Deep Sky 04

The Plane of the Galaxy

The Milky Way is our galaxy seen edge-on from inside. Find its midline, its centre in Sagittarius, and the empty window above it.

Working out when this is best placed from where you are…

Latitude

Longitude

Which night

Date

Hour

Used for this lesson only — your map settings, and every other lesson, stay as they are.

Sources

ESA, The Hipparcos and Tycho Catalogues (SP-1200, 1997), vol. 1 §1.5.3 — the galactic frame in J2000: north galactic pole at RA 192.85948°, Dec +27.12825°; every galactic position, crossing and angle here is computed from it
GRAVITY Collaboration, A&A 625, L10 (2019) — distance to the Galactic centre, 8.178 ± 0.026 kpc (26,700 light-years)
HYG stellar database v4.2 — star positions and every separation quoted here
d3-celestial deep-sky catalogue, as this map ships it (public/data/dsos.10.json) — the 116 galaxies counted by galactic latitude
Binggeli, Sandage & Tammann, "Studies of the Virgo Cluster. II. A catalog of 2096 galaxies in the Virgo Cluster area", AJ 90, 1681 (1985)
Bortle, "The Bortle Dark-Sky Scale", Sky & Telescope, February 2001

← GUIDES

A March evening under a dark sky. The faint band arching from Cassiopeia, over Capella and down past Orion, is the Milky Way: the disc of our own galaxy, seen edge-on from inside it. The dashed line is its midline, the galactic equator, a great circle right round the sky. Run two hours: band and line turn together, fixed to the stars.

Places on the line are given as galactic longitude, l, counted from the centre. The callout, 4.2° from Elnath, is l = 180°: the anticentre, pointing straight away from it. We live 26,700 light-years out, so this way you look out through the outer disc, with less galaxy ahead, and the band is thin and faint.

Here in Monoceros, between Betelgeuse and Procyon, the galactic equator crosses the celestial equator at 62.9°. Nothing lines the two up: the Earth's spin and the galaxy's disc are unrelated. The angle is the frame's own: its north pole sits at declination +27.13°, and 90° − 27.13° = 62.87°.

The ecliptic, the Sun's yearly path, cuts the galactic equator at 60.2°, here in Taurus, 3.5° from Propus, and within a degree of the June solstice point. So every June the Sun stands in front of the Milky Way, and a planet crossing from Taurus into Gemini is seen against the band.

Now run six hours. The band sinks west until it lies low all the way round the horizon, and Coma Berenices, between Arcturus and Denebola, climbs toward the zenith. In it is the north galactic pole, 90° from the line: looking there, you look straight up out of the disc.

Out of the disc, little dust stands between you and other galaxies. Of the 116 this map draws, 27 lie in this ring, 6.7% of the sky; only 8 lie within 10° of the galactic equator, 17% of it. Here are the Coma and Virgo clusters: M100 is a Virgo member, and one survey of that area lists 2,096 galaxies.

Every galaxy this map draws, dropped into bands by distance from the Milky Way's midline. The middle is nearly empty: only eight lie within 10° of it, a strip that covers 17% of the sky. The dust in our own disc hides what lies behind it.

Before dawn Sagittarius rises, and with it l = 0°, the galactic centre, 26,700 light-years away, 4.6° from Alnasl. The Scorpius lesson walks you there from the Teapot's spout. This way you look in through the whole inner disc: the band is broadest and brightest here, and split by dust.

The ecliptic crosses back here, 6.4° from the centre and 4.0° from Polis, within a degree of the December solstice point. The two lines meet at opposite points of the sky, and the Sun reaches each in turn: in December it stands in front of the heart of the Milky Way. A planet in Sagittarius shines against its densest star clouds.