The 3 best locations for astronomy on planet Earth

Astronomers observe the Universe by looking up.

This photo shows Gemini South’s laser guide star system in action. The telescope is located atop Cerro Pachón in the Chilean Andes. The laser interacts with particles in the Earth’s upper atmosphere and allows for real-time corrections of the telescope’s optics to compensate for atmospheric turbulence. The laser light from the sodium laser actually goes up ‘only’ about 60 miles before colliding with a thin layer of sodium in our own atmosphere, which absorbs and re-radiates that light, creating an artificial guide star.

The best observations demand that many factors be overcome.

As light strikes the Earth from space, it must pass through the various layers of the atmosphere, where it’s distorted by turbulence, clouds, and molecules, and can be absorbed and/or reflected as well. The atmosphere distorts incoming light, which is a huge obstacle for ground-based astronomers.

For astronomy, gathering light undisturbed by the atmosphere is key.

Under a pristine night sky, the Milky Way’s center casts shadows. As light pollution worsens, nebulosity and stars disappear, until maybe a few dozen stars remain. The numbers 1 through 9 are the Bortle scale, which provides observers with a metric to measure the darkness and clarity of the sky overhead at their location. Only a few locations remain on Earth with pristine, dark skies.

Dark skies, free from light pollution, are mandatory.

Although the Keck Observatories on the summit of Mauna Kea offer some of the best views of the universe from Earth as far as dark skies are concerned, like all sites near the equator, they must now reckon with satellite pollution due to megaconstellations of communications satellites: largely from one company, Starlink. The effects of satellites on astronomy are severe, and are only worsening with more and more unregulated launches.

High altitudes are also ideal, located “above” the thickest part of the atmosphere.

From high enough altitudes, particularly atop mountains or mountain ranges, one can often find themselves above the cloud line, where they’re looking “down” on the clouds rather than up at them. From locations above the cloud line, astronomical observations can occur without the hindrance of these light-blocking nuisances.

You’ll want dry conditions, eliminating clouds and water vapor effects.

Features such as clouds, water vapor, and turbulence in the overhead atmosphere are all problems for astronomers, as they reduce the quality and clarity of seeing. Being above the cloud-line, having cirrus-free skies, and having laminar, not turbulent, airflow in the atmospheric layers above you are all necessary for good ground-based “seeing” conditions in astronomy.

And smooth, slowly-varying airflow overhead, without turbulence, is necessary.

In November of 2022, the largest active volcano in the world, Mauna Loa, erupted. Despite its high altitude, clear, still air, low light pollution, and other astronomically favorable conditions, the unstable ground located atop an active volcano rules this summit out as an astronomical site. The neighboring inactive peak, Mauna Kea, is far superior.

The location must also be accessible and stable enough to support an observatory.

This photograph of the under-construction ELT (ESO’s Extremely Large Telescope) atop Cerro Armazones, taken in 2023, shows the all-important roads that connect to the observatory and the summit. Such stable infrastructure is necessary to allow the construction and support of a world-class astronomical observatory.

Three locations on Earth meet these criteria better than all others.

A series of observatories line the summit of the highest Canary Island: La Palma, Spain. The peak, Roque de los Muchachos, is located an impressive 2400 meters above sea level.

3.) La Palma summit.

Since scientific observations began at Gran Telescopio Canarias in 2009, it has been the world’s largest single-aperture optical telescope, surpassing even the Keck telescopes atop Mauna Kea. Atop La Palma at the Roque de los Muchachos Observatory, it will someday be surpassed by 30-meter class telescopes, two (GMT and ELT) of which are currently under construction.

At 2400 meters (7874 feet) above sea level, it’s well above the typical cloud line.

This photograph, outside the Gran Telescopio Canarias, showcases an outside view of the world’s largest optical telescope as of 2024. Atop the summit of La Palma, it’s one of Earth’s three great astronomical sites, although either the ELT or the GMT, both in the Chilean Andes, will someday surpass it as the world’s largest optical telescope.

Over a dozen observatories populate the summit, including the world’s largest optical telescope.

This expansive photograph showcases the Milky Way, the two Magellanic Clouds, and several other impressive celestial features at the Cerro Tololo Inter-American Observatory. Although it’s located at an impressive 2200 meters (7200 feet) above sea level, many other mountains within the Andes mountain range have even higher altitude summits.

2.) Atop the Andes mountains.

The University of Tokyo Atacama Observatory (TAO) Project has completed construction atop Cerro Chajnantor and, as of 2024, has set a new record for highest-altitude observatory ever: at 5640 meters (18,500 feet) in the Atacama Desert of northern Chile.

Separating Chile and Argentina, this high-altitude range boasts clear skies and dry air.

This image shows the Vera C. Rubin Observatory atop Cerro Pachón in Chile, with fairly typical cloud-free conditions accompanying it. When complete, Rubin Observatory will use its 8.4-meter telescope and 3200-megapixel camera to conduct an unprecedented, multi-color, decade-long survey of the optical sky called the Legacy Survey of Space and Time (LSST). The camera will be the largest ever constructed for astronomy, weighing an incredible 2.8 tonnes.

The Vera Rubin observatory, atop Cerro Pachón, and the ELT, atop Cerro Armazones, will soon revolutionize astronomy.

The summit of Mauna Kea is home to many of the greatest astronomical observatories ever to exist on Earth. The summit is also an inextricable part of the native Hawaiian culture, history, and heritage that has historically been erased by US and European interests. Here, the domes of the Subaru Telescope (left), the twin W. M. Keck Telescopes (center), and the NASA Infrared Telescope Facility (right) all stand out.

1.) Mauna Kea summit.

This aerial view shows the summit of Mauna Kea with a series of astronomical observatories and roads connecting them to one another and to the Mauna Kea access road. These observatories, on one of the most sacred locations in Hawaiian society and culture, should not continue to exist as a legacy of an unethical past, but can only continue to exist with the consent of the native population.

It’s the highest, driest, clearest, and most equatorial mountaintop on planet Earth.

The summit of Mauna Kea holds many of the world’s most powerful optical telescopes, including Gemini North, shown here. A sodium laser can be seen emerging from another observatory (not pictured), while the Milky Way and the streak of a Perseid meteor also appear in this 2021 image.

At 4205 meters (13,796 feet) above sea level, the Mauna Kea Observatories offer unparalleled conditions.

This facility photo of the Subaru Telescope and its nearby vicinity at the summit of Mauna Kea shows a common occurrence during the winter months: snow at the summit. The Subaru Telescope at right, is next to the twin Keck Telescopes, with other observatories visible farther to the left in this image. The summit of Mauna Kea is the greatest location in the world for optical ground-based astronomy due to a combination of Mauna Kea’s equatorial location, high altitude, quality seeing, and the fact that it’s generally, but not always, above the cloud line.

Even with the largest-aperture telescopes, adaptive optics, and modern instruments, astronomy still requires pristine locations.

First light, on April 26, 2016, of the 4LGST. This is arguably the most advanced adaptive optics system in use aboard a modern observatory, and helps astronomers produce, in many ways, superior-quality images to what even a space-based observatory like Hubble can obtain.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.