Depending on where you are on Earth, yesterday or today is defined as the equinox. Solstices are fairly easy to identify; they represent the longest day or night of the year, depending on your hemisphere. But the definition of equinoxes is that the equator is facing directly towards the Sun.
The name equinox, meaning “equal night” in Latin, may originally be misunderstood. This leads to the idea that at equinoxes there is an equal amount of day and night all over the world. One obvious reason why this is wrong can be seen in obvious points like the poles, but as Space.com points out, the definition also has issues that require more information to spot.
What is the equinox?
The equinox is defined in Wikipedia as follows: “The equinox (equality of day, equality of day and night, or equinox) is the moment when the circle of enlightenment passes through the poles as a result of the sun’s rays striking the Equator. It is the situation where the day and night are equal. Twice a year, in spring and autumn. repeated over and over.”
One of the biggest misunderstandings of the equinox is that it ignores twilight and thinks that anything that is not day is night, or dark. Sunlight scatters from particles in the upper atmosphere before sunrise and after sunset, still providing a bright environment. The amount of extra light varies with location, with twilight being much shorter in the tropics than in temperate areas. However, if you want a full 12-hour night, you’ll have to wait until the depths of winter.
Even if your definition of night includes twilight, it will not equal the amount of daylight at the equinox due to the Sun not being a point source. Before the midpoint of our local star rises and after it sets, a powerful source of light still covers a quarter of the sky above us all.
Once again, the Sun sets much more slowly at high latitudes, so no universal number is sufficient to describe how long a portion of the Sun stays above the horizon. At the equator at the equinox, this minimum number is two minutes, but at the poles there is a long period of more than 24 hours, during which only part of the Sun is visible.
Altitude can also complicate things. The tops of the mountains stay bright a little longer than the plains, making the day a little longer.
Even if you’re on a small boat in the ocean, Earth has one more trick to play to give you precious moments of light. This is because the atmosphere scatters light as well as refracts it, so when the Sun is slightly below the horizon, it appears to be just above the horizon.
In short, the definition of equinox does not mean 12 hours of light – 12 hours of darkness. It is the name given to the day when the Sun can be seen for exactly 12 hours and it changes according to your latitude.
The year doesn’t last exactly 365 days, and so the timing of equinoxes and solstices moves a bit. It will take place at 01:04 UTC on September 23 this year. This means that while the equinox happens on Friday, September 23 for Europe, Asia, Australia and Africa, it happens on Thursday, September 22 for the Americas.
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