Why is it that astronomers and their telescopes are commonly perched on the highest mountaintops? Atmospheric observations are often easier to make higher up, when there isn’t so much distortion or pollution in the way of an accurate look at the night sky. Closer to sea level atmospheric distortion can include heat, dust, moisture, air pollution, and other materials that cloud the air. Up high these distortions are minimized.
One way to see this kind of distortion is to stand near sea level and hold your thumb up to the sky, in front of the sun. The sun’s rays, or aureole, will expand far beyond what your finger can block. This blur of the sun’s boundaries is caused by atmospheric distortions like dust, heat, air pollution, moisture, haze, and other materials in the air. A fingertip held out at arm’s length is about one-half a degree of sky, which is about the same amount of space that the sun and moon take up from the same perspective on Earth.
Up high, these atmospheric distortions are much less. The atmosphere is much thinner higher up, making it difficult for distortions to appear and stay for very long. From their positions on high mountaintops, astronomers can view the sky both at night and during the day without things getting in the way. Mountaintop viewpoints like Mona Lea in Hawaii give scientists the ability to look deep into space to the extent of their technological abilities.
At an elevation of 10,000 feet above sea level (or 3,050 meters), the sun looks very different. The sun’s disk can be easily placed behind your thumb if you hold it out at arm’s length. The sun’s aureole is a lot smaller because there is less atmospheric distortion from dust, pollution, or moisture spreading out the sun’s visible rays.
Sun Size and Altitude, Universities Space Research Association.