Astronomy & Celestial Mechanics
the sky, computed
Concept

Galactic coordinates — where it appears

The latitude and longitude system with its equator along the Milky Way, fixed in 1958 from the neutral hydrogen's plane and the radio source at the Galaxy's centre. Its equator passes through the Sun, which sits a little above the disc's midplane.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

Three planes on one sky: the Milky Way crosses the celestial equator at 62.87° and its pole sits at declination 27.13°. The whole sky in equatorial coordinates — right ascension decreasing to the right, declination up — with the celestial equator (grey), the ecliptic (orange) and the galactic equator, b = 0 (blue), each a great circle drawn by rotating its own frame into the equatorial one. The north galactic pole lies at right ascension 192.86° and declination 27.13°, so the galactic plane is inclined at 62.87° to the celestial equator; the zero of galactic longitude, towards the centre, lies at 266.40°, -28.94° in Sagittarius. All three are conventions tied to physical planes — the Earth's equator, its orbit, the Galaxy's disc — and only the first two are defined by measurements that can be repeated to the precision of the coordinates themselves.

A frame fixed before its centre was found

Galactic longitude and latitude look like the Galaxy's own coordinates, and they are a convention agreed in 1958: a pole taken from the radio map of neutral hydrogen, a zero of longitude put on a radio source then catalogued in Sagittarius, an equator drawn through the Sun. The black hole at the centre was found afterwards, three arcminutes away, and nobody moved the origin. The Sun sits twenty parsecs above the disc, so latitude zero is not the Galaxy's plane except far away — and over eighty million years the Sun will swing a hundred parsecs either side of it.

sky · Celestial sphere The Sun's height, against the radius it is measured over. The Sun's height above the midplane as a survey would measure it, fitting the midplane to the stars within a projected radius R, against R, for a corrugation of 15 pc on a 2500-pc wavelength with the Sun 15 pc above the midplane beneath it. Nearby the answer is 15 pc whatever the phase; over several kiloparsecs it is the height above the mean plane: 30 pc with the Sun on a crest, 0 pc with it in a trough. Between them every intermediate value is a correct answer to a different question — a spread of 31 pc, comparable with the five to thirty parsecs the published measurements span.

A midplane that depends on how far it is averaged

The Sun sits a little above the middle of the Galaxy's disc — twenty parsecs, by the most careful recent counts, and anywhere from five to thirty by others. If the disc were flat that scatter would be error. It is not flat: the disc near the Sun ripples vertically by tens of parsecs on kiloparsec wavelengths, and on a rippled disc the Sun's height above 'the' midplane depends on how far around it the midplane is averaged. Every value in the scatter can be a correct answer to a different question.

sky · Celestial sphere

Named alongside it

The objects these essays reach for when they reach for this one.

Galactic planeReference frameCelestial sphereDark matterLocal standard of restPhase spiralPrecessionRotation matrixSagittarius a starSagittarius dwarfScale heightStar counts

All concepts