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When is the time of the year when all places on the Earth have the same length of nighttime? During equinox, of course! The word ‘equinox’ actually comes from two Latin words, aequus (meaning equal) and nox (meaning night).
Two equinoxes occur every year. One is called the vernal equinox, which takes place in March, while the other is called autumnal equinox, which takes place in September.
Other terms used in lieu of vernal equinox include March equinox and Northward equinox. On the other hand, terms like September equinox or Southward equinox are sometimes used in favor of autumnal equinox.
The reason why March or September equinox is used is because vernal (wherein ‘ver’ is derived from the Latin word which means spring) and autumnal are biased towards inhabitants of the northern hemisphere. You see, when vernal equinox occurs, it is spring in the northern places. And when autumnal equinox occurs, it is autumn in the northern places.
However, the exact opposite is experienced by southern dwellers, wherein vernal equinox coincides with their autumn, while autumnal equinox coincides with their spring.
Now, if you’ve noticed, it is during the equinoxes that the climate is pleasant, i.e., not too hot and not too cold. And that holds true to practically all parts of the world. This is because the tilt of the Earth during these times allow for an even distribution of heat from the Sun.
During other times of the year, the tilt of the Earth exposes more of one side to the Sun – either the northern or southern hemisphere – while hiding the other. This happens despite the Earth’s constant spinning about its axis. It’s like constantly turning a giant marshmallow but keeping only one side near the fire.
Thus, at times, the northern countries experience longer days than those in the South. Then at other times, the opposite happens.
During an equinox, however, you can picture the tilt of the Earth as being perpendicular to an imaginary straight line joining the Earth and Sun – still much like turning a giant marshmallow but this time, right above the fire. So what do you get? The result is an evenly cooked camping meal.
Equinox events are not monopolized by the Earth. Other planets which have substantial axial tilts also get to go through them. Saturn is one of them. During an equinox, it would be difficult to observe the rings of Saturn because they would face the Sun edge-on.
More information can be found at NASA:
Check out this podcast at Astronomy Cast:
NASA: Seasons of the Year