DIAGRAM SHOWING POSITION OF THE SUN IN RELATION TO THE ZENITH OF LONDON AT THE NORTHERN WINTER SOLSTICE.
DIAGRAM SHOWING POSITION OF THE SUN IN RELATION TO THE ZENITH OF LONDON AT THE NORTHERN SUMMER SOLSTICE.
We get the equal altitudes at the equinoxes, and the greatest and the least at the solstices.
These altitudes depend upon the change of the sun's declination. The change of declination will affect the azimuth and amplitude of the sun's rising and setting; this is why, in our northern hemisphere, the sun rises and sets most to the north in summer and most to the south in winter. At the equinoxes the sun has always 0° Decl., so it rises and sets due east and west all over the world. But at the solstices it has its greatest declination of 23½° N. or S.; it will rise and set, therefore, far from the east and west points; how far, will depend on the latitude of the place we consider. The following are approximate values:
| Latitude of Place. | Amplitude of Sun at Solstice. | |
|---|---|---|
| ° | ° | ′ |
| 25 | 26 | 5 |
| 30 | 27 | 24 |
| 35 | 29 | 8 |
| 40 | 31 | 21 |
| 45 | 34 | 40 |
| 50 | 38 | 20 |
| 55 | 44 | 0 |
At Thebes, Lat. 25° 40′ N., representing Egypt, we find that the amplitude of the sun at rising or setting at the summer solstice will be approximately 26° N. of E. at rising, and 26° N. of W. at setting.
These solstices and their accompaniments are among the striking things in the natural world. At the winter solstice we have the depth of winter, at the summer solstice we have the height of summer; while at the equinoxes we have but transitional changes; in other words, while the solstices point, out for us the conditions of greatest heat and greatest cold, the equinoxes point out for us those two times of the year at which the temperature conditions are very nearly equal, although of course in the one case we are saying good-bye to summer and in the other to winter. In Egypt the summer solstice was paramount, for it occurred at the time of the rise of the Nile, the beginning of the Egyptian year.
Did the ancients know anything about these solstices and these equinoxes? Were the almost mythical Hor-shesu or sun-worshippers familiar with the annual course of the sun? That is one of the questions which we have to discuss.