TARGET 02 · l = 30°, b = 0° · Aquila/Scutum, near the Scutum arm tangent and the W43 complex
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.
- RA / Dec (J2000)
- 18h 46m 05s
−2.6° - Transits at midnight
- 4 Jul
local mean time - Model vLSR range
- −63 to +110
km/s, flat rotation - LSR correction over a year
- −11 to +44
km/s at 40° N, 75° W
If you only point at one place in the inner Galaxy, point here. This is the longitude we used for the worked example in LAB-013, and it is the one we come back to when a change to the receive chain needs testing against a known spectrum.
Why it works. Along l = 30° the line of sight passes through the inner Galaxy and grazes a circle of radius R₀ sin 30°, half the Sun’s distance from the centre. Gas at that tangent point moves straight along your line of sight, so it has the highest positive velocity on the sightline. In the spectrum that is the high-velocity edge, and the flat-curve model puts it near +110 km/s. The difference between your measured edge and the model is one point on a rotation curve.
What you’ll see. Expect a strong, broad profile from roughly zero up to that edge, plus a smaller negative-velocity tail from gas beyond the solar circle on the far side. The Scutum arm tangent sits around this longitude, so the high-velocity end is usually well populated, which makes the edge easier to place than at higher l.
Practicalities. The declination is just south of the equator, so it rises well from almost anywhere on Earth. At 40° N it culminates near 47° and spends close to twelve hours above the horizon. It transits at local midnight in early July, so from late summer it is an evening target, with the Sun well away from your sidelobes. Use the Doppler calculator for the LSR correction on the night you observe; the yearly swing below shows how much it moves.
Radial velocity (LSR) of gas at each distance along l = 30°, 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 1419.885 to 1420.705 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 | 27° | 11.4 h | 5.3 h |
| 50° N | 37° | 11.6 h | 7.2 h |
| 40° N | 47° | 11.7 h | 8.1 h |
| 30° N | 57° | 11.8 h | 8.7 h |
| 20° N | 67° | 11.8 h | 9.0 h |
| 0° | 87° | 12.0 h | 9.3 h |
| 20° S | 73° | 12.1 h | 9.3 h |
| 35° S | 58° | 12.2 h | 8.9 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.
- Jan12:02
- Feb10:01
- Mar08:10
- Apr06:09
- May04:11
- Jun02:09
- Jul00:11
- Aug22:05
- Sep20:03
- Oct18:05
- Nov16:03
- Dec14:05
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.