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Years | Months | Days | Solstice | Equinox | GMT & Zulu Scientists often used complicated terms when discussing time. Here are a few definitions you might find helpful: YearsA normal Earth year on a calendar (called a tropical year) is the time it takes the planet to make one orbit around the Sun in relation to the vernal equinox, and it is equal to 365 days, 5 hours, 48 minutes and 46 seconds. This is also called a mean solar year. A sidereal year is measured by the Sun's movement relative to a fixed star, and is 365 days, 6 hours, 9 minutes and 11 seconds. Most of us don't need to know this. MonthsThe Moon takes 29 days, 12 hours and 44 minutes to orbit the Earth. Sort of. This is a synodic month, and it's measured between in relation to the Sun. Each time the Sun, Earth and Moon are lined up, there is a full moon and one synodic month has passed. This is the only month most of us care about. A sidereal month, if you must know, is how long it takes the Moon to orbit the Earth in relation to a fixed star, and it equals 27 days, 7 hours and 43 minutes. DaysBecause Earth does not rotate uniformly, and because the planet's path around the Sun is not in line with the plane of the equator, the time from sunrise to sunset varies. A mean solar day is, therefore, an average, and it equals 24 hours, 3 minutes and 57 seconds. A sidereal day is measured in relation to another star, and it equals 23 hours, 56 minutes and 4 seconds. SolsticeIn the Northern Hemisphere, the summer solstice (the first day of summer) is the longest day of the year, because the Sun is at its greatest angular distance north (it's really high in the sky). It occurs on or about June 21 (In 1999: 3:49 p.m. EDT on June 21). Winter solstice (the first day of winter) is the opposite, and occurs on or about December 21 (In 1999: 2:44 a.m EST on December 22). Technically, the solstice is not a day, but rather an exact instant in time. EquinoxThe equinox is the exact instant when neither the North Pole nor the South Pole is inclined toward the Sun. Day and night are of equal length. In the Northern Hemisphere, the vernal equinox is on or about March 21 (In 1999: 8:46 p.m. EST on March 20), and the autumnal equinox is on or about September 23 (In 1999: 7:31 a.m. EDT on September 23). GMT and ZuluGreenwich Mean Time (GMT) is the standard used throughout the scientific community, and is measured at the Greenwich meridian, which runs through Greenwich, England. It is often called Zulu time, or "Z." GMT is five hours ahead of Eastern Standard Time, and 4 hours ahead of the East Coast during Daylight Savings Time in the summer. ez Explore
more Other science links from explorezone.com Text by Robert Roy Britt
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