Welcome to spring! Tuesday is vernal equinox ... the official start of spring!
During the equinox, the sun’s rays are most heavily concentrated right over the equator. Equinox means "equal night," but if you look at the sunrise/sunset times for D/FW International Airport below, you'll see that it’s not equal day or night.
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There is an 8-minute difference between the sunrise and sunset times. Why?
Most of the reason is because sunlight is refracted (or bent) as it enters the atmosphere. The composition and density of the atmosphere causes the light to change direction.This results in us seeing the sun before it has actually risen over the horizon.
The same is true as the sun sets. We can still see the sun even though technically it has already gone below the horizon.
This accounts for about six of the eight minutes of difference. The other two to three minutes of extra daylight is because of the what's defined as "sunrise" and "sunset."
Sunrise occurs when the first limb of the sun appears above the horizon. Sunset happens when the last bit of the sun disappears from view.
But if you measure when the midpoint of the sun crosses the horizon AND you were to discount the sunlight refraction, then the day would be 12 hours of day and 12 hours of night on the day of the equinox.
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Ellis, Johnson Co.