[thedoc (OP)]

Michael asked the Naked Scientists:
   
Is there anywhere in the world where the longest day the same length as the longest night and if not why not.

I started looking just out of interest at London and Edinburgh and each were different by about 7 minutes. So I thought maybe is the same at 45 degrees north as its half way, yet again there was a difference of a few minutes.

Love the radio show.

Your help would be very much appreciated.
Michael
   
What do you think?

[evan_au]

There is a nice interactive graphic daylength calculator here (move the latitude slider on the right):
http://astro.unl.edu/classaction/animations/coordsmotion/daylighthoursexplorer.html

If the Earth were an airless sphere on a circular orbit around the Sun, the length of the longest day would equal the length of the longest night, everywhere on the planet (bearing in mind that the length of a "day" needs a generalised definition in polar regions).

Atmosphere: The atmosphere lets us see the light of dawn before the center of the Sun's disk has risen, and see the light of sunset well after the center of the Sun has set. This would tend to make the longest day longer than the longest night.

Orbital Eccentricity: The Earth orbits the Sun in about 365.25 days, and the orbital motion of the Earth affects the length of the day by 24 hours/365.25=3.9 minutes per day, if the Earth were on a circular orbit.  However, the Earth's orbit is squashed by about 1.7%, into an ellipse. Earth's closest approach to the Sun is in January (perihelion), and the Earth has it's fastest angular velocity in its orbit at this time. This will add to maximum daylength in one hemisphere, and subtract from maximum daylength in the other hemisphere.

Local obstacles: At the trivial end of the scale, the Earth isn't a sphere, and if you have a high mountain range in the west, the Sun will set much earlier, tending to make the night longer than the day.

But overall, people living on the equator have very uniform daylength throughout the year - it doesn't vary enough to be noticeable.

[evan_au]

According to the US Naval Observatory:

Sunrise and sunset conventionally refer to the times when the upper edge of the disk of the Sun is on the horizon.

This is a different edge at sunrise and sunset.

The Sun's disk is 0.5 degrees wide, as seen from Earth. Since the Earth rotates 360° in 24 hours, it takes around 4.2 minutes for the Sun to rise or set.

In addition, atmospheric bending means:

the sunset appears to occur when the Sun’s disk is already about one diameter below the horizon.

Since this effect occurs at both sunrise & sunset, this suggests that in most locations (even a tropical island, right on the equator), the longest day should be around 8 minutes longer than the longest night.

If you defined a day and a night as having a maximum length of 24 hours, all polar regions would experience days and nights of exactly 24 hours. (Of course, some different definitions of the maximum length of a day will exclude all polar regions, since the Sun spends a long time apparently just on the horizon...)