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# Tides and Water Levels

## Tidal Variations - The Influence of Position and Distance

The moon is a major influence on the Earth’s tides, but the sun also  generates considerable tidal forces. Solar tides are about half as  large as lunar tides and are expressed as a variation of lunar tidal  patterns, not as a separate set of tides. When the sun, moon, and Earth  are in alignment (at the time of the new or full moon), the solar tide  has an additive effect on the lunar tide, creating extra-high high  tides, and very low, low tides—both commonly called spring tides. One  week later, when the sun and moon are at right angles to each other, the solar tide partially cancels out the lunar tide and produces  moderate tides known as neap tides. During each lunar month, two sets  of spring tides and two sets of neap tides occur (Sumich, J.L., 1996).

 Together, the gravitational pull of the moon and the sun affect the Earth’s tides on a monthly basis. When the sun, moon, and Earth are in alignment (at the time of the new or full moon), the solar tide has an additive effect on  the lunar tide, creating extra-high high tides, and very low, low tides  — both commonly called spring tides. One week later, when the sun and  moon are at right angles to each other, the solar tide partially  cancels out the lunar tide and produces moderate tides known as neap  tides. During each lunar month, two sets of spring and two sets of neap  tides occur.

Just as the angles of the sun, moon and Earth affect tidal heights over  the course of a lunar month, so do their distances to one another. Because the moon follows an elliptical path around the Earth, the  distance between them varies by about 31,000 miles over the course of a  month. Once a month, when the moon is closest to the Earth (at  perigee), tide-generating forces are higher than usual, producing  above-average ranges in the tides. About two weeks later, when the moon  is farthest from the Earth (at apogee), the lunar tide-raising force is smaller, and the tidal ranges are less than average. A similar  situation occurs between the Earth and the sun. When the Earth is  closest to the sun (perihelion), which occurs about January 2 of each  calendar year, the tidal ranges are enhanced. When the Earth is  furthest from the sun (aphelion), around July 2, the tidal ranges are  reduced (Sumich, J.L., 1996; Thurman, H.V., 1994).

*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).*

### Tides and Water Levels

              - Welcome
              - What are Tides?
              - What Causes Tides?
              - Gravity, Inertia, and Bulges
              - Changing Angles and Tides
              - The Frequency of Tides
              - Tidal Variations
              - Types and Causes of Tidal Cycles
              - What Else Affects Tides?
              - Monitoring the Tides
              - Old System
              - New System
              - References
              - Roadmap to Resources 
              - Subject Review

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