TARGET 04 · l = 80°, b = 0° · Cygnus, near the Cygnus X complex; Cygnus A lies a few degrees away
Cygnus, l = 80°
The easiest pointing in the northern sky: overhead from mid-northern latitudes, a long path down the local spiral arm, and a strong line. The fallback when the inner plane is down.
- RA / Dec (J2000)
- 20h 35m 53s
+40.7° - Transits at midnight
- 1 Aug
local mean time - Model vLSR range
- −125 to +3
km/s, flat rotation - LSR correction over a year
- 0 to +33
km/s at 40° N, 75° W
LAB-009 sends first-timers to l = 30° to 60°, because the inner-Galaxy line is broad and hard to mistake for anything instrumental. Cygnus is the other easy option, and the better one when the inner plane is down or too low, which from a northern site is much of the winter evening sky.
Why it is easy. At 40° N this stretch of the plane culminates almost exactly overhead and stays up for about eighteen hours a day. North of about 49° it never sets. High elevation means less ground noise in the sidelobes and a longer drift scan. The line of sight runs along the local spiral arm, so there is a lot of hydrogen in the beam, and the line is strong even for a small horn.
What you’ll see. At l = 80° the tangent point is nearly at the Sun’s own orbital radius, so the inner-Galaxy part of the profile is squeezed into a few km/s above zero. The flat-curve model puts that edge near +3 km/s. Almost everything else is outer-Galaxy gas at negative velocities, which the model extends to about −125 km/s at R = 20 kpc. In practice that means one strong peak near zero with a long shoulder on the negative side. If your peak is at a strongly positive velocity, check the sign convention and the LSR correction before you believe it.
Neighbours. The Cygnus X star-forming complex sits near this longitude, and Cygnus A, one of the strongest radio sources in the sky, is at l ≈ 76°, b ≈ +6°. With a wide beam you will see some of their continuum as a rise in total power. That is not the line, and it is a handy pointing check.
For the first-night procedure, the same steps from LAB-009 apply here. Use the transit planner to see when it crosses your meridian tonight.
Radial velocity (LSR) of gas at each distance along l = 80°, b = 0°, if everything moved on circular orbits at 220 km/s. The line stops where the sightline leaves R = 20 kpc.
In frequency, that model range is 1420.390 to 1420.996 MHz in the LSR frame. Your receiver sees it shifted by the correction in the last box above, so leave a margin of about 0.2 MHz either side when you set the span.
How high it gets
Altitude at meridian transit and time above the horizon per sidereal day, for this pointing at a range of observer latitudes. Below about 20° the beam starts picking up ground, so the last column is the one to plan on.
| Latitude | Transit altitude | Above horizon | Above 20° |
|---|---|---|---|
| 60° N | 71° | always | 16.7 h |
| 50° N | 81° | always | 14.5 h |
| 40° N | 89° | 18.1 h | 13.0 h |
| 30° N | 79° | 15.9 h | 11.8 h |
| 20° N | 69° | 14.4 h | 10.7 h |
| 0° | 49° | 12.0 h | 8.4 h |
| 20° S | 29° | 9.5 h | 5.0 h |
| 35° S | 14° | 7.1 h | 0 h |
Transit time by month
Local mean time of meridian transit on the 1st of each month. That is clock time without daylight saving, corrected by four minutes for every degree you sit east or west of your time zone's central meridian. The tools do it for your exact site. The transit comes about four minutes earlier each day.
- Jan13:52
- Feb11:50
- Mar10:00
- Apr07:58
- May06:00
- Jun03:58
- Jul02:00
- Aug23:54
- Sep21:53
- Oct19:55
- Nov17:53
- Dec15:55
Read alongside
Coordinates, visibility, transit times and the LSR swing are computed for this page with the same code as our calculators. The velocity model is circular rotation on a flat curve with R₀ = 8.5 kpc and V₀ = 220 km/s (IAU 1985), the textbook first approximation. Real spectra depart from it, and the notes above say where. Found an error? Tell us.