DALLAS — The fall equinox is here (Sept. 22), which we associate with equal amount of daylight and nighttime: 12 hours of daylight and 12 hours of nighttime as the sun is directly over the equator.
But did you know that on the equinox, there is actually more daylight than night?
There are a couple interesting reasons for why this is the case:
How we define "sunrise" and "sunset"
We define sunrise as when the moment the top edge of the sun appears above the horizon. And we define sunset as the exact moment the top edge of the sun disappears below the horizon.
Using this definition, we will have more daylight than night on the equinox.
If we measured sunrise and sunset by the moment the middle part of the sun hit the horizon, then the hours of daylight and night would be close to equal.
Sunlight refraction
The Earth's atmosphere refracts sunlight.
Refraction is the bending of light due to the light of the sun travelling from the vacuum of space into the gases of our atmosphere. We can't see it happen, but the light bends as it moves through the atmosphere.
This causes the top edge of the sun to appear slightly earlier than if the earth did not have an atmosphere.
Conversely, the top edge of the sun lingers above the horizon at sunset as well.
According to timeanddate.com, this causes every day on Earth to be around 6 minutes longer than if refraction did not exist.
So when are day and night equal?
Simply, it depends on where you are located.
In North Texas (where this article is being written), daylight and nighttime will be close to equal on September 26th. Four days after the fall equinox.
For areas close to the equator in the Northern Hemisphere, equal day and night happens weeks after the fall equinox.
For areas farther north, including most of the United States, it is usually 2 to 4 days after the equinox.