number of sines and cosines that you can find in e.g. [Barbieri (2006)](https://books.google.dk/books?id=JhU6V1yDRCgC&pg=PA153&lpg=PA153&dq=geocentric%20to%20heliocentric%20velocity&source=bl&ots=NBLRwDXsOW&sig=NyR5Kz4lechA7DqrWrnFaBRsydY&hl=en&sa=X&ved=0CB4Q6AEwADgKahUKEwiw8PiOlPLIAhVLcRQKHVtsAkE#v=onepage&q=geocentric%20to%20heliocentric%20velocity&f=false).

If you further want to convert from $V\_\mathrm{H}$ to the reference frame in which the Milky Way's center is at rest, the Local Group is at rest, or the Cosmic Microwave Background is isotropic (the "cosmic" frame), *then* you use the formula you link to, adding a similar term as described above, but instead using the velocity (i.e. speed and direction) of the Sun wrt. to the given frame.

> **Comments:**
> **username_0**: I know it's been a loooong time, thanks a lot!  

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