At some point in the vastness of geologic time, the Earth was hit by a gigantic space rock which knocked it off of a straight up and down orientation to one where it is pointing roughly 23.5 degrees off of vertical. This value varies slightly between 21 and 24 degrees over a period of tens of thousands of years, not because the Earth is tilting but because the distribution of ice at the poles varies, and all of that ice weighs enough to change the Earth’s gravity.
The Earth’s orbit is also eccentric, meaning that the highest and lowest points of its orbit around the Sun are slightly different. Or, put even more simply, it’s not quite a perfect circle but rather just a tiny bit of an oval.
Now, the effect of the tilt is that different portions of the Earth’s surface receive more or less solar heating depending on the time of year in a cycle from roughly July to January. This is why in one half of the Earth it is cold, while the other half is hotter, winter versus summer.
At two points in this yearly cycle, the Sun crosses the Earth’s equator so that both hemispheres receive the same amount of solar energy, making day and night have an equal length everywhere. This is called an equinox, which is Latin for “equal night”. In the Northern Hemisphere, the Autumnal Equinox happens on September 22 at 3:21 p.m. EDT. This means that the nights are getting longer, which is great for people who want to look at the stars.
Next week, we’ll talk more about just what stars you can see this time of year. Unless our writer Erik has bought another lens or telescope, in which case we’re all going to be plagued by clouds.
Why Earth Has 4 Seasons (EarthSky)