why isn't there 12 hours of daylight and 12 hours of darkness on the first day of spring?

Answer 1

The word equinox means "equal nights" and it does mean to indicate the day on which both day and night are 12 hours each.

Astronomers and navigators calculate the altitude of the Sun by using the centre of the disk and a mathematical horizon (defined as being everywhere at 90 degrees from the observer's zenith). They also correct their calculations to eliminate the effect of refraction.

Doing that allows them to use "simpler" spherical trigonometry equations such as:

Sin(H) = Sin(L)*Sin(D) + Cos(L)*Cos(D)*Cos(P)

H = Height (altitude)
L = Latitude (observer)
D = Declination (star, planet, Sun...)
P = Polar angle (diff. in longitude between observer and projection of object onto Earth's surface).


To find sunrise (or sunset), we set H=0. From the above, we get a much simpler equation:

Cos(P) = - Tan(L)*Tan(D)

Where P is the polar angle at sunrise (or sunset)
The rate at which P changes is 15 degrees per hour (the Earth rotates 360 degrees in 24 hours).

Time from sunrise to sunset = 2*P / 15 degrees per hour.

If you look at the equation, you see that if we set D=0 (giving us Tan(D)=0 ) we get Cos(P) = 0, meaning that P = 90 degrees.
This value is good for every latitude (except 90).

Length of day = 2*90 = 180 divided by 15 = 12 hours.

In reality, three things happen:

1) The Sun's disk has a certain apparent radius (one quarter of a degree). Most people do not call it "night" if a part of the Sun is still above the horizon giving us light. Therefore, the real night is a bit shorter at both ends: the top limb of the Sun rises a few minutes before the centre, and it sets a few minutes after the centre.

2) Air refraction: the varying densities of air that sunlight has to go through bends the rays. At the horizon, this effect is almost one degree. The Sun always appears a little higher than the calculated altitude. More minutes of daylight morning and evening.

Most published sunrise and sunset tables are corrected for 1 and 2.

3) Your horizon may vary. The theoretical horizon is very close to the real one for a navigator at sea on a small ship (or close to the waterline).
As you get higher above the waterline, the true horizon appears to "dip". The "dip" angle is another correction. More minutes for the Sun to go from the theoretical horizon to the visible horizon.
Because this last value changes quickly with your precise location (e.g., each floor of a building has a different dip correction), tables do not take it into consideration.

So the day of the equinox is when the centre of the Sun's disk has declination of 0 exactly.

The date on which you get 12-hours each of daylight and night is normally a few days on the winter side of the official date of the equinox.

If you are surrounded by mountains, then it may be the other way arround (your "dip" would be the wrong way).

Answer 2

Because even though the sun is below the horizon at rise and set, the atmosphere is still lit and we percieve it as being daylight.

Answer 3

I was wondering about that too, given that's pretty much the definition of the equinox(sp?)...
I assume it's something to do with the fact that the Earth's orbit isn't exactly 365.25 days, so it used to be the equinox, but isn't any more.

Answer 4

It's partly because the earth's atmoshpere refracts light, so we see the sun a little before it comes up to the horizon, and also because light persists a little after sunset.

Answer 5

You have to wait a little longer for the equinox day

Answer 6

there is if you live on the equator.

Answer 7

why should there be lol