Wwelcome to fall! The northern hemisphere’s autumnal equinox occurred on September 22. This is the day when the sun crosses the celestial equator and moves from the northern 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 shines directly above Earth’s equator, creating nearly equal hours of daylight and darkness across the globe. This is reflected in the name. The word “equinox” comes from the Latin words aequuswhich means equal, and nox, means 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 at night.
The practical consequence of this is that the length of the day is always extended by a few minutes at the expense of the time we spend in the dark. So, on the equinox itself, the day will always be a little 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).
