diamond geezer

 Sunday, September 23, 2007

Autumn Equinox (10:51 BST)

Well known autumnal fact 1: There are four seasons in a year
Officially, astronomically, the seasons all start and finish at either a solstice or an equinox. And today is the autumnal equinox, which makes today the start of autumn. It's all because the earth is tilted on its axis, which means that different parts of the planet get more direct sunlight at different times of the year. Yes, obviously. Now tell us something we don't know...

Little-known autumnal fact 1: Autumn is shorter than summer
It's true. There may be four seasons in a year, but they're not all of equal length. It's 94 days from the summer solstice to the autumn equinox, making summer the longest of all the seasons. But it's only 90 days from the autumn equinox to the winter solstice, making autumn four days shorter. Winter's even shorter still, just 89 days, the shortest season of all. We may be entering the cooler, darker half of the year (between now and next March), but it's also the shorter half of the year. Here are the precise figures.

21 Mar 07
21 Jun 07
23 Sep 07
22 Dec 07
21 Jun 07
23 Sep 07
22 Dec 07
20 Mar 08
Duration93 days94 days90 days89 days

So why aren't the four seasons equal? It's because the earth's orbit around the sun isn't circular, it's an ellipse, and so some bits of the earth's orbit are nearer to the sun than others. Kepler's Second Law says that planets travel a bit faster when they're closest to the sun, and a bit slower when they're further away. The earth happens to be closest to the sun in early January, which is when it travels fastest, so autumn and winter are relatively short. And the earth is furthest from the sun in early July, which is when it travels slowest, so spring and summer are relatively long. It's not by much, but it's enough to make a difference. Autumn isn't a quarter of a year long, it's really 1½ days shorter than that. Make the most of it.

Well known autumnal fact 2: Today the sun is directly overhead at the equator
Sorry to those of you with Seasonal Affective Disorder, but nine minutes to eleven this morning is the official moment when the overhead sun switches from the northern to the southern hemisphere. It's about to be spring in the latter, but it's about to be autumn in the former. Damn. Or, if you're Antipodean, hurrah.

Little-known autumnal fact 2: Today is not the day that day and night are of equal length
It's true. Sunrise in London this morning was at 6:47am, while sunset tonight is at 6:58pm. So that makes the day 22 minutes longer than the night. Equality comes in three days time, around September 26th. Just look how rapidly the gap between daylight and darkness is changing this week. It's always fast-changing like this around the equinoxes.

DateSep 21Sep 22Sep 23Sep 24Sep 25Sep 26Sep 27
by 36min
by 28min
by 22min
by 12min
by 2min
by 12min

So why aren't day and night equal today? It's because the sun isn't a point of light as seen from the earth, it's a disc. At sunrise the top of the sun peeps above the horizon about a minute before the centre. And at sunset the top of the sun dips below the horizon about a minute after the centre. Atmospheric refraction means that we can still see the sun even when it's really below the horizon, adding a few more minutes to both of those times. And it's that pair of extra minutes that make all the difference. Daylight today is therefore still 22 minutes longer than darkness. But the illusion won't last long. In just ten days time darkness will already be as much as a whole hour longer than daylight, and then it's downhill all the way to Christmas. Let joy be unconfined.

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