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Paranal Observatory — the VLT
Four 8.2 m unit telescopes that can be combined into one interferometer, on a mountain flattened for the purpose. Among the driest and darkest skies used for astronomy.
The sky from here
Measured at these coordinates the sky is zone 2: rural, which is Bortle 2–3 and a naked-eye limit of about 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.
Of the 110 Messier objects, 44 are an unaided-eye target from a sky this dark, 107 are comfortable in 10×50s and 107 in a 130mm reflector. There is nowhere darker worth driving to; this is already among the better skies on the planet.
Every night of the year reaches full astronomical darkness at this latitude; the Sun always gets its eighteen degrees below the horizon, which places much further from the equator cannot say.
At its highest the galactic centre stands 86° 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.
Lagoon Nebula, Trifid Nebula and Southern Pinwheel pass close to the zenith from this latitude — 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 24.63°S 70.40°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 Paranal Observatory?
Everything north of declination +65°. From 24.6°S that is a cap 25° in radius centred on the north celestial pole, which is 4.5% of the whole celestial sphere. Nothing about the calendar or the aperture changes that — the cap is below the horizon at every hour of every night. Its mirror image, the same 4.5% around the south celestial pole, is circumpolar here and never goes down.
How many hours of true darkness does Paranal Observatory get?
Between 7 hours 22 minutes on the shortest night of the year, around 21 December, and 10 hours 41 minutes on the longest, around 21 June. Astronomical darkness is the Sun more than eighteen degrees below the horizon, which is a stricter thing than the Sun being down. The sky here reaches it on every night of the year, which is not true much further from the equator.
When is the Milky Way best from Paranal Observatory?
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 Paranal Observatory from the other sites on this map?
The nearest is Extremely Large Telescope, about 22 km to the east. 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.