|MadSci Network: Astronomy|
Dear Dermot, You have asked a very good and thought provoking question. I think that the answer is 'yes', but lets think about it using a little geometry. First here are the assumptions: 1) We are going to ignore surface features like mountains 2) We are going to assume the Earth's orbit around the Sun is a perfect circle 3) We are going to assume a year is 365 days long (its actually 365.25 days) 4) We are going to assume that there are an equal number of days and nights for all seasons throughout the year. Now, consider the fact that every point on the Earth has a point that is symmetrically opposite to it through the center of the Earth. For example, a point at a 40 degree latitude in the Northern Hemisphere will correspond to a point at a 40 latitude in the Southern hemisphere. One of these points is dark when the other is light, always. Both of these points have the same latitude, and during the year will have the same seasons. Therefore each of the points will have the same amount of light and dark. Since one point is in the light while the other is in the dark, one of the points has the same amount of light as the other has dark. The symbols below represent the amount of time a point on Earth is in the light or the dark throughout the year: L1 = point 1 in light D1 = point 1 in dark L2 = point 2 in light D2 = point 2 in dark Remember, points 1 and 2 are at the same latitude but in different hemispheres. Now, lets make some relationships between the symbols: 1) L1 = D2 - while point 1 is in the light, point 2 is in the dark 2) L2 = D1 - while point 1 is in dark, point 2 is in light 3) L1 = L2 - points at the same latitude in opposite hemispheres receive the same amount of light in a year 4) L1 + D1 = 100% - it is either light or dark Therefore, L1 = D1 = L2 = D2 - from 1,2 & 3 5) Substitute the above result into equation 4 then L1 + L1 = 100% L1 = 50% This works for any pair of points on earth, so all points on Earth get 50% light over the course of a year. The length of any individual day is dependent on the season and the latitude. Also, this discription is just considering the amount of time each location is in the light or dark and not the amount of light incident on the surface of the Earth. At the higher latitudes, the Sun might be up for the same amount of time as for the lower latitudes but it will be lower in the sky. The amount of sunlight incident on the ground will be much lower than if the Sun were overhead. I hope this is helpful. Angelle Tanner UCLA Astronomy
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