Galactic coordinates

Equatorial coordinates (RA and Dec) are tied to the Earth’s spin. For hydrogen-line work the natural frame is the Galaxy itself. Galactic latitude b is zero along the mid-plane of the Milky Way, positive to the north. Galactic longitude l is measured along the plane, starting at l = 0° toward the Galactic centre and increasing eastward through Aquila (l ≈ 30°), Cygnus (l ≈ 80°), Cassiopeia (l ≈ 120°) and the anticentre in Taurus/Auriga at l = 180°.

Because neutral hydrogen is concentrated in a thin disk, nearly everything interesting for a small station happens at b ≈ 0. Pointing is a matter of choosing l.

The four quadrants

How the gas moves depends on which quarter of the Galaxy you look through, and the quadrants are why spectra at different longitudes look so different:

• Quadrant I (0° to 90°): you look inward of the Sun’s orbit; gas inside the solar circle shows up at positive LSR velocities. This is where the tangent-point method works for northern observers.
• Quadrant II (90° to 180°): outer Galaxy; mostly negative velocities.
• Quadrant III (180° to 270°): outer Galaxy; mostly positive.
• Quadrant IV (270° to 360°): inner Galaxy again, negative velocities; better placed for southern observers.

Converting to RA and Dec

Our tools do the conversion with the standard J2000 rotation. Some reference points on the plane: l = 30° is RA 281.52°, Dec −2.61°; l = 90° is RA 318.00°, Dec +48.33°; l = 180° is RA 86.40°, Dec +28.94°. From 40° N, l = 30° culminates at about 47° altitude, while the Galactic centre only gets to about 21°.

To find when a given longitude crosses your meridian, use the Galactic-plane transit planner. The Doppler calculator accepts l and b directly for the LSR correction.