Galactic-plane transit planner

Why plan around transits

Most first hydrogen-line antennas don't track. A horn on a sawhorse or a dish on a fixed mount points at one altitude and azimuth, and the sky turns past it. If you point due south (due north in the southern hemisphere) at the right elevation, a given part of the Galactic plane passes through the middle of your beam at its meridian transit, which is also when it is highest and you're looking through the least atmosphere and ground pickup. That makes transit time the one number worth planning a session around.

How it's worked out

Each Galactic longitude l at the chosen latitude b is converted to J2000 right ascension and declination with the standard IAU rotation. A target transits when local sidereal time equals its right ascension, so the planner finds your local sidereal time at the start of the window and steps forward at the sidereal rate:

Transits come about four minutes earlier each day, because a sidereal day is 23 h 56 m. Precession between J2000 and today moves things by a fraction of a degree, which doesn't matter for a beam several degrees wide.

Reading the result

• l = 0°, the Galactic centre, sits at declination −29°. From 40° N it culminates only about 21° up, and from much above 60° N it never rises. Southern-hemisphere sites get it nearly overhead.
• l = 30° to 90° is the stretch most northern amateurs actually observe: high transits, bright inner-Galaxy hydrogen at l = 30°, and the long sightline down our own Local Arm toward Cygnus, near l = 80°.
• The anticentre, l = 180°, is the quiet direction. Gas there moves almost across your line of sight, so the profile piles up near zero velocity. It makes a useful comparison spectrum.

The worked case in our Doppler calculator uses l = 30° from 40° N. On 6 October 2026 from 75° W, that point transits at about 22:44 UTC, 47° up. Measure the spectrum then and run the frequency through the Doppler calculator to get its velocity.

For what to expect once a longitude is in the beam, the sky targetspages cover ten standard pointings, from Cygnus to Norma, with transit times by month and the velocity range each one should show.