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Autumn Equinox 2013

Today, 22 September 2012, marks the moment of the Autumn Equinox. At 2044 UT (2144 BST) the Sun will cross from the northern hemisphere sky to the southern, and we’ll begin the slow approach to the Winter Solstice on 21 December.

The equinoxes (one in spring and one in autumn) are the two instances every year when the Sun makes that crossing from north to south and vice versa, and they’re commonly thought to be the days when day and night are equal length, but they’re really not, for reasons I’ve outline before:

  1. astronomers measure the timings of equinoxes, sunrises and sunsets based on the middle point of the Sun’s disk in the sky, so when you read a sunrise time it means the time that the centre of the Sun’s disk rises above the horizon. For a few minutes before that time the top of the Sun’s disk will already have risen, giving “daylight”.
  2. Even before this happens the sky is lit up by the Sun below the horizon, and we experience twilight. Most people would think that the sky is bright enough to call it “daytime” long before the Sun pops above the horizon, during the phase of civil twilight.

So today, even though day and night are said to be equal on the equinox, the “daytime” (i.e the start of civil twilight) started about 0630BST in Glasgow (where I am) and will end this evening around 2000BST, giving me 13.5 hours of “daylight”. (Londoners will have from about  0615 until 1930BST, or approx. 13.25 hours of “daylight”).

The day this year where I have exactly 12 hours of “daylight” (i.e. between the morning start and the evening end of civil twilight) is 11 October and this day is called the equilux. (In London the equilux falls on 12 October).

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