TARGET 03 · l = 50°, b = 0° · Aquila, a little south of Sagitta, near the W51 complex
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.
- RA / Dec (J2000)
- 19h 23m 19s
+15.1° - Transits at midnight
- 13 Jul
local mean time - Model vLSR range
- −97 to +51
km/s, flat rotation - LSR correction over a year
- −5 to +42
km/s at 40° N, 75° W
l = 50° is the natural next step after l = 30°. It is still inside the solar circle, so the tangent-point method works, but the geometry is less forgiving.
What changes. The tangent point is now at R = R₀ sin 50°, about three quarters of the way out to the Sun’s orbit. The flat-curve model puts the terminal velocity near +52 km/s, roughly half what it is at l = 30°. That edge sits much closer to the bright local emission near zero, so separating it needs decent spectral resolution and a clean baseline. If your channel width is several km/s, you will feel this.
What you’ll see. A profile that is narrower on the positive side than at l = 30°, with a longer negative-velocity tail from the far outer Galaxy, which the model extends to about −97 km/s at R = 20 kpc. The Sagittarius arm tangent is near this longitude; the W51 star-forming complex lies just below l = 50°.
Practicalities. At +15° declination it culminates higher than l = 30° for anyone north of the equator, about 65° at 40° N, so ground pickup is less of a worry. It transits around midnight in mid July. Doing l = 30°, 40°, 50°, 60° on the same night gives you four rotation-curve points with the same receiver state, which is worth more than one point measured carefully on four different nights.
Radial velocity (LSR) of gas at each distance along l = 50°, 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 1420.162 to 1420.865 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 | 45° | 15.7 h | 10.1 h |
| 50° N | 55° | 14.5 h | 10.2 h |
| 40° N | 65° | 13.7 h | 10.2 h |
| 30° N | 75° | 13.2 h | 10.0 h |
| 20° N | 85° | 12.7 h | 9.8 h |
| 0° | 75° | 12.0 h | 9.2 h |
| 20° S | 55° | 11.2 h | 8.2 h |
| 35° S | 40° | 10.5 h | 6.9 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.
- Jan12:40
- Feb10:38
- Mar08:48
- Apr06:46
- May04:48
- Jun02:46
- Jul00:48
- Aug22:42
- Sep20:40
- Oct18:42
- Nov16:40
- Dec14:42
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.