Tides change monthly primarily due to the Moon's varying distance from the Earth.
The Moon's orbit around the Earth is not a perfect circle; it's an ellipse. This elliptical path means the distance between the Earth and the Moon fluctuates throughout the month, varying by approximately 31,000 miles.
Key Monthly Tidal Change:
- Perigee: Once a month, the Moon reaches its closest point to the Earth in its orbit. This point is called perigee.
- Effect on Tides: According to the provided information, "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."
- Result: This stronger gravitational pull at perigee results in tides that have a larger difference between high and low water levels compared to the average monthly tides. These are sometimes referred to as "perigean tides."
Understanding the Monthly Cycle:
The monthly tidal cycle is influenced by this distance variation. While the daily (and semi-daily) tides are primarily driven by the Moon's phase (new/full causing spring tides, quarter moons causing neap tides) and its position relative to the Earth and Sun, the magnitude of these tides is modulated monthly by the Moon's distance.
Think of it this way:
- Daily/Fortnightly: The phase of the Moon (new, quarter, full) determines if you have stronger-than-average tides (spring tides) or weaker-than-average tides (neap tides) every two weeks.
- Monthly: The distance of the Moon from Earth (perigee/apogee) influences how strong those spring or neap tides are. A spring tide occurring near perigee will be stronger than a spring tide occurring near apogee (when the Moon is farthest).
Practical Impact:
- Above-average tidal ranges at perigee can lead to higher high tides and lower low tides, potentially affecting coastal activities, navigation, and even contributing to coastal flooding during storms, especially when combined with other factors like strong winds.
In summary, the main monthly change in tides is driven by the change in the Moon's distance from Earth due to its elliptical orbit, resulting in higher tide-generating forces and above-average tidal ranges when the Moon is at perigee.
