On the 22nd the Sun reaches the celestial position along the ecliptic that marks the end of northern hemisphere summer. The ecliptic is the Sun’s apparent path throughout the year and is used to delineate the position for the ending and beginning of each season. The ecliptic path, relative to the equator, is a curved line that crosses the celestial equator twice—once as it moves northward toward a maximum distance from the celestial equator, and again as it moves south of the celestial equator toward its maximum distance south.

    Picture a curved line wrapping around the Earth that goes from the equator to the Tropic of Cancer, back down to and across the equator to the Tropic of Capricorn and then back up to and crossing the equator to the Tropic of Cancer. This curved line represents the Sun’s apparent path along the ecliptic throughout the year. The Sun, from our perspective, appears to be moving eastward relative to the stars in the background, following the curving path of the ecliptic.

    This curve is a result of the Earth's axial tilt of 23.5 degrees. Each of our seasons starts or ends when the Sun reaches a particular set of coordinates along the ecliptic. The September equinox, for example, is the day when the Sun crosses the celestial equator moving southward. The coordinates for the crossing point is 0 degrees, and 12 hours of right ascension.