Expert: Philip Stahl - 6/3/2008

Question
Philip:

If the earth was not tilted, would the daylight period on the earth be the same year round?  My first thought is yes it would be.... but I am not sure.

Answer
Hello,

If Earth was not tilted, then effectively there'd be no ecliptic. Or, to put it another way, the ecliptic would coincide with the celestial equator.  Thus, if one were on the Earth's equator (lived there) the path taken by the Sun would always be across an imaginary line in the sky passing directly overhead at one's zenith. The time taken for this passage at the equator would be 12 hrs. (To transit from East to the West cardinal point of the horizon).

Now, what this means for any given latitude, is that the Sun would trace out a diurnal circle in the sky proportional to the latitude circle (for O deg) *at that location*.

For example, if you were at lat. 45 N, the Sun would be visible for you making a circle in the sky 45 degrees SOUTH of your zenith. (This is where the ecliptic-celestial equatos resides) The length of this day would be reduced by a factor cos(theta) = cos (45) ~ 0.7 from the equatorial value of the day.

Or, 12 hr. x (0.7) = 8.4 hrs.

Thus, the length of EACH day at THIS latitude would equal 8.4 hrs.

Thus, the daylight period would always be the same for EACH latitude, but not the same for each location all over the world.

Take another case, the North Pole (lat. 90).

By this rule, cos 90 = 0 and R(90) - or diurnal radius at lat. 90 =
R(E) x cos 90

that is, the radius at the equator (6400 km) x cos 90 = 0.

Thus there is no length of day! WHY? Because the Sun is never above your horizon!

Again, at the N. Pole the 90 degree line (for latitude) goes overheaed at your zenith. The Sun is always at 0 degrees which is 90 degrees SOUTH of this. But 90 degrees South puts it right on the Southern horizon, viewed from the N. Pole.

Hence, no length of day since it has no altitude above the horizon!

Hope this helps, but if not be sure to ask a follow -up.