Equinox

Today is the Spring Equinox.

equinox /ˈɛkwɪˌnɒks/  (noun)

An equinox is when “the sun crosses the plane of the earth’s equator, making night and day of approximately equal length (12 hours) all over the earth”. Due to the angle of the Earth’s rotation this happens in Spring and again in Autumn.

This year (2018) this happened today (20th March) at 16:15 UTC/GMT.

The word equinox came to English through French and Medieval Latin, with its origin in Latin:

aequinoctium” – from aequus = “equal” + nox = “night“.

So it simply means “Equal night“.


A bit of “science”:

In reality the day and night on the day of the “equinox” are not the same length!
It is the centre of the sun that is above the horizon for 12 hours, but because:

  1. the sun appears as a “disc” (not a dot),
  2. is much bigger than the Earth, and
  3. the Earth’s atmosphere refracts light so that daylight might reach the ground before the sun has actually risen,

the days of the equinoxes are always slightly longer than the night. By how much longer depends on where you are on Earth.

In Guildford, for example the length of daylight today will actually be 12 hours, 9 minutes and 48 seconds (according to sunrise/sunset times.  I am not sure if this includes the lightness caused by refraction). Tomorrow, 21st March, will be 12 hours, 13 minutes and 44 seconds. The length of day is currently increasing by just under 4 minutes per day.

In fact a day having more nearly equal length of day and night will always be towards the winter side of each equinox, and can only happen at places far enough from the equator for the day length to vary throughout the year by at least seven minutes.

In Guildford, the day having closest length of day and night (according to sunrise/sunset times) is usually on the 18th March; this year the length of daylight was 12 hours and 1 minute and 55 seconds.


Back to some English:

Note how I have been using the word “day”.
I have used it in two senses:

One sense meaning a “calendar day” of 24 hours, which includes light and dark time;
and the second sense meaning “the light time” during that period, as opposed to night.

I can also say, enjoy your “day”


This post was modified from one that I wrote back in 2015 for the Guildford Quest Facebook Page.

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