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Land with a View:
Finding the Best Places on Earth to Study the Heavens
by Jake Widman

Chichen Itza, Mexico
Chichen Itza, Mexico

The atmosphere that surrounds our planet is generally a good thing, everyone agrees. Besides providing air for us to breathe, is also protects us from potentially damaging radiation from space. The ozone layer, for instance, protects us from the ultraviolet rays of the sun.

When it comes to observing space, though, the atmosphere becomes a problem. Stars, galaxies, and other heavenly bodies broadcast information all along the electromagnetic spectrum, but much of that information--like some of the sun's UV rays--never gets to the planet's surface. And the part that does (visible light, for instance) can be distorted as it passes through the atmosphere. The history of astronomical observatories is partly the history of trying to eliminate the difficulties of seeing through the atmosphere.

Eyes and Ears on the Heavens

When we hear the word "observatory," most of us think of a site dedicated to visual observation of the sky through what's called an "optical" telescope.

Scientists using optical telescopes have two main obstacles to overcome. One is the effect of light pollution--nearby light that makes it hard to see the sky. This effect is familiar to anyone who's ever compared the number of stars visible in the country to the number of stars visible in the city. The lack of environmental light in a less-inhabited area makes it possible to see vastly more heavenly objects in the night sky.

This is partly why most observatories with optical telescopes are located out in the country, usually on top of a mountain. That way, even the lights that might happen to be around will be below the observatory, where they won't interfere. Kitt Peak National Observatory, for instance, is situated more than 6,000 feet (1830 meters) above the Arizona desert, a 90-minute drive from Tucson. In Chile, the Cerro Tololo Inter-American Observatory, 50 miles (80 kilometers) east of La Serena, is located at an altitude of over 7200 feet (2200 meters). Observatories can be found in remote areas all over the world, from the outback of New Zealand to the desert peaks of Uzbekistan.

Kitt Peak
Kitt Peak Observatory, Arizona

The other main obstacle to visual observation of the sky is light refraction caused by the atmosphere. The air around our planet is constantly heating up and cooling down, rising and falling and swirling around, forming layers of different densities. Each layer acts like a lens, and as light passes from one of these layers to the next, the light gets bent, or "refracted," even if only a little. The effect is similar to the way a straw in a glass of water will appear to be shifted sideways where it breaks the surface.

In the atmosphere, the shift from one layer of air to the other isn't nearly that great, but the accumulation of many tiny shifts is enough to scatter the light and affect the sharpness of an observed image. Because of this effect, the way we see objects outside the atmosphere will always be slightly blurred. Astronomers refer to the quality of their view as their "seeing": a place that has "good seeing" offers a less-distorted view than a place that has "bad seeing." How severe the problem is depends on the atmospheric conditions in a given location.

The result is that there's a practical upper limit on how sharp any observation from earth can be. As a rule of thumb, astronomers estimate than no earth-bound telescope, no matter how big it is, can create an image sharper than one made by a six-inch-diameter telescope (that’s about a hand-span across). Increasing the size of a telescope's mirror can gather more light to make faint objects brighter, but the image won’t get any sharper.

Most important, putting an observatory on a mountaintop means there’s less air to see through, so the "seeing" is better. On a mountaintop, there’s also less air above you to absorb light from the stars.

The best earthbound locations for optical telescopes, in fact, are on mountaintops in the middle of the ocean near the tropics, where temperature variations are small. That's why, for instance, the Mauna Kea Observatory is situated on the summit of Mauna Kea, the "white mountain" on the island of Hawaii, and the Teide Observatory was built almost 7900 feet (2400 meters) above Tenerife, in the Canary Islands.

The same kinds of locations can also benefit other kinds of observations. In the 1930s, astronomers began to aim antennas at the sky to pick up radio waves from outer space. Unfortunately, though, nearby human activity can interfere with radio reception the same way light interferes with visual observation. Television and radio broadcasts, electric motors, even the signals from car ignitions can be picked up by radio telescopes. For that reason, this kind of observatory also functions best in isolated areas. The Very Large Array, for example, is located 7000 feet high in a remote area west of Socorro, New Mexico. In Puerto Rico, the Arecibo Observatory is nestled in a natural valley surrounded by mountains that shield it from human activity.

Eyes and Ears in the Sky

Hubble Telescope
Hubble Telescope, NASA image
Some electromagnetic waves--including gamma rays, X-rays, and some kinds of ultraviolet light--don't get through our atmosphere at all, or are so scattered by it as to be unreadable. Only from mountaintops (or higher) do astronomers stand any chance of observing these wavelengths of light.

To solve these problems, astronomers have long tried to make observations from locations off the planet's surface. In 1911-12, the Austrian researcher Viktor Hess sent instruments up in balloons. He detected radiation that led him to hypothesize (correctly) the existence of cosmic rays. After World War II, American scientists used captured German V-2 rockets to send instruments up above the ozone layer, enabling them make ultraviolet and X-ray observations for the first time. Serious work with aircraft-based observatories began in 1966 and led to the discovery that Uranus had rings.

In 2009, the Next Generation Space Telescope will take us to a new level of cosmological sophistication. But today, the Hubble Space Telescope represents the current culmination of these efforts. For the first time, scientists have a permanent astronomical observatory that's not affected by the problems of light, atmosphere, or extraneous signals. Of course, operating a telescope in space creates its own problems, such as the need for regular servicing missions. But most astronomers would agree that the results are worth it.




© 2001 The Exploratorium