Places · Observatory
Gemini South
The southern half of a matched pair — its twin is on Maunakea — giving one programme coverage of both hemispheres with identical 8.1 m optics.
- Where
- Cerro Pachón, Chile
- Coordinates
- 30.24°S 70.74°W
- Since
- 2000
- On the map
- Show me
- More
- noirlab.edu
The sky from here
Measured at these coordinates the sky is zone 3: rural transition, which is Bortle 3–4 and a naked-eye limit of about magnitude 6.2–7.0. The Milky Way is clearly there but loses its detail near the horizon. Light domes are obvious in several directions.
Of the 110 Messier objects, 38 are an unaided-eye target from a sky this dark, 106 are comfortable in 10×50s and 106 in a 130mm reflector. You would have to get roughly 7 km west before the sky reaches Bortle 2.
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 89° 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 4 never rise.
Southern Pinwheel, Butterfly Cluster and Ptolemy's Cluster 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 30.24°S 70.74°W. Darkness is estimated rather than measured; everything else follows from the latitude and holds along the whole parallel. How accurate this is.
Nearby
Questions
What part of the sky never rises at Gemini South?
Everything north of declination +60°. From 30.2°S that is a cap 30° in radius centred on the north celestial pole, which is 6.8% of the whole celestial sphere. That part of the sky is not merely a winter object or a difficult one; it is geometrically unavailable from this latitude. The equivalent cap around the south celestial pole is the compensation, and it never sets.
How many hours of true darkness does Gemini South get?
Between 6 hours 40 minutes on the shortest night of the year, around 21 December, and 10 hours 57 minutes on the longest, around 21 June. That counts only proper astronomical night, with the Sun eighteen degrees down; the merely-dark hours either side of it are longer. 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 Gemini South?
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 Gemini South from the other sites on this map?
Its closest neighbour on this map is Vera C. Rubin Observatory, 1.2 km away to the west. That is also the nearest other observatory, with the next one 10 km beyond it. Distances are great-circle, so they are what a signal covers rather than what a road does.