called the "Horizon System."
The north-south great circle traveling from horizon to horizon through the zenith is the meridian. The cast-west great circle traveling from horizon to horizon through the zenith, and making a right angle with the meridian, is the 14 prime vertical."
A point in the sky can then be said to be so I many degrees (positive) above the horizon or so many degrees (negative) below the horizon, this being the "altitude."
Once that is determined, the exact point in the sky can be located by measuring on that altitude the number of degrees westward from the southern half of the meridian.
At least astronomers do that. Navigators and surveyors measure the number of -degrees eastward from the north end of the meridian. (In both cases the direction of meas ure is clockwise.)
The number of degrees west of the southern edge of the meridian (or east of the northern edce, depending on the system used) is the azimuth. The word is a less corrupt form of the Arabic expression from which "zenith" also comes.
If you set north as having an azimuth of O', then east has an azimuth of 90', south an azimuth of 180', and west an azimuth of 270'. Instead of boxing the compass with outlandish names you can plot direction by degrees.
And as for myseff?
Why, I have an azimuth of isaac. Naturally.
Part I Of Time And Space 4. The Heavenly Zoo
On July 20, 1963 there was a total eclipse of the Sun, visible in parts of Maine, but not quite visible in its total aspect from my house. In order to see the total eclipse I would have had to drive two hundred miles, take a chance on clouds, then drive back two hundred miles, braving the traffic congestion produced by thousands of other New Englanders with the same notion.
I decided not to (as it happened, clouds interfered with seeing, so it was just as well) and caught fugitive glimpses of an eclipse that was only 95 per cent total, from my backyard. However, the difference between a 95 per cent eclipse and a 100 per cent eclipse is the difference between a notion of water and an ocean of water, so I did not feel very overwhelmed by what I saw.
V-/bat makes a total eclipse so remarkable is the sheer astronomical accident that the Moon fits so snugly over the Sun. The Moon is just large enougb. to cover the Sun completely (at times) so that a temporary night falls and the stars spring out. And it is just small enough so that during the Sun's obscuration, the corona, especially the brighter parts near the body of the Sun, is completely visible.
The apparent size of the Sun and Moon depends upon both their actual size and their distance from us. The diameter of the Moon is 2160 miles while that of, the Sun is 864,000 miles. The ratio of the diameter of the Sun to that of the Moon is 864,000/2160 or 400. In other words, if both were at the same distance from us, the Sun would appear to be 400 times as broad as the Moon.
However, the Sun is farther away from us than the Moon is, and therefore appears smaller for its size than the Moon does. At great distances, such as -those which characterize the Moon and the Sun, doubling the distance halves the apparent diameter. Remembering that, consider that the average distance of the Moon from us is 238,000 miles while that of the Sun is 93,000,000 miles. The ratio of the distance of the Sun to that of the Moon is 93,000, 000/238,000 or 390. The Sun's apparent diameter is cut down in proportion.
In other words, the two effects just about cancel. The Sun's greater distance makes up for its greater size and the result is that the Moon and the Sun appear to be equal in size. The apparent angular diameter of the Sun averages 32 minutes of arc, while that of the Moon averages 31 minutes of arc.
These are average values because both Moon and Earth possess elliptical orbits. The Moon is closer to the Earth (and therefore appears larger) at some times than at others, while the Earth is closer to the Sun (which therefore appears larger) at some times than at others. This variation in apparent diameter is only 3 per cent for the Sun and about 5 per cent for the Moon, so that it goes unnoticed by the casual observer.
There is no astronomical reason why