Friday, September 18, 2015

How Long is a Day

The Autumn Equinox this year is Wednesday September 23rd.  This is the day when the night and day are nearly exactly the same length - 12 hours - all over the world.  Equinox comes from Latin, meaning 'equal night'.

The equinoxes marks the exact moment twice a year when the Earth's axis is not tilted toward or away from the Sun at all. However, the axis tilt of around 23.4 degrees toward the celestial pole, remains the same.

On the equator, the day and night stay approximately the same length all year round, but the day will always appear a little longer than 12 hours, due to the reasons below.
On the equinoxes, the geometric center of the sun is above the horizon for 12 hours, and you might think that the length of the day (hours of daylight) would be 12 hours too.
However, ‘sunrise’ is defined as the moment the upper edge of the sun's disk becomes visible above the horizon – not when the center of the sun is visible. In the same sense, ‘sunset’ refers to the moment the Sun's upper edge, not the center, disappears below the horizon. The time it takes for the sun to fully rise and set, which is several minutes, is added to the day and subtracted from the night, and therefore the equinox day lasts a little longer than 12 hours.

Another reason why the day is longer than 12 hours on an equinox is that the Earth's atmosphere refracts sunlight.
This refraction, or bending of the light, causes the Sun’s upper edge to be visible from Earth several minutes before the edge actually reaches the horizon. The same thing happens at sunset, when you can see the sun for several minutes after it has actually dipped under the horizon. This causes every day on Earth – including the days of the equinoxes – to be at least 6 minutes longer than it would have been without this refraction.

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