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Marking the
beginning of spring, the sun will cross the plane of the earth's equator
on Monday morning, making day and night of approximately the same
duration. The event, known as equinox, is eagerly awaited by space
enthusiasts as it takes place only twice a year, on March 21 and September 23.Mar 21, 2011
What is an Equinox?
Earth spins around its own axis approximately every 24 hours (a sidereal day is 23 hours, 56 minutes, 4 seconds). It orbits the sun in around 365 days. And, like a spinning top, it wobbles around on its axis, making a complete rotation every 26,000 years (precession of the equinoxes).Earth is actually tilted at an angle of around 23.4 degrees toward the celestial pole, which is a a certain point in the sky. As the Earth makes its yearly orbit, one hemisphere faces the sun more than the other, the side that has summer.
On any other day of the year, the Earth's axis also tilts a little away from or towards the Sun. However, 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.
Latitude Determines Day Length
Even if day and night aren’t exactly equal on the day of the equinox, there are days when day and night are both very close to 12 hours.| Approx date of "Equal Day & Night" | ||
|---|---|---|
| Latitude | March | September |
| 60° North | Mar 18 | Sep 25 |
| 55° North | Mar 17 | Sep 25 |
| 50° North | Mar 17 | Sep 25 |
| 45° North | Mar 17 | Sep 25 |
| 40° North | Mar 17 | Sep 26 |
| 35° North | Mar 16 | Sep 26 |
| 30° North | Mar 16 | Sep 27 |
| 25° North | Mar 15 | Sep 27 |
| 20° North | Mar 14 | Sep 28 |
| 15° North | Mar 12 | Sep 30 |
| 10° North | Mar 8 | Oct 4 |
| 5° North | Feb 24 | Oct 17 |
| Equator | No equal day and night | |
| 5° South | Apr 14 | Aug 29 |
| 10° South | Apr 1 | Sep 10 |
| 15° South | Mar 28 | Sep 14 |
| 20° South | Mar 26 | Sep 16 |
| 25° South | Mar 25 | Sep 17 |
| 30° South | Mar 24 | Sep 18 |
| 35° South | Mar 24 | Sep 19 |
| 40° South | Mar 23 | Sep 19 |
| 45° South | Mar 23 | Sep 19 |
| 50° South | Mar 23 | Sep 20 |
| 55° South | Mar 23 | Sep 20 |
| 60° South | Mar 22 | Sep 20 |
More Than 12 Hours
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.
Refraction: Light Lingers
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.
The extent of refraction depends on atmospheric pressure and temperature. Our calculations in the Sunrise and Sunset Calculator assumes the standard atmospheric pressure of 101.325 kilopascal and temperature of 15°C or 59°F.