TARGET 08 · l = 300°, b = 0° · Crux, near the Coalsack
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.
- RA / Dec (J2000)
- 12h 25m 48s
−62.7° - Transits at midnight
- 29 Mar
local mean time - Model vLSR range
- −29 to +110
km/s, flat rotation - LSR correction over a year
- −26 to +10
km/s at 40° N, 75° W
From mid-southern latitudes this part of the plane is circumpolar, which makes it the southern counterpart to Cassiopeia as an always-available reference.
What the model says. l = 300° is in the fourth quadrant, inside the solar circle, so there is a tangent point, but its velocity is negative. The flat-curve model puts the terminal velocity near −30 km/s, at R = R₀ |sin 300°|, about 7.4 kpc from the centre. Beyond the solar circle on the far side the sightline picks up receding gas, and the model runs out to about +110 km/s at R = 20 kpc.
What you’ll see. A profile that leans the opposite way to the northern inner-Galaxy pointings. The tangent-point edge sits close to the local gas at this longitude, so for rotation-curve work l = 330° gives a cleaner edge. Use this pointing as a reliable reference whenever the plane closer to the centre is down.
Practicalities. At −63° declination it never sets south of about 27° S and culminates at 62° from 35° S. From 20° N it barely clears the horizon, and from 30° N it doesn’t rise at all. It transits at midnight in late March.
Radial velocity (LSR) of gas at each distance along l = 300°, 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 1419.887 to 1420.545 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 | never rises | 0 h | 0 h |
| 30° N | never rises | 0 h | 0 h |
| 20° N | 7° | 6.0 h | 0 h |
| 0° | 27° | 12.0 h | 5.5 h |
| 20° S | 47° | 17.9 h | 11.3 h |
| 35° S | 62° | always | 15.5 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.
- Jan05:43
- Feb03:41
- Mar01:51
- Apr23:45
- May21:47
- Jun19:46
- Jul17:48
- Aug15:46
- Sep13:44
- Oct11:46
- Nov09:44
- Dec07:46
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.