Since it was equinox time again this week, I became a little curious and did some research. Nowadays, it has become quite easy to find answers to open questions, even if you can't always be sure that they are completely correct.
I wondered whether day and night really are the same length everywhere in the world during the time of the spring and autumn equinox. Since we unfortunately only have 24 hours in a day, the light and dark periods of the day should be exactly 12 hours each.
And as I thought – on the two days of the equinox, i.e. in spring (around March 20) and in autumn (around September 22/23), day and night are almost the same length – approximately 12 hours. And this applies worldwide, but with slight variations.
But why isn't it exactly 12 hours everywhere?
This is due to atmospheric refraction. Yes, it's getting scientific, and I'm venturing into uncharted territory and very thin ice. The Earth's atmosphere refracts sunlight so that the sun is visible before it actually appears above the horizon and remains visible after it has set. This makes the day slightly longer than the night, usually by 6–8 minutes.
It also depends on the definition of sunrise and sunset. Sunrise is when the upper edge of the sun appears—not the center. This also slightly lengthens the day.
Another factor is geographical latitude, so that the differences are minimal near the equator. The closer you get to the poles, the greater the effect of atmospheric effects and the curvature of the Earth.
Here are three examples of a few global differences:
Tokyo, Japan: approx. 12 hours 7 minutes of daylight at the equinox
Quito, Ecuador (close to the equator): almost exactly 12 hours
Oslo, Norway: approx. 12 hours 15 minutes of daylight
In conclusion, we can say that the equinox brings almost equal lengths of day and night, but not exactly 12 hours everywhere.
The differences are small but measurable – and a nice example of how complex and fascinating our planet is.
