Sun Size and Altitude
April 21, 2010
Astronomers are frequently asked why we have our observatories on high mountain tops. A big part of looking into deep space has to do with atmospheric transparency and freedom of particulates along with heat that causes blurring. A simple but effective understanding of this seeing is to put a finger at arm’s length in front of the Sun and observe the aureole that’s produced. Held at arm’s length, a finger tip subtends about one half of a degree of sky – nearly the same amount of space that both the Sun and Moon take up. At sea level, observing is often compromised by the build up of heat, dust, moisture, haze, pollution and aerosols that include ash and even salt. Higher up, there’s less of this to deal with since there’s less atmosphere to peer through. These three photos were taken on the Hawaiian island of Maui at (left to right) Baldwin Beach, Kula and Haleakala Observatory, respectively. The disk of the Sun is completely hidden by my index fingertip at 10,000 ft (about 3,050 m). Note, I can positively verify that my finger didn’t increase in size as a result of the thinner air.