It take the Earth 365.242 days for the Earth to orbit the Sun. The Moon completes one orbit around the Earth in 27.3 days (called the sidereal month). However, due to the Earth's motion around the Sun it has not finished a full cycle until it reaches the point in its orbit where the Sun is in the same position (29.53 days) - this is the time from one full moon to the next.
However, both the Earth and the Moon have orbits that are slightly elliptical (not circular). This has an influence on the intensity of tide cycles (Figure 11.6).
- Perigee is when the Moon is closest to the Earth.
- Apogee the Moon the farthest from the Earth.
- Perihelion is when Earth is closest to the Sun (in early January).
- Aphelion Earth is farthest from the Sun it is called (in early July).
Because the Moon has a greater influence on tides, the highest tides happen at perigee when there is a full or new moon. This happens a couple times a year and are called king tides. King tides occur when the Earth, Moon and Sun are aligned at perigee and perihelion, resulting in the largest tidal ranges seen over the course of a year.