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Ciencias y Artes Patagonia www.sun45.org International Year of Astronomy 2009 |
International Sunbeam and Shadow Project “Sun 45 Equi-umbra” What / When is Equi-umbra or Equiumbra ? |
"Equi-umbra" (or "Equiumbra" pronounced "equi-umbra") means equal shadow (equi = equal: umbra=shadow) in the same way that "Equinox" means equal night. Equinox refers to a time when the length of the night is approximately equal to the length of the day as measured from sunrise to sunset. Equi-umbra refers to a time when the length of the horizontal shadow of a vertical object is equal to its height.
Equi-umbra Each day the sun rises to its highest altitude at Midday.
Midday (Solar Noon or Transit) in the Northern hemisphere, North of the Tropic of Cancer, is when the sun is exactly due south .In the Southern hemisphere below the Tropic of Capricorn it is when the sun is due North. The altitude of the sun at Midday depends on the time of year and the latitude of the location. At Midsummer (the summer solstice) the sun at Midday is at its highest altitude of the year. At Midwinter (the winter solstice) the sun at Midday is at its lowest altitude of the year. In some parts of the world (within most of the Arctic and Antarctic circles) the sun never reaches 45 degrees at Midday at any time of the year, even Midsummer. In contrast, even in Midwinter, in most of the area between the Tropic of Cancer and the Tropic of Capricorn the sun is never less than 45 degrees at Midday.
In the rest of the world the altitude of the sun is higher than 45 degrees for a certain length of time during the summer. Unlike for Eqinoxes, the dates of Equiumbra are strongly dependent on the latitude of the particular location. The exact dates of Equiumbra are specific for a particular local location. Spring Equi-umbra in a particular localation is the first day after Midwinter when the length at Midday of the horizontal shadow of a vertical object is equal, or very slighly less, than its vertical height. On this day, the VERTICAL ANGLE (ALTITUDE) of the sun at Midday when it is exactly South, is equal, or more, than 45 degrees, whereas on the day before it was less than 45 degrees. Autumn or Autumnal Equi-umbra is the last day of the year after Midsummer when the altitude of the sun at Midday is equal or more than 45 degrees. The following day it is less than 45 degrees and the shadows of vertical objects at Midday again begin to become longer than their vertical heights In the Northern Hemisphere, Midsummer falls around June 20 and Midwinter around 20 December. In the Southern hemisphere it is the reverse. Thus in the Northern Hemisphere the date of Spring Equiumbra in most cases occurs in the months January to June and the Autumn Equiumbra in the months July to December. In the Southern Hemisphere it is again the reverse. For example:
What / When are Morning Equi-umbra and Afternoon Equi-umbra ?
On each day between the Spring and Autumn Equi-umbra
there are two moments,
one before Midday (Morning Equi-umbra) and
one after Midday (Afternoon Equi-umbra) when the Altitude of the Sun is 45 degrees and the length of the horizontal shadow of an object is equal to its vertical height.
The times of Morning and Afternoon Equi-umbra vary strongly with the date and the latitude and longitude of the place.
The intervals of time betwen the Equiumbra and Midday (Solar Noon or Transit) are equal.
For example:
From: United States Naval Observatory (USNO)
N 38-53 W 77-02 Washington DC 2009 July 4
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