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Some believe that it is always warmer during daytime at the equator because the Sun is directly overhead at midday and is able to warm everything equally underneath it. In reality, there are only two instances in the entire year when the Sun can be directly above an area at the equator. One is the autumnal equinox while the other is the vernal equinox.
To understand how this is so, allow me to refer to the image above. If you notice, the Earth is always tilted towards one direction. This degree of tilt changes over a period of thousands of years. Thus, it would be safe to say that the tilt stays the same as the Earth revolves around the Sun throughout an entire year (or even for several decades).
There are two instances during a full revolution when the Sun’s behavior at one end of the Earth is the exact opposite of that at the other. The two instances are called solstices.
At one solstice (summer solstice), inhabitants at the northernmost regions will experience longer days, while those at the southernmost regions will experience longer nights. In fact, the Sun will be up 24 hours at the north pole and hidden 24 hours at the south.
During the other solstice (winter solstice), the exact opposite happens, with northernmost dwellers experiencing shorter days and southernmost dwellers shorter nights.
Solstices happen when the Earth’s axial tilt is farthest or nearest from the Sun. Halfway between the solstices, as seen on the image, is the time when the equinoxes occur. If we were to draw an imaginary line joining the center of the Sun and the center of the Earth, the only time that the line would make a 90 degree angle with the Earth’s rotational axis would be during equinoxes.
During vernal equinox, as well as in autumnal equinox, the Sun lights up all parts of the Earth at equal durations. In other words, sunlight shines on all parts of the world at approximately 12 hours each. That is why, in a vernal equinox, which happens in March during spring, the temperature is neither too warm nor too cold.
The word ‘vernal’ comes from the Latin word ‘ver’, which means spring. In more recent text, the term vernal equinox is replaced with the more neutral-sounding March equinox. The reason is because it is only in the northern hemisphere that it is spring during the so-called vernal equinox. Down south, it is fall or autumn.
More information can be found at NASA:
Check out this podcast at Astronomy Cast:
Solar Altitudes on Equinoxes and Solstices