Autumnal Equinox

This past September 23rd, the northern hemisphere experienced the beginning of our fall season.

In terms of astronomy, what does this mean?

In the sky, if we could see the ecliptic, the path the Sun travels on as it appears to cross the sky, the point of the autumnal equinox occurs when the Sun crosses the celestial equator as it travels south on the ecliptic. This marks the beginning of autumn for all of us in the northern hemisphere, and the beginning of the spring for those in the southern hemisphere.

Another way to look at it, is that the Sun is directly overhead at the equator, which means that the Earth itself is tilted in the plane perpendicular to the Sun. Neither the northern hemisphere or the southern hemisphere is tilted towards the Sun. But at this point, the southern hemisphere will begin to receive more sunlight as the southern hemisphere is beginning to be tilted more towards the Sun.

The Autumnal Equinox

September 23, 2014 at 02:29 UTC (10:29 EDT) marks the official beginning of autumn (autumnal equinox) in the northern hemisphere and spring (spring equinox) in the southern hemisphere.

If you were to imagine the sky above the earth to be a giant sphere with the Earth at the center, the path the Sun travels on this sphere is the ecliptic. The sphere itself is the celestial sphere, and is like a map of the nighttime sky. The line direction above the equator is the celestial equator. When the ecliptic and the celestial equator intersect, we have the equinoxes. If the Sun is travelling north on the celestial sphere when it crosses the equator, we have the northern hemisphere's vernal, or spring, equinox. When the Sun is travelling south on the ecliptic when it crosses the celestial equator, the autumnal equinox occurs, like it is doing today. If the Earth were not tilted with respect to the ecliptic, the celestial equator and the ecliptic would coincide and we would not have the seasons we have. But because it is tilted at 23.5° with respect to the ecliptic, we have the four seasons we know and love (unless you are like me and hate winter).

This is the time of year for us in the northern hemisphere start cursing the weather getting colder, especially those of us in the northern latitudes, and those in the southern hemisphere are beginning to get warmer weather.