Welcome to autumn! The northern hemisphere’s autumnal equinox took place on 22 September. This is the day on which the sun crosses the celestial equator, moving from the northern celestial hemisphere to the southern.
The celestial equator is the projection of the Earth’s equator up into the sky. So, on the equinox, the sun is shining directly above Earth’s equator, and this creates nearly equal hours of daylight and darkness across the globe. This is reflected in the name. The word “equinox” comes from the Latin words aequus, meaning equal, and nox, meaning night.
A subtlety is that because the Earth’s atmosphere bends sunlight, a process known as refraction, every day we can see the sun a few minutes before it rises and a few minutes after it sets in the evening.
The practical upshot of this is that the length of the day is always elongated by a few minutes at the expense of the time we spend in darkness. So, on the equinox itself, the day will always be a bit longer than the night.
A few days later as the nights get longer, we experience true equal day and night length. This is known as the equilux (equal light).
