<p>The zero of Galactic latitude (i.e. the Galactic plane) was defined by a working group of the International Astronomical Union. The observations used to do this were 21 cm radio observations of atomic hydrogen, which are unaffected by dust absorption near the plane. The details of the observations are found in <a href="http://adsabs.harvard.edu/abs/1960MNRAS.121..132G" rel="noreferrer">Gum, Kerr &amp; Westerhout (1960)</a>. This paper uses least squares fits to the 21cm data <em>assuming</em> that the greatest concentration of neutral hydrogen is found in the Galactic plane itself. Whether this <em>is</em> the dynamical plane was considered by <a href="http://adsabs.harvard.edu/abs/1960MNRAS.121..150G" rel="noreferrer">Gum &amp; Pawsey (1960)</a>, as part of the same IAU study. They concluded that it was (bar the location of the Sun with respect to the plane), citing the flatness and relative narrowness of the neutral hydrogen and the coincidence of the source Sgr A*, which even then was thought to be at the Galactic centre, with zero latitude.</p>

<p>Dark matter was not well explored or widely considered in the dynamics of individual galaxies in the late 1950s, however it is not unexpected that neutral hydrogen <em>should</em> lie in the Galactic plane, as defined by the symmetry of the Galactic potential. A rotating mass of gas, capable of dissipative interactions, <em>should</em> settle to a plane perpendicular to its angular momentum vector.</p>

<p>The 21cm definition of Galactic coordinates cannot itself define the position of the Galactic plane in 3d space unless the Sun is exactly in the plane. Detailed observations of star counts and the dynamics of stars around the Sun and perpendicular to the plane have revealed this isn't quite the case. The details are given in <a href="https://astronomy.stackexchange.com/questions/822/how-far-is-the-earth-sun-above-below-the-galactic-plane-and-is-it-heading-towar">How far is the Earth/Sun above/below the galactic plane, and is it heading toward/away from it?</a> ; the Sun is currently around 17 pc above the Galactic plane and heading upwards. This is close enough that it makes very little difference to the apparent flatness and narrowness of the 21cm data.</p>

CC BY-SA 4.0
2018-10-12T06:37:39.673Z
This is much more of an answer than I expected, thank you very much! I very much appreciate links to the original IAU working group papers.
uhoh
CC BY-SA 4.0
2018-10-19T15:00:53.163Z
I've just asked [Why the blank wedges in this very early 21 cm map of the Milky Way? (Oort et al. 1958)](https://astronomy.stackexchange.com/q/28083/7982)
uhoh
CC BY-SA 4.0
2018-10-12T06:23:51.900Z
2018-10-12T06:23:51.900Z
ProfRob
<p>As pointed out in <a href="https://astronomy.stackexchange.com/a/16340/7982">this answer</a> <em>the axes</em> of <a href="https://en.wikipedia.org/wiki/Galactic_coordinate_system" rel="noreferrer">galactic coordinates</a> are defined such that galactic longitude and latitude (0, 0) point toward the <a href="https://en.wikipedia.org/wiki/Galactic_Center" rel="noreferrer">galactic center</a>. But then the plane defined by galactic latitude equal to zero would be parallel to the galactic plane only if the Sun were <em>in the galactic plane as well</em>. </p>

<p>Is that the case? </p>

<p>More generally, by what procedure was the galactic plane actually defined, considering a substantial part of it is obscured by dust (at least in the visible) and most of the mass is <a href="https://en.wikipedia.org/wiki/Dark_matter" rel="noreferrer">dark matter</a> and therefore not observed at all (directly at least)?</p>

<p><strong>Question:</strong> How was the galactic plane established? Primarily what kinds of observations were used, and how were they analyzed to help decide how this plane should be chosen?</p>

<galaxy><milky-way><coordinate>
How was the galactic plane established?
uhoh
# How was the galactic plane established?

(Asked by: username_0 on 2017-12-12)

As pointed out in [this answer](https://astronomy.stackexchange.com/a/16340/7982) *the axes* of [galactic coordinates](https://en.wikipedia.org/wiki/Galactic_coordinate_system) are defined such that galactic longitude and latitude (0, 0) point toward the [galactic center](https://en.wikipedia.org/wiki/Galactic_Center). But then the plane defined by galactic latitude equal to zero would be parallel to the galactic plane only if the Sun were *in the galactic plane as well*.

Is that the case?

More generally, by what procedure was the galactic plane actually defined, considering a substantial part of it is obscured by dust (at least in the visible) and most of the mass is [dark matter](https://en.wikipedia.org/wiki/Dark_matter) and therefore not observed at all (directly at least)?

**Question:** How was the galactic plane established? Primarily what kinds of observations were used, and how were they analyzed to help decide how this plane should be chosen?

**Tags:** galaxy, milky-way, coordinate


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## 1 answers

### Answer by username_1 on 2018-10-12. Score: 8 | Accepted answer

The zero of Galactic latitude (i.e. the Galactic plane) was defined by a working group of the International Astronomical Union. The observations used to do this were 21 cm radio observations of atomic hydrogen, which are unaffected by dust absorption near the plane. The details of the observations are found in [Gum, Kerr & Westerhout (1960)](http://adsabs.harvard.edu/abs/1960MNRAS.121..132G). This paper uses least squares fits to the 21cm data *assuming* that the greatest concentration of neutral hydrogen is found in the Galactic plane itself. Whether this *is* the dynamical plane was considered by [Gum & Pawsey (1960)](http://adsabs.harvard.edu/abs/1960MNRAS.121..150G), as part of the same IAU study. They concluded that it was (bar the location of the Sun with respect to the plane), citing the flatness and relative narrowness of the neutral hydrogen and the coincidence of the source Sgr A\*, which even then was thought to be at the Galactic centre, with zero latitude.

Dark matter was not well explored or widely considered in the dynamics of individual galaxies in the late 1950s, however it is not unexpected that neutral hydrogen *should* lie in the Galactic plane, as defined by the symmetry of the Galactic potential. A rotating mass of gas, capable of dissipative interactions, *should* settle to a plane perpendicular to its angular momentum vector.

The 21cm definition of Galactic coordinates cannot itself define the position of the Galactic plane in 3d space unless the Sun is exactly in the plane. Detailed observations of star counts and the dynamics of stars around the Sun and perpendicular to the plane have revealed this isn't quite the case. The details are given in [How far is the Earth/Sun above/below the galactic plane, and is it heading toward/away from it?](https://astronomy.stackexchange.com/questions/822/how-far-is-the-earth-sun-above-below-the-galactic-plane-and-is-it-heading-towar) ; the Sun is currently around 17 pc above the Galactic plane and heading upwards. This is close enough that it makes very little difference to the apparent flatness and narrowness of the 21cm data.

> **Comments:**
> **username_0**: This is much more of an answer than I expected, thank you very much! I very much appreciate links to the original IAU working group papers.  
> **username_0**: I've just asked [Why the blank wedges in this very early 21 cm map of the Milky Way? (Oort et al. 1958)](https://astronomy.stackexchange.com/q/28083/7982)  

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