Thursday, 19 June 2008
Equinox?????
Hey I got that bit wrong, didn't I? I should have said the summer solstice, which is when we have the most hours of daylight! Here in the south of Scotland that means it doesn't get black dark till after 11p.m. and is beginning to get light again around 3.15 in the morning. At the winter-solstice in December it's the opposite. It's mid-afternoon when it gets dark and after 8.30 a.m when it gets light again. Of course daylight savings affect those times too. In summer we are an hour ahead of Greenwich Mean Time!!!
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For your information so you don't make the same mistake again!!
In astronomy, equinox can have two meanings:
Either of the two events when the Sun is positioned directly over the Earth's equator and, by extension, the apparent position of the Sun at that moment, or the time that it happens.
The time at which the vernal point, celestial equator, and other such elements are taken to be used in the definition of a celestial coordinate system—see Equinox (celestial coordinates).
An equinox in astronomy is that moment in time (not a whole day) when the centre of the Sun can be observed to be directly above the Earth's equator, occurring around March 20 and September 23 each year.
More technically, at an equinox, the Sun is at one of two opposite points on the celestial sphere where the celestial equator (i.e. declination 0) and ecliptic intersect. These points of intersection are called equinoctial points—the vernal point and the autumnal point. By extension, the term equinox may be used to denote an equinoctial point.
There is either an equinox (autumn and spring) or a solstice (summer and winter) on approximately the 21st day of the last month of every quarter of the calendar year. On a day which has an equinox, the centre of the Sun will spend a nearly equal amount of time above and below the horizon at every location on Earth and night and day will be of nearly the same length. The word equinox derives from the Latin words aequus (equal) and nox (night). In reality, the day is longer than the night at an equinox. Commonly the day is defined as the period that sunlight reaches the ground in the absence of local obstacles. From Earth, the Sun appears as a disc and not a single point of light; so, when the centre of the Sun is below the horizon, the upper edge is visible. Furthermore, the atmosphere refracts light; so, even when the upper limb of the Sun is below the horizon, its rays reach over the horizon to the ground. In sunrise/sunset tables, the assumed semidiameter (apparent radius) of the sun is 16 minutes of arc and the atmospheric refraction is assumed to be 34 minutes of arc. Their combination means that when the upper limb of Sun is on the visible horizon its centre is 50 minutes of arc below the geometric horizon, which is the intersection with the celestial sphere of a horizontal plane through the eye of the observer. These effects together make the day about 14 minutes longer than the night at the equator, and longer still at sites toward the poles. The real equality of day and night only happens at places far enough from the equator to have at least a seasonal difference in daylength of 7 minutes, and occurs a few days towards the winter side of each equinox.
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