The dates on which day and night are each 12 hours occur a few days before and after the equinoxes. So why are sunrise and sunset not exactly 12 hours apart on the Equinox?ĭay and night are not exactly of equal length at the time of the March and September equinoxes. Everyplace in the world has about 12 hours of daylight and 12 hours of night. This corresponds to the Fall and Spring Equinox (equinox is Latin for "equal night"). For the Southern Hemisphere, this is reversed.įor both hemispheres, the earth is 90 degrees away from the sun around March 21 and then again around September 21. This corresponds to the Winter and Summer Solstice (solstice is Latin for "the sun stands"). Since the earth's axis is tilted, the sun is higher when you are on the part of the earth where the axis points more towards the sun, and lower on the part of the Earth where the axis points away from the sun.įor the Northern Hemisphere, the axis points most toward the sun in June (specifically around June 21), and away from the sun around December 21. When the sun is lower in the sky, the light gets more spread out over the surface of the earth, and less heat (per square centimeter) can be absorbed.
When the sun is overhead, the light is falling straight on you, and so more light (and more heat) hit each square centimeter of the ground. In other words, the amount of light per square centimeter drops (the number of square centimeters increases, while the total amount of light stays the same). All the light is still in that ellipse, but the ellipse is spread out over more paper. Now slowly tilt the paper, so the circle elongates into an ellipse. All the light from the flashlight is in that circle. Shine the light from the flashlight straight onto the paper, so you see an illuminated circle. Why should this tilt of the Earth's axis matter to our weather? To understand this, take a piece of paper and a flashlight. This means that on this date, day and night have about the same length: 12 hours each, more or less. Midway between these two times, in spring and autumn, the spin axis of the earth points 90 degrees away from the sun. Since the tilt of the axis is 23 1/2 degrees, the North Pole never points directly at the Sun, but on the summer solstice it points as close as it can, and on the winter solstice as far as it can. When the earth's axis points away, winter can be expected. When the earth's axis points towards the sun, it is summer for that hemisphere. The earth's spin axis is tilted with respect to its orbital plane. As the earth spins on its axis, producing night and day, it also moves about the sun in an elliptical (elongated circle) orbit that requires about 365 1/4 days to complete.
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