<p>Yes, the difference is exactly that. The precise meaning here cannot be determined with certainty, it is a matter of convention.</p>
<p>With meteors (and their parents bodies in this context) true geocentric speed <span class="math-container">$v_g$</span> is most commonly reported. Meteors can only be detected in the Earth's atmosphere, and this is the speed measured from ground-based observations. It might or might not be corrected for the Earth's rotation (<span class="math-container">$\approx 460 \text{ m/s} \cdot \cos \phi$</span> where <span class="math-container">$\phi$</span> is the geographical latitude, a relatively small but measurable effect).</p>
<p>It is also common to calculate and report the &quot;geocentric speed at infinity&quot;, that is, outside the Earth's gravity well but otherwise at the same location. From conservation of mechanical energy that is <span class="math-container">$v_\infty = \sqrt{v_g^2 - v_0^2}$</span>, where <span class="math-container">$v_0 \approx 11.2 \text{ km/s}$</span> is the escape speed from the surface.</p>
<p>When talking about perihelions or orbits of comets, the obvious choice is the heliocentric or barycentric coordinate system. Geocentric speeds far from the Earth can be computed as well, but are not very useful. I would interpret the sentence &quot;the speed of a comet at perihelion is <span class="math-container">$71 \text{ km/s}$</span>&quot; as a heliocentric / barycentric speed, but I cannot comment on whether this is what the author actually meant.</p>

CC BY-SA 4.0
2024-03-26T11:48:17.943Z
There's not much difference between heliocentric & barycentric speed, since the Sun's barycentric speed is quite small, around 9 to 16 m/s. See the graph at the end of https://astronomy.stackexchange.com/a/28036/16685
PM 2Ring
CC BY-SA 4.0
2024-03-26T11:55:48.253Z
Of course, in this context the difference is negligible for all practical purposes (I can think of).
Martin 'Kvík' Baláž
CC BY-SA 4.0
2024-03-26T10:57:38.207Z
2024-03-26T10:57:38.207Z
Martin 'Kvík' Baláž
<p>Yes, the difference is a difference of coordinate systems.  The Earth moves with respect to the sun, and velocities depend on the choice of coordinate system.</p>
<p>In your particular case, it seems likely that 71 km/s is a geocentric velocity as it is the maximum geocentric velocity that a body could reach at one AU from the sun, if it falls from rest.</p>
<p>But, of course, a comet that has a perihelion closer to the sun could have a heliocentric velocity that is 71km/s.</p>
<p>Neither the geocentric, nor the heliocentric coordinate systems define an inertial frame of reference.  This means that in these frames of reference, Newton's first law doesn't work. But the sun is close to the centre of mass of the solar system, which is very nearly an inertial frame.</p>

CC BY-SA 4.0
2024-03-26T06:50:18.670Z
2024-03-26T06:50:18.670Z
James K
<p>Is the difference between heliocentric and geocentric velocities a matter of the coordinate system? Or are these kinds of velocities different?
For example, the speed of a comet at perihelion is 71km/s. Is this speed geocentric or heliocentric?</p>
<p>Best regards</p>

CC BY-SA 4.0
2024-03-25T23:57:32.733Z
My initial thought is heliocentric, because perihelion is in reference to the sun, but this depends on the context of the information. Is there a particular text that you saw this in? Could you provide a relevant quote?
Greg Burghardt
CC BY-SA 4.0
2024-03-25T23:59:00.713Z
Also, I'm not an astronomer, so there might be a convention I'm not aware of that provides a dead obvious answer.
Greg Burghardt
CC BY-SA 4.0
2024-03-26T00:11:01.877Z
Generally it's barycentric.  But as Greg said above, context means a lot.
Greg Miller
CC BY-SA 4.0
2024-03-26T00:24:49.223Z
Both speeds are mentioned in a german metor book, but in relation to comets geocentric velocity at perihelion is used.
Robert
CC BY-SA 4.0
2024-03-26T01:06:42.537Z
That speed *could* be either heliocentric or geocentric. Note that heliocentric escape speed near Earth orbit is ~42 km/s, so if such a body is on a head-on collision course for Earth its geocentric speed is ~71 km/s. FWIW, the heliocentric perihelion speed of Halley's comet is ~54.5 km/s.
PM 2Ring
<solar-system><comets>
the difference between the heliocentric and the geocentric
Robert
# the difference between the heliocentric and the geocentric

