I had intended to write this post on Monday, but as often happens, time has ran from me and other things required my attention.
Monday, of course, was the official day of the Vernal (Fall) Equinox (in the Northern hemisphere). Technically this is considered to be the day when we have equal lengths of daylight and nighttime and marks the beginning of the period when the longer days of Summer give way to the longer nights of Winter. Or so many of us have been lead to believe.
Like an increasing number of things these days, we’ve accepted it as being true without really checking the facts behind the ‘theory’. Anyone who pays attention to a decent weather report given with the evening TV news hour (or half hour – depending) is easily able to prove for themselves that this ‘well known’ fact is in fact, false. (I hesitate to say ‘fake news’).
My news on the 23rd clearly showed that the sun ‘rose’ at 6:05 am and set at 6:13 pm that evening – making 12 hours and 8 minutes of ‘daylight’ where the sun would be visible and leaving only 11 hours 52 minutes when it is not in the 24 hour period from 12:00 pm on the 22nd to 12:00pm on the 23rd. Dawn and twilight, when technically the day has not yet started or finished respectively, yet we can still see refracted sunlight, are not included in this unbalanced calculation.
The reason for this ‘anomaly’ has to do with how astronomers and meteorologists define the start and ending point of any given ‘day’. They define it, not when the Sun first becomes visible, or when it finally sets below the horizon, as you might expect, but as when the centre of the Sun’s disk passes the horizon, rising or setting.
So with the day not officially starting until fully half of the sun is visible at the horizon, nor ending until half of it has set, both equinoxes do not actually have 12 hours of sunlight, but in fact have around 8 minutes more and must therefore have a similar shortage of ‘night-time’ in the 24 hour long day, as our clocks measure it!
At around the next equinox in March you may want to check sunrise and sunset times and see just when a day really is 12 hours of daylight and 12 of night – i bet it won’t be on the day people believe it to be. 😉
There is another ‘false’ fact regarding ‘equal’ amounts of sunshine and darkness that you have probably always got wrong before and it concerns not earth, but the Moon!
Whenever we see an exact ‘half moon’, the disk of the Moon having 50% in sunlight with 50% in darkness, and a straight line of shadow dividing it into two equal halves…
… it doesn’t!!
Only around 48% of the Moon’s surface is in shadow leaving about 52% still sunlit.
How is this possible? It’s because the Sun is many, many times the size of the Moon and so part of the Sun can cast sunlight further past a line circling the Moon that would normally be dividing it in two as seen from Earth as this diagram shows (not to scale!)
So, when we see a shadowline on the Moon that is perfectly straight we are seeing not half a moon but 52% of it lit up!
And when we’re supposed to have equal length daytime and nighttime there is really 16 minutes more sunshine than the 50/50 split you’d expect! (For the same reason).
‘This’ does not equal ‘that’!! 😉