TARGET 07 · l = 270°, b = 0° · Vela; the Vela supernova remnant lies near l ≈ 264°
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.
- RA / Dec (J2000)
- 09h 12m 01s
−48.3° - Transits at midnight
- 8 Feb
local mean time - Model vLSR range
- 0 to +126
km/s, flat rotation - LSR correction over a year
- −30 to 0
km/s at 40° N, 75° W
This one is for southern observers. From 40° N it barely clears the horizon, and from 50° N it never rises.
What the model says. At l = 270° the line of sight points opposite to the Sun’s direction of orbital motion and stays outside the solar circle. Under circular rotation every parcel of gas along it is receding, so the flat-curve model predicts velocities from zero up to about +127 km/s at R = 20 kpc, and nothing negative. It is the mirror image of looking toward l = 90° from the north, where everything is approaching.
What you’ll see. A strong peak near zero with a shoulder on the positive side. If you also observe l = 120° or l = 80° from a northern site, the asymmetry flips sign. That flip is a nice, simple demonstration that the Galaxy rotates, and it doesn’t need any rotation-curve maths.
Practicalities. At −48° declination it culminates at about 77° from 35° S and is up for most of a day. It transits at midnight in early February. The Vela supernova remnant sits nearby; its continuum may raise your total power, which is separate from the line.
Radial velocity (LSR) of gas at each distance along l = 270°, 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 1419.806 to 1420.406 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 | never rises | 0 h | 0 h |
| 50° N | never rises | 0 h | 0 h |
| 40° N | 2° | 2.6 h | 0 h |
| 30° N | 12° | 6.6 h | 0 h |
| 20° N | 22° | 8.8 h | 2.3 h |
| 0° | 42° | 12.0 h | 7.9 h |
| 20° S | 62° | 15.2 h | 10.9 h |
| 35° S | 77° | 18.9 h | 13.2 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.
- Jan02:30
- Feb00:28
- Mar22:34
- Apr20:32
- May18:34
- Jun16:32
- Jul14:34
- Aug12:32
- Sep10:31
- Oct08:33
- Nov06:31
- Dec04:33
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.