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.

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.