NASA Science
Places · Observatory
Mount Wilson Observatory
Hubble measured the distance to Andromeda from the 100-inch here and ended the argument about whether the Milky Way was the whole universe.
- Where
- California, United States
- Coordinates
- 34.23°N 118.06°W
- Since
- 1917
- On the map
- Show me
- More
- mtwilson.edu
The sky from here
The sky over this spot is zone 6 — bright suburban, Bortle 6–7, with a naked-eye limit around magnitude 4.7–5.5. The sky within thirty degrees of the horizon glows greyish white. Only the brighter constellations survive.
Against that sky 37 of the 110 Messier objects are straightforward in 10×50 binoculars and 37 in a 130mm, with 9 reachable by eye alone. The nearest sky of Bortle 2 or better lies about 141 km north-east of here.
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.
The centre of the galaxy reaches 27° at its highest, which puts the brightest part of the Milky Way clear of the horizon murk without ever being overhead. Of the 110 Messier objects, 5 never set from here and every one of them rises at some point in the year.
The sky here is kindest to Ring Nebula, Hercules Cluster and Triangulum Galaxy, 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.
Bode's Galaxy and Cigar Galaxy never set from this latitude, so they are available on any clear night of the year rather than for one season of it.
Read at 34.23°N 118.06°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 Mount Wilson Observatory?
Everything south of declination −56°. From 34.2°N that is a cap 34° in radius centred on the south celestial pole, which is 8.7% 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 8.7% around the north celestial pole, is circumpolar here and never goes down.
How many hours of true darkness does Mount Wilson Observatory get?
11 hours 8 minutes on the best night, around 21 December, falling to 6 hours 3 minutes around 21 June. The measure is astronomical darkness — the Sun a full eighteen degrees under — not simply the hours between sunset and sunrise. Every night of the year gets there eventually, which is the quiet advantage of not being any further from the equator than this.
When is the Milky Way best from Mount Wilson Observatory?
Around 17 June, when the galactic centre transits at local midnight, and usefully from about May to September — earlier each evening as the season goes on. 34.2°N is close enough to the equator that the sky reaches full astronomical darkness on every night of the year, so unlike sites further north there is no part of the season where the core is high and the sky refuses to cooperate.
How far is Mount Wilson Observatory from the other sites on this map?
The nearest is Jet Propulsion Laboratory, about 11 km to the west. The nearest other observatory is Palomar Observatory, 147 km to the south-east. Measured as the great circle between coordinates, which is the shortest path across the surface and rarely the one you could travel.