Jeff Matthews Posted March 19, 2016 Share Posted March 19, 2016 ... tonight is Houston's Vernal Equinox? Quote Link to comment Share on other sites More sharing options...
mungkiman Posted March 19, 2016 Share Posted March 19, 2016 Late tonight, as in tomorrow? Quote Link to comment Share on other sites More sharing options...
Jeff Matthews Posted March 19, 2016 Author Share Posted March 19, 2016 At 11:30 p.m... ... on the day of an equinox, the Sun rises everywhere on Earth (except at the poles) at about 06:00 and sets at about 18:00 (local time). These times are not exact for several reasons: The Sun is much larger in diameter than the Earth, so that more than half of the Earth could be in sunlight at any one time (due to unparallel rays creating tangent points beyond an equal-day-night line). Most places on Earth use a time zone which differs from the local solar time by minutes or even hours. For example, if the Sun rises at 07:00 on the equinox, it will set 12 hours later at 19:00. Even people whose time zone is equal to local solar time will not see sunrise and sunset at 06:00 and 18:00. This is due to the variable orbital speed of the Earth and the inclination of its orbit, and is described as the equation of time. It has different values for the March and September equinoxes (+8 and −8 minutes respectively). Sunrise and sunset are commonly defined for the upper limb of the solar disk, rather than its center. The upper limb is already up for at least a minute before the center appears, and the upper limb likewise sets later than the center of the solar disk. Also, when the Sun is near the horizon, atmospheric refraction shifts its apparent position above its true position by a little more than its own diameter. This makes sunrise more than two minutes earlier and sunset an equal amount later. These two effects combine to make the equinox day 12 h 7 min long and the night only 11 h 53 min. Note, however, that these numbers are only true for the tropics. For moderate latitudes, the discrepancy increases (e.g., 12 minutes in London); and closer to the poles it becomes very much larger (in terms of time). Up to about 100 km from either pole, the Sun is up for a full 24 hours on an equinox day. Night includes twilight. If dawn and dusk are instead considered daytime, the day would be almost 13 hours near the equator, and longer at higher latitudes. Height of the horizon changes the day's length. For an observer atop a mountain the day is longer, while standing in a valley will shorten the day. Quote Link to comment Share on other sites More sharing options...
Jeff Matthews Posted March 19, 2016 Author Share Posted March 19, 2016 Quote Link to comment Share on other sites More sharing options...
mungkiman Posted March 19, 2016 Share Posted March 19, 2016 Learned something today... Quote Link to comment Share on other sites More sharing options...
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