Since the dawn of time, human beings have relied on the cycles of the sun, the moon, and the constellations through the zodiac in order to measure time. The most basic of these was the motion of the Sun as it traced an apparent path through the sky, beginning in the East and ending in the West. This procession, by definition, is what is known as a Solar Day. Originally, it was thought that this motion was the result of the Sun moving around the Earth, much like the Moon, celestial objects and stars seemed to do. However, beginning with Copernicus’ heliocentric model, it has since been known that this motion is due to the daily rotation of the earth around the Sun’s polar axis.
Up until the 1950’s, two types of Solar time were used by astronomers to measure the days of the year. The first, known as Apparent Solar Time, is measured in accordance with the observable motion of the Sun as it moves through the sky (hence the term apparent). The length of a solar day varies throughout the year, a result of the Earth’s elliptical orbit and axial tilt. In this model, the length of the day varies and the accumulated effect is a seasonal deviation of up to 16 minutes from the mean. The second type, Solar Mean Time, was devised as a way of resolving this conflict. Conceptually, Mean solar time is based on a fictional Sun that is considered to move at a constant rate of 360° in 24 hours along the celestial meridian. One mean day is 24 hours in length, each hour consisting of 60 minutes, and each minute consisting of 60 seconds. Though the amount of daylight varies significantly throughout the year, the length of a mean solar day is kept constant, unlike that of an apparent solar day.
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The measure of time in both of these models depends on the rotation of the Earth. In both models, the time of day is not plotted based on the position of the Sun in the sky, but on the hour angle that it produces – i.e. the angle through which the earth would have to turn to bring the meridian of the point directly under the sun. Nowadays both kinds of solar time stand in contrast to newer kinds of time measurement, introduced from the 1950s and onwards which were designed to be independent of earth rotation.
We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.