I have always supposed that on the day of the spring equinox (i.e. yesterday) the name and night both became 12 hours long - or, at least, it was the day in which they were most-equal; so that the next day was the first on which the day was longer than the night.
But this is not true; as I realized when I was checking the sunrise and sunset times.
It turns-out that on 20 March, the day of the vernal equinox, there were 12 hours and 10 minutes of daylight; and the most equal date wrt. day-night was either the 17th (11:58 h day) or 18th (12:02 h day).
So I was wrong about the definition of an equinox - which is actually when the sun rises in due-east and sets due-west.
(And also when the sun is positioned exactly above the equator, but that would never have been apparent to people up here at 55 degrees North.)
When I think about it; I already knew about the east-west definition, but had assumed that this also meant that day and night were equal length - and the day-length aspect of it seemed to be more important.
Furthermore, I'm pretty sure I have been told that the vernal equinox was the exact time of year when light began to overcome dark... A very poetic, not to say spiritual, conceit.
Well, I guess the timing is "close enough for government work"; but I am rather sad that it is not, after all, exact.
And it emphasizes my cumulative recognition that exact astronomical measures are not humanly-valid segmentations of time - the reality is more like a phase or season; and we ought not to make as much of specific days as we typically do.
*Stonehenge is Not aligned on the Spring Equinox
6 comments:
I found this hard to visualise but I think I get it now. 12 hour night and sun-rising-in-the-exact east *ought* to coincide but generally don't because the earth doesn't rotate exactly around its poles -- the axis of rotation itself annoyingly rotates about the poles too. So much for the music of the spheres. Astronomers feel free to jump in and correct me!
@Ron - Errr - no comment. I tried to read the Wiki article, but my mind went blank. Could be what you say, or refraction by the atmosphere, or something else?
But if Bonald (from the Orthosphere) happens to read this, maybe he would condescend to comment (he is an astrophysicist).
Thinking again, I didn't get it quite right. The earth does rotate about the poles, but the axis wobbles slightly ('precession').
Imagine taking an apple up into space, removing the stalk, and driving a metal skewer through its centre from the point where the stalk was. Set it spinning about the skewer in front of you in zero-gravity. Now give the skewer a light tap sideways at one end. The apple will continue to rotate around the skewer, but now the skewer will now 'wobble' slightly, tracing out a cone shape. You could shine a torch at the apple from the other end of the space cabin to simulate the sun.
Perhaps the earth was 'tapped' by an asteroid, anyhow I think this is why when the day is precisely 12 hours long the sun doesn't rise precisely in the East, but it does if you take the average over many years.
There's a dramatic demo of how axes of freely rotating bodies can move, even flip right around:
https://www.youtube.com/watch?v=1n-HMSCDYtM
@Ron - One of the effects of advancing age (and entropy) is that I am (willy nilly) in a perpetual "energy conservation mode" - that prevents me from thinking seriously about anything I find difficult.
...Unless I am convinced there is a really good reason to do so - in which case I enjoy nothing more!
So, having established what Equinox is and is not; I am happy to leave the explanation to others...
"Stonehenge is Not aligned on the Spring Equinox." Thanks to precession perhaps?
@ap - I meant that it is supposed to be aligned with the solstices, not the equinoxes.
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