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The analemma is caused by two phenomena: the tilt of the earth on its axis and the elliptical shape of the earth's orbit. An ellipse is similar to an oval. If the earth moved in a circle, it would travel at the same speed all year, but because it moves at different speeds at different times during the year, the sun appears to be "moving" in the sky at the same time each day. The sun will drift in the westward direction when the speed is less than if it was on a circular orbit, and it will drift in the east ward direction when the speed is greater than if it was on a circular orbit. The earth is tilted approximately 23.5 degrees on its axis. This causes the top half of the earth to be closer to the sun while the bottom half of the earth is farther from the sun, and vice versa. The tilt will cause the sun to be more toward the north in the summer and more toward the south in the winter. These two factors combined form the figure-eight shape known as the analemma. |
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