The Moon: Master of the Tides
For thousands of years, coastal communities have observed the intimate connection between the Moon and the sea. Ancient civilizations recognized that tides followed the Moon's rhythms, though the scientific explanation would not come until Isaac Newton formulated his theory of gravitation in 1687. Today, we understand in precise detail how the Moon's gravitational pull creates the tidal movements that shape our coastlines.
The Moon exerts a gravitational force on every part of the Earth, but its effect on the oceans is most visible because water is fluid and can respond freely to this force. The Moon's gravitational influence on Earth's tides is approximately 2.2 times stronger than the Sun's, despite the Sun being vastly more massive, because the Moon is so much closer to us.
Understanding the Lunar Cycle
The lunar cycle — also called the synodic month — lasts approximately 29.53 days, during which the Moon passes through eight distinct phases. Each phase represents a different geometric relationship between the Sun, Moon, and Earth, and directly influences the character of our tides.
New Moon
During a new moon, the Moon is positioned between the Earth and the Sun. All three bodies are approximately aligned (a configuration called syzygy), meaning the gravitational forces of the Moon and Sun pull in the same direction. This alignment produces spring tides — the largest tidal ranges of the lunar cycle. The Moon is invisible in the night sky during this phase because its illuminated side faces away from Earth.
First Quarter
About seven days after the new moon, the Moon has traveled one quarter of its orbit and appears as a half-moon in the sky. At this point, the Sun and Moon are at right angles relative to the Earth (a configuration called quadrature). Their gravitational forces partially oppose each other, producing neap tides — the smallest tidal ranges. High tides are lower and low tides are higher than average.
Full Moon
At full moon, the Earth is positioned between the Moon and the Sun. Once again, the three bodies are approximately aligned in syzygy, though this time the Moon is on the opposite side from the Sun. The gravitational forces still combine constructively, producing another set of spring tides with large tidal ranges comparable to those at new moon.
Third Quarter
The Moon has completed three quarters of its orbit and again appears as a half-moon, but now the opposite half is illuminated compared to the first quarter. The Sun and Moon are again at right angles, producing another period of neap tides with minimal tidal ranges.
The Moon's Orbit: Perigee and Apogee
The Moon's orbit around the Earth is not a perfect circle — it is an ellipse. This means the distance between the Moon and the Earth varies continuously throughout each orbit, with significant consequences for tidal forces.
Perigee: When the Moon Is Closest
Perigee is the point in the Moon's orbit when it is closest to Earth, at a distance of approximately 356,500 kilometers. At perigee, the Moon's gravitational pull is stronger than average, increasing tidal forces by roughly 20%. When perigee coincides with a new or full moon, the resulting tides — known as perigean spring tides — can be 5 to 15 centimeters higher than typical spring tides. The Moon completes one orbit (perigee to perigee) in approximately 27.55 days, known as the anomalistic month.
Apogee: When the Moon Is Farthest
Apogee is the opposite extreme — the point when the Moon is farthest from Earth, at approximately 406,700 kilometers. At apogee, the Moon's gravitational pull is weaker, resulting in below-average tidal forces. Spring tides occurring near apogee will have smaller ranges than those occurring near perigee.
The Variation in Distance
The difference between perigee and apogee is substantial — about 50,200 kilometers, or roughly 14% of the average Earth-Moon distance. This variation means that tidal forces at perigee are approximately 20% stronger than at apogee, creating a noticeable month-to-month variation in spring tide magnitudes.
Supermoons and Their Effect on Tides
A supermoon occurs when a full moon or new moon coincides with the Moon being at or near perigee. The term was coined by astrologer Richard Nolle in 1979, though the astronomical phenomenon has been understood for much longer.
What Makes a Supermoon Special?
During a supermoon, the Moon appears approximately 14% larger and 30% brighter in the sky compared to when it is at apogee. More importantly for coastal communities, the combined effect of the full/new moon alignment and the close perigee distance creates particularly strong tidal forces.
Supermoon Tides
Tides during a supermoon are called perigean spring tides. They typically produce tidal ranges that are 5-8% larger than average spring tides. While this may sound modest, in areas with already large tidal ranges — like the Bay of Fundy or the English Channel — this can translate to an additional 30-50 centimeters of water level difference, which is significant for navigation, coastal flooding risk, and marine activities.
When Do Supermoons Occur?
Supermoons occur several times per year, though the exact number depends on the definition used. The closest perigee-full moon alignments — producing the most dramatic supermoons — typically occur two to four times annually. The effects on tides are most pronounced when the supermoon occurs near an equinox (March or September), as the Sun's equatorial position adds to the tidal forces.
The Lunar Nodal Cycle: An 18.6-Year Influence
Beyond the monthly cycles, the Moon has a longer-term influence on tides through its nodal cycle. The Moon's orbital plane is tilted about 5.1 degrees relative to the Earth's orbital plane around the Sun, and the points where these planes intersect (the nodes) gradually rotate over an 18.6-year cycle.
This cycle subtly modulates tidal ranges over nearly two decades. At the peak of the nodal cycle, the Moon can reach higher declinations (up to 28.6 degrees from the equator), producing slightly different tidal patterns. At the minimum, the Moon's maximum declination is limited to about 18.3 degrees. Scientists and coastal engineers account for this cycle when planning long-term coastal infrastructure.
Practical Applications: Using Moon Phases to Plan Coastal Activities
Understanding the relationship between moon phases and tides has practical value for numerous activities:
Fishing
Many experienced anglers plan their fishing trips around the lunar cycle. The increased water movement during spring tides (new and full moons) tends to stimulate fish feeding. The days around the full moon are often considered the best for night fishing, as the moonlight illuminates the water and triggers feeding in many species.
Surfing
Surfers use moon phase knowledge to predict tidal conditions. Some surf breaks work best at specific tide levels, and knowing whether you are in a spring or neap tide period helps in choosing the right time to paddle out.
Shellfish Gathering
The lowest tides of the month — occurring during spring tides — expose the most seabed, making these the optimal times for gathering mussels, clams, and other shellfish. The very lowest tides of the year occur during equinoctial spring tides in March and September.
Photography and Nature Observation
Low spring tides reveal rock pools, marine organisms, and coastal features that are normally hidden underwater. Combined with the dramatic lighting of a full moon or the golden hours around sunrise and sunset, these conditions create exceptional opportunities for coastal photography.
Moon Phase Calendar and Tides
By keeping track of moon phases, you can anticipate tidal patterns weeks in advance without consulting a tide table:
- New Moon and Full Moon: Expect spring tides with the largest tidal ranges 1-2 days later.
- First Quarter and Third Quarter: Expect neap tides with the smallest tidal ranges 1-2 days later.
- Check for perigee: If the new/full moon falls near perigee, expect even larger than normal spring tides.
- Equinox months: March and September spring tides will typically be the year's largest.
The Moon has governed the rhythm of our tides since the formation of the Earth-Moon system over four billion years ago. By understanding this celestial relationship, we can better predict, prepare for, and appreciate the extraordinary power of our ocean's tides.