Seasons and Tropics

If the Earth were upright, so its north-south axis of rotation intersected the plane of the ecliptic at a right angle, the plane of the ecliptic would divide the Earth exactly at the equator. The person wishing to stand directly beneath the Sun could choose any point on the equator to do so. In fact, though, the Earth's rotational axis is tilted from the vertical by 23.5°. Consequently, as the Earth travels around its orbit, the Sun appears to move north and south, and it is directly overhead at the equator on only two days each year. Those days are known as the equinoxes and at present they fall on about March 21 and September 23 (the date varies by a day or two from year to year). They are called equinoxes—from a Latin word meaning "equal nights"—because when the Sun is directly above the equator it is above the horizon for precisely 12 hours and below it for precisely 12 hours as seen from anywhere on Earth.

The Sun is farthest from the equator at the solstices—June 21 and December 23—when it appears overhead at noon at a point on a circle in latitude 23.5° N or S. These circles, shown in the illustration, are known as Tropics. The dates of the solstices change (see the section "Milutin Milankovitch and his astronomical cycles" on pages 52-60), but in ancient times the Sun was in the constellation of Cancer at the June solstice and in the constellation of Capricorn at the December solstice, and that is how the Tropics earned their names. The northern circle is the tropic of Cancer and the southern circle is the tropic of Capricorn. Not surprisingly, the latitude of the Tropics is the same as the angle of tilt of the Earth's rotational axis.

The combination of the axial tilt and the Earth's orbit produces our seasons, as shown in the diagram. When the Sun is over the tropic of Can

Why the Sun is overhead in the Tropics. As the Earth travels around the Sun in the course of a year, the line where the plane of the ecliptic meets the surface—and therefore where the Sun is directly overhead at noon—moves from the tropic of Cancer, across the equator, to the tropic of Capricorn, then back again.

Seasons and the Earth's orbit. Because of the tilt of the Earth's axis, when it is summer in one hemisphere it is winter in the other. As the Earth travels around the Sun, these alternate.

cer, the Northern Hemisphere is more intensely illuminated than the Southern Hemisphere, and it is summer in the north. During summer, there are more hours of daylight than there are hours of darkness. On the longest day of the year, New York (40.72° N) enjoys 15 hours 6 minutes when the Sun is above the horizon. (Daylight lasts longer than this because light is scattered by the atmosphere, and so the Sun provides some illumination even when it is a few degrees below the horizon.) Inside the Arctic Circle, at 66.5° N, there is a period in midsummer when the Sun does not sink below the horizon at all—the time of the "midnight Sun." When it is summer in one hemisphere, it is winter in the other and the hours of darkness exceed the hours of daylight. The shortest day in New York lasts for 9 hours, 15 minutes, and inside the Arctic Circle the Sun does not rise above the horizon at all, although scattering of light means that the Arctic "darkness at noon" is twilight rather than total darkness.

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