Vernal and Autumnal Equinoxes
Ok, so here's my first attempt at answering a "viewer question".
Ken writes to ask:
"Why don't Vernal and Autumnal Equinoxes have exactly equal night and day? Usually the Equinoxes fall around 22nd of March/Sept. Ideally, there should be 12 hrs separating sunrise and sunset. It's close, but not exact. Usually, the 12hr sunrise/sunset days fall a few days before or after the equinoxes. Why is this?"
First of all, you usually hear of two definitions of the equinox:
The "terrestrial" definition is when the sun rises exactly due east and sets exactly due west. It is believed that at this moment we get exactly 12 hours of daylight and darkness. In fact, the word equinox comes from the Latin "equi nox" meaning "equal night".
The "astronomical" or "celestial" definition refers to the moment when the center of the sun crosses the celestial equator in the sky. The celestial equator is the imaginary circle in the sky that lies above the imaginary equator on earth. It is this definition that is more commonly used today.
Both of these definitions are equivalent. Both result in the same moment in time being defined as the equinox.
It is when you bring "daylight" into the picture that it gets all messed up!
We know that due to the tilt of the earth's axis relative to its orbit, the sun spends most its time either north of south of the equator and for this reason the days are usually longer or shorter than 12 hours most of the time. We expect that when the sun is directly over the equator that we should get a perfect 12 hour day.
Shouldn't we?
Not if we take daylight into account, because of 3 reasons:
As humans we generally measure the "day" as the time between sunrise and sunset. Sunrise is defined as when the TOP EDGE of the sun breaks the horizon and sunset is defined as when the TOP EDGE disappears below the horizon. Equinox is measured based on the CENTER of the sun's disk, not at the edge. This difference means that at the equinox moment, the day is actually slightly LONGER than 12 hours.
Not only that, but the earth's atmosphere refracts (bends) sunlight coming over the horizon so that the sun appears to have risen even before it really has. This adds 7-8 more minutes to the length of the day at the equinoxes.
Both of these effects combine so that the true 12-hour day for high northern latitudes can occur several days before the Vernal Equinox and several days after the Autumnal equinox. The effect is lessened for latitudes closer to the equator. And the opposite holds true for those latitudes in the southern hemisphere.
In addition, since the earth is not rotating at the actual moment of the astronomical equinox (can't rotate if there is no time change) then only certain points on earth even see a sunset or sunrise at the ACTUAL moment of the equinox. And then the other end of the day for those locations would be before or after the equinox anyway. So no one location on earth can possibly see a sunrise and sunset at the equinox moment.
Still, this third "effect" does not contribute to the lengthening the day as much as the other two.
Hope this answers it!
The U.S. Naval Observatory provides tables for earth's perihelion, aphelion, equinoxes and solstices until 2020. Notice that these equinoxes change from year to year due to the earth's precession (the "wobble" as it spins on its axis). Notice that in a leap year the day changes by 1 for the obvious reasons.
WORD OF WARNING: Never confuse equinox with the 12-hour day. The former happens at the same time for everyone on earth. The latter is dependent on your location.
Now I won't pretend that I knew all that off the bat! But having some ideas and after a thorough internet search revealed the true answer.
Another USNO reference: http://aa.usno.navy.mil/faq/docs/equinoxes.html
Good Night!
Ken writes to ask:
"Why don't Vernal and Autumnal Equinoxes have exactly equal night and day? Usually the Equinoxes fall around 22nd of March/Sept. Ideally, there should be 12 hrs separating sunrise and sunset. It's close, but not exact. Usually, the 12hr sunrise/sunset days fall a few days before or after the equinoxes. Why is this?"
First of all, you usually hear of two definitions of the equinox:
The "terrestrial" definition is when the sun rises exactly due east and sets exactly due west. It is believed that at this moment we get exactly 12 hours of daylight and darkness. In fact, the word equinox comes from the Latin "equi nox" meaning "equal night".
The "astronomical" or "celestial" definition refers to the moment when the center of the sun crosses the celestial equator in the sky. The celestial equator is the imaginary circle in the sky that lies above the imaginary equator on earth. It is this definition that is more commonly used today.
Both of these definitions are equivalent. Both result in the same moment in time being defined as the equinox.
It is when you bring "daylight" into the picture that it gets all messed up!
We know that due to the tilt of the earth's axis relative to its orbit, the sun spends most its time either north of south of the equator and for this reason the days are usually longer or shorter than 12 hours most of the time. We expect that when the sun is directly over the equator that we should get a perfect 12 hour day.
Shouldn't we?
Not if we take daylight into account, because of 3 reasons:
- The sun is not a finite point source of light.
- The earth has an atmosphere.
- At the moment of the equinox, the earth is not rotating.
As humans we generally measure the "day" as the time between sunrise and sunset. Sunrise is defined as when the TOP EDGE of the sun breaks the horizon and sunset is defined as when the TOP EDGE disappears below the horizon. Equinox is measured based on the CENTER of the sun's disk, not at the edge. This difference means that at the equinox moment, the day is actually slightly LONGER than 12 hours.
Not only that, but the earth's atmosphere refracts (bends) sunlight coming over the horizon so that the sun appears to have risen even before it really has. This adds 7-8 more minutes to the length of the day at the equinoxes.
Both of these effects combine so that the true 12-hour day for high northern latitudes can occur several days before the Vernal Equinox and several days after the Autumnal equinox. The effect is lessened for latitudes closer to the equator. And the opposite holds true for those latitudes in the southern hemisphere.
In addition, since the earth is not rotating at the actual moment of the astronomical equinox (can't rotate if there is no time change) then only certain points on earth even see a sunset or sunrise at the ACTUAL moment of the equinox. And then the other end of the day for those locations would be before or after the equinox anyway. So no one location on earth can possibly see a sunrise and sunset at the equinox moment.
Still, this third "effect" does not contribute to the lengthening the day as much as the other two.
Hope this answers it!
The U.S. Naval Observatory provides tables for earth's perihelion, aphelion, equinoxes and solstices until 2020. Notice that these equinoxes change from year to year due to the earth's precession (the "wobble" as it spins on its axis). Notice that in a leap year the day changes by 1 for the obvious reasons.
WORD OF WARNING: Never confuse equinox with the 12-hour day. The former happens at the same time for everyone on earth. The latter is dependent on your location.
Now I won't pretend that I knew all that off the bat! But having some ideas and after a thorough internet search revealed the true answer.
Another USNO reference: http://aa.usno.navy.mil/faq/docs/equinoxes.html
Good Night!
posted by Dan at 7:07 PM
2 Comments:
Thanks for the detailed explanation! You're on your way to be the "Carl Sagan" of the blogosphere!
-Ken
The irony is that at the equinox moment, when the sun crosses the celestial equator, no point on earth sees a 12-hour day, or experiences "equi nox".
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