Places · Dark sky
Big Bend National Park
The least light-polluted national park in the contiguous United States, on a stretch of the Rio Grande far from any city on either side of the border.
The sky from here
This is a zone 1 sky — pristine on the nine-grade scale, Bortle 1–2, reaching roughly magnitude 7.2–8.0 unaided. The Milky Way casts a shadow. Airglow is visible, the zodiacal light reaches across the sky, and the darkest places on Earth look like this.
That grade puts 107 of the catalogue within easy reach of 10×50 binoculars and 107 within a 130mm — 58 of them need no instrument at all. There is nowhere darker worth driving to; this is already among the better skies on the planet.
There is no season here in which the sky fails to get properly dark — the Sun clears eighteen degrees below the horizon on all 365 nights.
The centre of the galaxy reaches 32° 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, 3 never set from here and every one of them rises at some point in the year.
Of everything in the catalogue, Triangulum Galaxy, Ring Nebula and Pleiades are the best served here, climbing to within a few degrees of straight up — 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 29.25°N 103.25°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 Big Bend National Park?
Everything south of declination −61°. From 29.3°N that is a cap 29° in radius centred on the south celestial pole, which is 6.4% 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 6.4% around the north celestial pole is the same geometry inverted, and never sets.
How many hours of true darkness does Big Bend National Park get?
10 hours 54 minutes on the best night, around 21 December, falling to 6 hours 48 minutes 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. 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 Big Bend National Park?
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. 29.3°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 Big Bend National Park from the other sites on this map?
Launch Site One landing pad is the nearest thing to it here — 282 km to the north-west. The nearest other dark sky is Cherry Springs State Park, 2,670 km to the north-east. Measured as the great circle between coordinates, which is the shortest path across the surface and rarely the one you could travel.