Why the spring equinox is not really

Today is the spring equinox, which is one of two times in the year when the Sun is exactly above the Equator and day and night are of equal length all over the Earth. (The word equinox is Latin for equal night.) But if you look at the times of sunrise and sunset today, you will see that we actually have 12 hours and nine minutes of daylight. You have to go back to March 16 or 17 to get closest to exactly 12 hours.

It is the same for the fall equinox, which this year officially falls on September 23 but September 25 is when we have equal lengths of day and night.

Why is this? This article explains.

The Equinox is when the sun is in line with the Equator. The reason we do not see equal dark and daylight on the equinox has to do with how we view the sun. Sunrise is defined as when we first see the upper outer ring of the sun, of course sunset would be the exact opposite.

Atmospheric gases play a large role of when we see these outer rings of the sun to determine sunrise and sunset. The waves of light from the sun pass through atmospheric gas causing them to bend, reflect and refract. These changes to the sunlight can lead to a few minutes of change when sunrise and sunsets are observed.


  1. sundoga says

    Plus, as far as those of us in the southern hemisphere are concerned, it’s the Autumnal Equinox.

  2. says

    Well, those of you in the southern hemisphere are still being punished for Ken Ham and Mel Gibson, and this new pope may add required penance. So you just hush and turn your globes back over.

  3. jufulu says

    This kinda reminds of a question asked on QI (a British Quiz show hosted by David Fry). Briefly, when you watch the sun going down, at what point has the Sun dropped below the horizon. The answer is when the bottom of the sun touches the horizon. The atmosphere bends light by the amount that is the apparent diameter of the sun.

    The sun goes down, the sun stays up. You can’t explain this.

  4. Jockaira says

    So, when is Easter Sunday? I’m reluctant to ask anyone I know, they might invite me to a Sunday Service and then I would have to violate my carefully constructed image of a “Reasonable, Likeable atheist” by explaining why I couldn’t attend.
    (JOKE!) but really when is it. I thought it was on or near the vernal equinox which according to googled sources is on the first of April (appropriately All Fools’ Day).

  5. bad Jim says

    Easter is the first Sunday after the first full moon after the vernal equinox.

    It should follow that Wester is the first Sunday after the first full moon after the autumnal equinox, but strangely no one observes this.

    (Wikipedia is now using northwards equinox, northern solstice, southwards equinox and southern solstice to denote what in the northern hemisphere are called the spring, summer, autumn and winter events.)

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