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ALMA
Sixty-six millimetre-wave dishes at 5,000 m, moved around the plateau on custom transporters to change the array's resolution. It resolves the dust gaps in planet-forming discs.
- Where
- Chajnantor Plateau, Chile
- Coordinates
- 23.03°S 67.75°W
- Since
- 2013
- On the map
- Show me
The sky from here
The sky over this spot is zone 2 — rural, Bortle 2–3, with a naked-eye limit around magnitude 6.7–7.5. The Milky Way is structured and obvious. Light domes sit low on the horizon over distant towns, and the rest of the sky is genuinely dark.
Against that sky 107 of the 110 Messier objects are straightforward in 10×50 binoculars and 107 in a 130mm, with 44 reachable by eye alone. There is nowhere darker worth driving to; this is already among the better skies on the planet.
The Sun drops far enough below the horizon here for full astronomical darkness on every night of the year — which is not true of anywhere much further from the equator.
At its highest the galactic centre stands 84° up, which is the difference between a Milky Way with depth in it and a smear along the horizon. Of the 110 Messier objects, none are circumpolar and 2 never rise.
The sky here is kindest to Trifid Nebula, Lagoon Nebula and Southern Pinwheel, each of which passes within a few degrees of overhead — an object at the zenith is being seen through the least air it will ever be seen through, and the difference against the same object low in the south is not subtle.
Against that, Bode's Galaxy and Cigar Galaxy never clear the horizon here at all. No amount of aperture or patience changes that; the only remedy is to travel.
Read at 23.03°S 67.75°W. Darkness is estimated rather than measured; everything else follows from the latitude and holds along the whole parallel. How accurate this is.
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Questions
What part of the sky never rises at ALMA?
Everything north of declination +67°. From 23.0°S that is a cap 23° in radius centred on the north celestial pole, which is 4.0% of the whole celestial sphere. It is out of reach permanently rather than seasonally: no time of year and no instrument brings it above the horizon. The matching 4.0% around the south celestial pole is the same geometry inverted, and never sets.
How many hours of true darkness does ALMA get?
From about 7 hours 33 minutes at the thin end of the year, near 21 December, up to 10 hours 36 minutes around 21 June. The measure is astronomical darkness — the Sun a full eighteen degrees under — not simply the hours between sunset and sunrise. Unlike sites further from the equator, no part of the year here is written off entirely.
When is the Milky Way best from ALMA?
Midwinter — around 17 June, when the centre of the galaxy is on the meridian at local midnight. South of the equator that date brings the longest nights of the year rather than the shortest, so the best part of the sky is highest exactly when there is most darkness to see it in. Nothing about instrumentation compensates for that; it is why the serious optical telescopes went south.
How far is ALMA from the other sites on this map?
Its closest neighbour on this map is Extremely Large Telescope, 303 km away to the south-west. That is also the nearest other observatory, with the next one 323 km beyond it. These are straight-line distances over the surface, not driving distances, and in this part of the world the two can differ considerably.