Seasons on Earth

Recently, everyone on Earth went from one season to the next. In the northern hemisphere, we went from winter to spring and in the southern hemisphere, summer gave way to the fall (autumn). How exactly are the seasons defined?


Just this past Friday, the northern hemisphere experienced the vernal equinox, while in the southern hemisphere, the autumnal equinox occurred. We already know why they call these the equinoxes as they were explained in the previous posts. However, why does the northern hemisphere experience one season, while the southern hemisphere experiences the opposite (spring-fall and summer-winter)? It all has to do with the Earth's axial tilt.


The Earth rotates on its axis once a day. Most everyone knows that. However, the rotational axis of the Earth is tilted with respect to its orbital axis by about 23.5º.

So what does this mean?


When one half of the Earth is tilted towards the Sun, that half will receive more direct sunlight. The other half will not. It is like when you take a flashlight and aim it at a wall. If you keep the flashlight parallel to the wall, the light circle is tight and compact, if you tilt the flashlight away from the wall, the light circle becomes elongated. The same amount of light is hitting the wall, but in a larger area.

 

Another interesting thing about the seasons is that the time of year they occur change over time. However, none of us will be alive when January will be summertime in the northern hemisphere. This will occur in about 13,000 years. If you can wait until 28,500 CE (common era, formerly known as AD), April will be the middle of summer for the northern hemisphere. This is because the Earth wobbles on its axis, causing precession of the equinoxes (which in turn, also causes the solstices to precess). Currently, the summer solstice occurs when the Sun is in the constellation Gemini, but will someday be in the constellation Sagittarius.

 

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.