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The elliptical orbits of the moon around the Earth and the Earth around the sun have a substantial effect on the the Earth’s tides. Once a month, at perigee, when the moon is closest to the Earth, tide-generating forces are higher than usual, producing above average ranges in the tides. About two weeks later, at apogee, when the moon is farthest from the Earth, the lunar tide-raising force is smaller, and the tidal ranges are less than average. When the Earth is closest to the sun (perihelion), around January 2 of the calendar year, tidal ranges are enhanced. At aphelion, when the Earth is furthest from the sun, around July 2, tidal ranges are reduced (Sumich, J.L., 1996; Thurman, H.V., 1994).

Diagram of earth's elliptical orbit
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