Catherine Pelton Durrell '25: Archives & Special Collections Library

The surface of the Sun (called the photosphere) has a temperature of 6000K (about 10,000 degrees F). The outer layers are convective, with rising and falling gas just as in the convective motions of the Earth’s atmosphere. As the gas from lower levels rises, it expands and cools. But the Sun has a magnetic field, which at the start of a solar cycle is a dipole (like a bar magnet). Since the Sun is a fluid, gas at the equator rotates faster than gas at the poles, which tangles the magnetic fields. A sunspot occurs at a place where the field has buckled, so convection is impeded. The cool gas trapped at the surface is about 2000K cooler than the rest of the photosphere, so appears black by comparison.

When the Moon passes in front of the Sun, it temporarily blocks the Sun because by chance the Moon’s apparent size is about the same as the Sun’s apparent size (the Moon is of course much smaller than the Sun but proportionately closer to the Earth). The solar corona, the 1,000,000K gas that extends above the photosphere, is so low density that it is normally not visible. During an eclipse, its intricate structure can be studied because the Moon blocks the bright light from the photosphere.

Venus is closer to the Sun than the Earth. A transit is when Venus crosses it front of the Sun. Note that Venus is about 1/100th the size of the Sun, so appears as a small black dot. Venus transits were historically interesting because they were used to calculate the Earth’s distance from the Sun: if the transit of Venus were viewed from two different places on Earth, then timing of exactly when the transit occurred would allow the orbit to be calculated. The transits of 1874 and 1882 that Maria Mitchell and her students observed were the first ones to be recorded on photographs.

>Mercury transits can also be used to measure the distance between the Earth and the Sun. Another use during Maria Mitchell’s time was to determine the longitude of the observer through timing of the transit. Latitudes were easy to determine from measurements with sextants of star elevations as a function of time, but longitudes were more difficult to determine. Maria Mitchell, having grown up in the seafaring Nantucket community, was well versed in determining latitudes for navigational purposes. Even as a young girl, she helped her father by timing solar eclipses to determine their longitude. Timing the transits of Venus and Mercury serve the same purpose. Maria’s measurements were used as input for the American Ephemeris and Nautical Almanac.