Autumnal equinox.
21 September 2010, 09:03 AM
#1
Autumnal equinox.
Is today, apparently. I was under the impression then, that since "equinox" means equal length of daylight, and night, the sun would rise at 0700 and set at 1900, ging 12 hours of each? (It would be 0600 and 1800 if we weren't still on BST).
Not so: where I am, it rises at 0645 and sets at 1905, so 20 minutes more light than dark.
And NOWHERE in the UK does it do the 12/12 hour thing. Even in Greenwich, it rises at 0644 and sets at 1900.
Not so: where I am, it rises at 0645 and sets at 1905, so 20 minutes more light than dark.
And NOWHERE in the UK does it do the 12/12 hour thing. Even in Greenwich, it rises at 0644 and sets at 1900.
21 September 2010, 10:49 AM
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I dont usually cut and paste but it is convenient and quicker to do it for this subject.
On a day of the equinox, the center of the Sun spends a roughly equal amount of time above and below the horizon at every location on the Earth, night and day being of roughly 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 when sunlight reaches the ground in the absence of local obstacles. From the Earth, the Sun appears as a disc rather than a single point of light, so when the center of the Sun is below the horizon, its 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 center 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 cumulative effects make the day about 14 minutes longer than the night at the Equator and longer still towards the Poles. The real equality of day and night only happens in places far enough from the Equator to have a seasonal difference in day length of at least 7 minutes, actually occurring a few days towards the winter side of each equinox.
The date at which sunset and sunrise becomes exactly 12 hours apart is known as the equilux. Because sunset and sunrise times vary with an observer's geographic location (longitude and latitude), the equilux likewise depends on location and does not exist for locations sufficiently close to the Equator. The equinox, however, is a precise moment in time which is common to all observers on Earth.
That should explain it anyway.
Les
On a day of the equinox, the center of the Sun spends a roughly equal amount of time above and below the horizon at every location on the Earth, night and day being of roughly 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 when sunlight reaches the ground in the absence of local obstacles. From the Earth, the Sun appears as a disc rather than a single point of light, so when the center of the Sun is below the horizon, its 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 center 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 cumulative effects make the day about 14 minutes longer than the night at the Equator and longer still towards the Poles. The real equality of day and night only happens in places far enough from the Equator to have a seasonal difference in day length of at least 7 minutes, actually occurring a few days towards the winter side of each equinox.
The date at which sunset and sunrise becomes exactly 12 hours apart is known as the equilux. Because sunset and sunrise times vary with an observer's geographic location (longitude and latitude), the equilux likewise depends on location and does not exist for locations sufficiently close to the Equator. The equinox, however, is a precise moment in time which is common to all observers on Earth.
That should explain it anyway.
Les
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