(Asked by: username_0 on 2024-03-25)

Is the difference between heliocentric and geocentric velocities a matter of the coordinate system? Or are these kinds of velocities different?
For example, the speed of a comet at perihelion is 71km/s. Is this speed geocentric or heliocentric?

Best regards

**Tags:** solar-system, comets
> **Comments:**
> **username_1**: My initial thought is heliocentric, because perihelion is in reference to the sun, but this depends on the context of the information. Is there a particular text that you saw this in? Could you provide a relevant quote?  
> **username_1**: Also, I'm not an astronomer, so there might be a convention I'm not aware of that provides a dead obvious answer.  
> **username_2**: Generally it's barycentric.  But as Greg said above, context means a lot.  
> **username_0**: Both speeds are mentioned in a german metor book, but in relation to comets geocentric velocity at perihelion is used.  
> **username_3**: That speed *could* be either heliocentric or geocentric. Note that heliocentric escape speed near Earth orbit is ~42 km/s, so if such a body is on a head-on collision course for Earth its geocentric speed is ~71 km/s. FWIW, the heliocentric perihelion speed of Halley's comet is ~54.5 km/s.  

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## 2 answers

### Answer by username_4 on 2024-03-26. Score: 3 | Accepted answer

Yes, the difference is exactly that. The precise meaning here cannot be determined with certainty, it is a matter of convention.

With meteors (and their parents bodies in this context) true geocentric speed $v\_g$ is most commonly reported. Meteors can only be detected in the Earth's atmosphere, and this is the speed measured from ground-based observations. It might or might not be corrected for the Earth's rotation ($\approx 460 \text{ m/s} \cdot \cos \phi$ where $\phi$ is the geographical latitude, a relatively small but measurable effect).

It is also common to calculate and report the "geocentric speed at infinity", that is, outside the Earth's gravity well but otherwise at the same location. From conservation of mechanical energy that is $v\_\infty = \sqrt{v\_g^2 - v\_0^2}$, where $v\_0 \approx 11.2 \text{ km/s}$ is the escape speed from the surface.

When talking about perihelions or orbits of comets, the obvious choice is the heliocentric or barycentric coordinate system. Geocentric speeds far from the Earth can be computed as well, but are not very useful. I would interpret the sentence "the speed of a comet at perihelion is $71 \text{ km/s}$" as a heliocentric / barycentric speed, but I cannot comment on whether this is what the author actually meant.

> **Comments:**
> **username_3**: There's not much difference between heliocentric & barycentric speed, since the Sun's barycentric speed is quite small, around 9 to 16 m/s. See the graph at the end of https://astronomy.stackexchange.com/a/28036/16685  
> **username_4**: Of course, in this context the difference is negligible for all practical purposes (I can think of).  

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### Answer by username_5 on 2024-03-26. Score: 2

Yes, the difference is a difference of coordinate systems. The Earth moves with respect to the sun, and velocities depend on the choice of coordinate system.

In your particular case, it seems likely that 71 km/s is a geocentric velocity as it is the maximum geocentric velocity that a body could reach at one AU from the sun, if it falls from rest.

But, of course, a comet that has a perihelion closer to the sun could have a heliocentric velocity that is 71km/s.

Neither the geocentric, nor the heliocentric coordinate systems define an inertial frame of reference. This means that in these frames of reference, Newton's first law doesn't work. But the sun is close to the centre of mass of the solar system, which is very nearly an inertial frame.


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