Reel 10
Sky Targets
Real radioWhere we point, and where we tell people to point. Ten standard directions for a small hydrogen-line antenna, each with the numbers you need before you go outside: coordinates, how high it gets from your latitude, when it crosses the meridian, and what the spectrum should look like.
Longitude increases to the left, as on any all-sky map. Red marks are best from the north, white from the south, outlined from either.
All targets
- 01 · l = 0° · 17h 45m −28.9°Galactic centreStraight at the middle of the Milky Way. Bright in continuum, broad and messy in the line, and the one direction where the simple rotation model tells you nothing.40° N: 21° · 35° S: 84° · v 0 to 0 km/s
- 02 · l = 30° · 18h 46m −2.6°Inner Galaxy, l = 30°The best rotation-curve pointing for most northern observers: a clear tangent point, a high-velocity edge well separated from local gas, and a declination near the equator.40° N: 47° · 35° S: 58° · v −63 to +110 km/s
- 03 · l = 50° · 19h 23m +15.1°Aquila, l = 50°A second rotation-curve point, higher in the northern sky than l = 30°, where the tangent-point edge sits closer to the local gas and the measurement gets harder.40° N: 65° · 35° S: 40° · v −97 to +51 km/s
- 04 · l = 80° · 20h 35m +40.7°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.40° N: 89° · 35° S: 14° · v −125 to +3 km/s
- 05 · l = 120° · 00h 25m +62.7°Cassiopeia and the Perseus arm, l = 120°An outer-Galaxy pointing where a spiral arm shows up as its own peak. The easiest way to see Galactic structure in a home-made spectrum.40° N: 67° · 35° S: never · v −110 to 0 km/s
- 06 · l = 180° · 05h 45m +28.9°Galactic anticentre, l = 180°Straight away from the centre, where circular rotation gives no radial velocity at all. A narrow line near zero makes it the best calibration check on the sky.40° N: 79° · 35° S: 26° · v 0 to 0 km/s
- 07 · l = 270° · 09h 12m −48.3°Vela, l = 270°A southern outer-Galaxy pointing where circular rotation puts all the gas at positive velocities, the mirror image of the northern l = 90° direction.40° N: 2° · 35° S: 77° · v 0 to +126 km/s
- 08 · l = 300° · 12h 25m −62.7°Crux, l = 300°A fourth-quadrant pointing high in the southern sky, with a tangent point at negative velocity and a long positive tail from the far side of the Galaxy.40° N: never · 35° S: 62° · v −29 to +110 km/s
- 09 · l = 330° · 16h 04m −52.4°Norma, l = 330°The southern twin of l = 30°: a clean tangent point with a terminal velocity near −110 km/s, well separated from local gas. The rotation-curve pointing for the south.40° N: never · 35° S: 73° · v −110 to +63 km/s
- 10 · b = 90° · 12h 51m +27.1°North Galactic PoleStraight up out of the disk, through the least hydrogen you can find in the northern sky. The off-plane comparison every on-plane spectrum needs.40° N: 77° · 35° S: 28°
All ten at a glance
Transit altitudes for two common latitudes, the model velocity range under flat rotation, and the date each one crosses the meridian at local midnight.
| Target | l, b | RA / Dec | Alt at 40° N | Alt at 35° S | Model v (km/s) | Midnight transit |
|---|---|---|---|---|---|---|
| Galactic centre | 0°, 0° | 17h 45m / −28.9° | 21° | 84° | 0 to 0 | 18 Jun |
| Inner Galaxy, l = 30° | 30°, 0° | 18h 46m / −2.6° | 47° | 58° | −63 to +110 | 4 Jul |
| Aquila, l = 50° | 50°, 0° | 19h 23m / +15.1° | 65° | 40° | −97 to +51 | 13 Jul |
| Cygnus, l = 80° | 80°, 0° | 20h 35m / +40.7° | 89° | 14° | −125 to +3 | 1 Aug |
| Cassiopeia and the Perseus arm, l = 120° | 120°, 0° | 00h 25m / +62.7° | 67° | never | −110 to 0 | 28 Sep |
| Galactic anticentre, l = 180° | 180°, 0° | 05h 45m / +28.9° | 79° | 26° | 0 to 0 | 18 Dec |
| Vela, l = 270° | 270°, 0° | 09h 12m / −48.3° | 2° | 77° | 0 to +126 | 8 Feb |
| Crux, l = 300° | 300°, 0° | 12h 25m / −62.7° | never | 62° | −29 to +110 | 29 Mar |
| Norma, l = 330° | 330°, 0° | 16h 04m / −52.4° | never | 73° | −110 to +63 | 24 May |
| North Galactic Pole | 0°, 90° | 12h 51m / +27.1° | 77° | 28° | n/a | 5 Apr |
Which one first
North of about 30°: start with l = 30°, as LAB-009 does; the line is broad and unmistakable. When the inner plane is down, Cygnus passes nearly overhead and is just as easy. Add l = 50° for a second rotation-curve point, and l = 120° to see a spiral arm as its own peak.
South of about 20° S:Norma is your rotation-curve target, Crux is circumpolar and always available, and the Galactic centre passes almost overhead.
Everyone: do the anticentre once to check your velocity calibration, since the answer there is close to zero, and the North Galactic Pole to see what a weak line looks like with your setup.
How the numbers are made
Every figure on these pages is computed when the site is built, with the same code that runs our calculators. Galactic to equatorial conversion uses the standard J2000 rotation. Transit altitude is 90° minus the difference between your latitude and the declination, and time above the horizon comes from the hour angle at which the target sets. The velocity model assumes circular orbits on a flat rotation curve with the IAU 1985 constants, R₀ = 8.5 kpc and V₀ = 220 km/s, out to 20 kpc from the centre. That is the model behind the tangent-point method and LAB-013. Where the real sky departs from it, the target page says so.
The yearly LSR correction on each page is sampled daily for a site at 40° N, 75° W. For your own site and night, use the Doppler calculator; for tonight's transit times, the transit planner.