I go out most mornings before sunrise, ride down a narrow lane that’s more like a footpath and has a “no bicycle” sign (but there’s no one else around at this time), along a causeway between a river and a millstream, and up a steep path through a churchyard where, if I’m too late, the Sun will be like a rifle aimed into my eyes. I come to a cliff edge and see, across a bay, a long line of other cliffs.
The point of sunrise moves along this horizon. At the summer solstice it’s as far left as Black Ven; day by day it moves rightward, to the Charmouth gap, over Stonebarrow, along the forested top of Langdon, and reaches a famous summit called Golden Cap. That’s where it will rise on the morning of the autumn equinox.
Afterwards the sunrise point will roll over Doghouse Hill and Thorncombe Beacon and down a line of cliffs that diminish in height and fade into the distance; will silhouette the Isle of Portland, which is not an island but a thread-necked peninsula, twenty-five miles away; and by the winter solstice the rising Sun will at last, off the end of Portland Bill, reach a sea horizon.
The equinox will happen on Tuesday September 23, so I need to tell you about it – just in case you didn’t know – on September 20 or 21; but on just those days I have to be away on a brief long journey (not by bicycle) to the other side of the ocean. So I have to use a painting of the equinox dawn of a couple of years ago and write this in advance, and learn how to “schedule” it so that it appears later. If the solar system can schedule an eclipse or an equinox, I ought to be able to schedule a weblog post.
The equinox comes around and around, so my explanation of it in my Astronomical Calendar has to be either the same every time or slightly better, and will reach perfection in Astronomical Calendar 2015, so what more is there to say?
Well, here’s a diagram to bring out one feature that isn’t so easily thought of: the equinox is the time when the Sun crosses most steeply upward (or, at sunset, downward) through the horizon. So at and around this date, the time of sunrise is getting most rapidly later, and twilight is shortest and daylight begins and ends most suddenly.
The chart is of sunrise on the morning nearest to the instant of the equinox. That instant is Sep. 23 2:29 Universal Time, which is back in Sep. 22 for American time zones, but still the nearest sunrise is that of Sep. 23.
The thick light-blue line sprouting rightward from the Sun is the celestial equator – the line in the sky that is overhead for places along Earth’s equator. The lines of the same color parallel to it are lines of declination, or north-and-southness, like lines of latitude on the Earth. They curve because we are at latitude 40 on the spherical Earth, and have chosen to project the map so that the horizon is a straight line.
You can see that these lines of declination intersect the horizon at different angles. To the left they intersect at smaller angles until they become merely tangent to the horizon and then leave it, to become circles around the north pole of the sky. Rightward they shrink until there would come one that’s tangent to the horizon from below, and they surround the out-of-view south pole of the sky. It’s evident that the middle line of declination – the celestial equator – must be the one that intersects at the steepest angle. Actually it rises at 50 degrees, the complement of our latitude of 40 degrees.
The other thick curve, white, is the ecliptic, along which the Sun always travels. The equinox point is the point where the ecliptic cuts the celestial equator, and the equinox instant is when the Sun arrives southward to this point.
The next curve of declination to the left is at 23.4 degrees north, and the Sun was on it (at the point where the ecliptic touches it) on June 21, the summer solstice.
So at June 21 the Sun rises for us at the shallowest angle, and twilight is longest; and on September 23 it rises at the steepest angle, and twilight is shortest. For our latitudes of 40 and 50 north there isn’t a great deal of difference. But for latitude 60 or 70, where the celestial equator itself rises at a shallower angle, the effect is much greater. At these northern latitudes, in June twilight lasts all night.
Day starts when even a point of the Sun breaks the horizon, so the picture should show broad daylight; but I mark a few stars as clues to where in the celestial realm all this happens. Regulus is in Leo and the equinox point is in Virgo, whose great star Spica is below the horizon. The Moon, which will be at its New position in front of the Sun next day, shows far too slender a crescent to see; Venus is only about 8 degrees from the Sun, behind which it will pass one month later. Ths Sun and Moon are both exaggerated 4 times in size.
It’s getting easier to be out there before the Sun. At the June solstice, sunrise at latitude 50 was about 4 by local time – which was 3 in the Daylight-Shifting Time forced by law on our clocks for much of the year. So I had to get up not long after the clock said 2. At the equinox the Sun rises about two hours later, and at the December solstice another two hours later. Plus, by then clocks will have returned to truer time, another hour later. For latitude 40, the solstice times cluster closer together to the equinox time – June sunrise is nearly an hour later, and December sunrise nearly an hour earlier, than it is farther north. That makes sense when you remember that at the equator sunrise and sunset times hardly differ at all throughout the year. If I lived in Iceland I’d have to get up really early to see the midsummer sunrise.
All this is another instance of what a nuisance Daylight-Shifting Time is. It’s not just that it made me get up before 3 instead of before 4; it makes this whole subject, and many others, more complex to explain and understand.
There are two kinds of sunrise, because there are two kinds of horizon, the geometric and the solid. The Sun reaches altitude zero at its appointed time and then makes its slanting journey upward behind the hill. Golden Cap at its distance of two and a half miles is about three degrees high, a span the Sun travels in twelve minutes. So I could be twelve minutes late getting to my sunrise-viewing point. But it’s worth being out at least twelve minutes before that moment of moments and watching through the mounting suspense, the silent swift changes of cloud and color that precede the arrival of our lord of light.