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 2 January, tidal ranges are enhanced. At aphelion, when the Earth is furthest from the sun, around 2 July tidal ranges are reduced.
Last Update: 6 Jul 2011 (AMB)