The distance of the Sun (or of any heavenly body) from the equator is called its "declination."

The ecliptic, the tropics and the polar circles are based on the apparent annual course of the Sun. Actually the Sun has nothing to do with the seasonal change. It only stands still and lets our small Earth describe a path which takes the shape of an ellipse with the Sun at one focal point. Kepler discovered and Newton proved mathematically that if two bodies are revolving around each other under the force of gravity, the line joining them sweeps out equal areas in equal times. Strictly speaking, of course, both bodies revolve around their common center of gravity, but in this case the center of gravity is within the body of the Sun itself, so for all practical purposes we may speak of the Earth revolving around the Sun. This explains why the Earth travels faster when it is near the Sun, and relaxes its speed at a distance.

In the present time, the Earth reaches the closest point to the Sun, and is said to be at "perihelion" early in January. It moves then at its swiftest speed. It arrives at "aphelion," the point farthest from the Sun, early in July, and it is then traveling most leisurely. This combination has the effect of making the Sun appear to stay north of the equator seven days longer than it stays south. Our winters in the Northern Hemisphere are shorter than our summers, irrespective of the weather.

Apian gave the rule for finding the date of the autumnal equinox in these words:

"If you add three days, 0 hours, 42 m., to that date in March when the equinox occurs, you will have the day, hour and minute of the autumnal equinox which occurs in September."

That rule gives an excellent way of prediction, if a calendar is not handy. The figure was not exact in Apian's time, and is still less exact today. It is never constant, but the variations are quite small. In the past ten years, the amount to be added has varied from two days, ten hours and seven minutes, to two days, ten hours and thirty-nine minutes.

The fact that we are nearest the Sun in winter has another effect, quite apart from the long summers in the Northern Hemisphere. The nearer we are to the Sun, the more heat we receive. As our northern winters come when the Earth is closest to the Sun, they are more moderate than the winters of the Southern Hemisphere; and since we spend our summers at aphelion, like sensible people, we never feel the direct rays of the Sun except from near our greatest distance.

Certain scientists have thought that the glacial periods came when the Sun was farthest from the Earth during the northern winter. Then the winters would be colder and longer than they are today. If this variation does regulate the glacial epochs, we may just as well face the facts and admit that we are due for another in about twelve thousand years.

Of more immediate consequence are the effects of the altitude of the Sun. It is a fair assumption that the quantity of heat given off by the Sun is reasonably constant. The amount received on the Earth per square foot or per acre differs appreciably with the angle at which the heat rays strike. The greatest amount of heat is received when the rays of the Sun are coming at right angles to our acre. As the rays strike with a more oblique angle, fewer of them are intercepted by the ground and the heating effect is decreased.

The distance of the Sun (or of any heavenly body) from the equator is called its "declination." It corresponds to latitude on the Earth. As the Sun travels from south to north, starting at the winter solstice, its southern declination decreases from 23° 27' South to 0°. At zero it is crossing the equator and this is the point of the spring equinox. Then its declination grows, up to 23° 27' North. There the Sun pauses and retraces its path.

Declination is used in the determination of latitudes and longitudes. So that an ordinary surveyor or traveler may be equipped with accurate data, full and complete lists of declination are given in the Nautical Almanacs. The lists are very useful, and far more precise than any field work need require. With them at hand, no traveler need worry over the accuracy of declination. But he should never, under any conditions whatsoever, claim that he can equal their accuracy with his own observations. If he makes such a claim, his knowledge and his reliability are alike open to question. There is one famous example.

Dr. F. A. Cook and Commander Peary each made a polar expedition in the year 1909. Dr. Cook returned first and claimed that he had reached the North Pole. The scientists demanded proof, and he submitted a document which was printed in all the daily papers. In this essay he said that he continued to go north, finding his latitude by altitudes of the Sun, until one day his observations told him that he was at 89° 59' 46¨.

As soon as I had read those figures, I bought copies of every daily paper in Chicago to see if there had been a misprint. Surely no eminent explorer would claim an accuracy of one second of arc in his determination. But all the papers agreed on the 46¨ and I was satisfied that Dr. Cook had not reached the North Pole. Obviously he did not know even the elements of navigation or of astronomy.

No reliable scientific man would have tried to claim an accuracy far beyond anything he could ever achieve. If he had said 45¨, I might have believed him, for that is three-quarters of a minute and can be realized. But the reading to 1¨ with a sextant was entirely too precise. The fact that he claimed to have walked 14¨ farther north and planted a flag at the North Pole was only one of those "corroborative details, intended to give artistic verisimilitude to an otherwise bald and unconvincing narrative."

When Commander Peary returned, he found that Dr. Cook had literally stolen all his glory. Some months passed before the imposter was discredited. Peary, in the text of his book, The North Pole, gave his results to minutes only. His readings were taken with a sextant which was read to ten seconds; the facsimile pages of his notebook showed that he worked to seconds yet, as a scientific man, he knew that results obtained under such difficult conditions were not reliable. Peary's accounts were accepted, and he was given full credit for his wonderful performance in being the first to reach the North Pole.

The co-ordinate, which goes with declination, is "right ascension." These two bear exactly the same relationship in the sky that latitude and longitude have on Earth. Declination and latitude are measured north and south from the equator. Right ascension is measured from the first point of Aries (the vernal equinox) and longitude from the meridian of Greenwich; but, whereas longitude is measured both east and west, right ascension is always measured eastward.