From Glossary of Meteorology
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The point on the orbit of the earth (or any other body in orbit about the sun) that is farthest from the sun; the opposite of perihelion.

At present, aphelion occurs about July 1, when the earth is about three million miles farther from the sun than at perihelion, but the seasons in which aphelion and perihelion fall undergo a cyclic variation with a period of twenty-one thousand years. The date of aphelion passage is advancing slowly (toward dates later in the calendar year) at a rate of about one-half hour each year. This is a consequence of the eastward rotation of the line of apsides at a rate of about 11 sec of arc per year and the precession of the equinoxes of about 50 sec of arc per year, for a total of 61.9 sec of arc per year.
See apocenter, apogee.