## Earths Seasons

The Sun's path across the sky is highest in summer and lowest in winter. The altitude of the Sun above the horizon at noon varies during the year because Earth's axis is tilted to the plane of its orbit around the Sun (Figure 1.11).

Earth's equator remains tilted at about 23.5° to its orbital plane all year long. So as Earth travels around the Sun, the slant of the Earth-Sun line

September

September

Earth-Sun line

June

December

December

Earth-Sun line

-Earth's equator-.

March

Earth's

0ir^

equator-.

March

Figure 1.11. Because Earth's axis is tilted, each hemisphere gets varying amounts of sunlight during the year as our planet orbits the Sun.

changes. Sunlight pours down to Earth from different angles during the year, causing the change of seasons as well as seasonal variations in the length of days and nights.

Refer to Figure 1.11. Is the northern hemisphere tipped toward or away from the Sun (a) in December?_(b) in June?_

You can determine what the Sun's apparent position in the sky will be on any given day by checking the ecliptic on a celestial globe or a flat sky map like the one in Figure 1.12.

The vernal equinox, which occurs about March 20, is the Sun's position as it crosses the celestial equator going north. It is the point on the celestial sphere chosen to be the 0h of right ascension (see Section 1.9). The autumnal equinox, which occurs about September 23, is the Sun's position as it crosses the celestial equator going south. At the equinoxes, day and night are equal in length.

The summer solstice, which occurs about June 21, and the winter solstice, which occurs about December 21, are the most northern and most southern positions of the Sun during the year. At these times we have the longest and shortest days, respectively, in the northern hemisphere.