We have entered the period of the year when, as Sir Thomas Browne put it 300 years ago, "the night of time far surpasseth the day, and who knows when was the equinox?" And that last is a good question. Who indeed?
The equinox is defined as time when the Sun apparently crosses the Equator on its seasonal journey north or south. The word means equal night and implies that at this time of year day and night are the same length, 12 hours each.
It happens twice a year, in autumn and in spring, and few readers of Weather Eye will have been unaware that, officially, last Monday, September 22nd, was the autumnal equinox. However, if you consulted your Irish Times on that day it would have told you that sunrise was at 07.11 a.m. and sunset as 7.24 p.m.; the day was 12 hours and 13 minutes long, and the night, by default, a mere 11 hours and 47 minutes. Some equinox!
It all depends on what you mean by day and night. The concept of the equinox works well when viewed from outer space, but here on the surface of our planet, we have two complications to contend with, both concerning the definition of sunrise and sunset.
Firstly, by convention the term sunrise means the time at which the upper limb - or the upper edge - of the solar disc peeps above the horizon. This means that sunrise takes place when the centre of the Sun is still about a quarter of a degree below.
The Earth's atmosphere adds a second complication. When sunlight passes obliquely through the atmosphere, from the rarefied air near the top to the much denser air below, the rays curve downwards towards the surface of the Earth.
As a result, when the Sun is very low in the sky, a ray of light which seems to be approaching the eye almost parallel to the surface of the Earth has in fact begun its journey well below the horizon, and allows us to see the rising Sun some minutes before its upper limb is geometrically level with the horizon; we see a Sun which ought to be invisible.
This combination of convention and illusion gives us a spuriously early sunrise and lengthens the day by some five or six minutes. Sunset is similarly delayed by a corresponding amount. As a result, that which we call our day is artificially extended by about 12 minutes, time which, of course, must be borrowed from the night.