You’ve likely heard that the moon is responsible for tides on Earth, but have you ever actually thought about why? While the moon has the biggest effect on Earth’s tides, the sun and the ocean’s shape also have effects that increase or decrease the tide. The effects of the moon on Earth’s tides are called moon tides. Moon tides are the main focus of this article.
Everything in space exerts gravity on everything else. Since the moon is the closest celestial body to Earth, it exerts the greatest force on the Earth’s oceans. The force of the moon’s gravity pulls both the Earth and the water on Earth’s surface closer to the moon. When water is pulled towards the moon the tide goes up. Whatever part of the Earth is closest to the moon experiences the most intense lunar gravity, and therefore the most intense moon tide. But the gravitational pull of the moon doesn’t stop there! The moon also pulls the Earth slightly towards it, pulling the side of the Earth closest to the moon with the most force, and the side of the Earth furthest away from the moon with the least amount of force. The water on the side of the Earth furthest from the moon experiences the least amount of lunar gravity.
There are two high tides each day. As the Earth completes its 24-hour, day-long rotation, the side of the Earth which is closest to the moon constantly changes. This is why a high tide happens every 24 hours (actually, it’s 24 hours and 50 minutes, but more on that later).
Each day there is a second, lesser high tide. This second high tide occurs on the side of Earth furthest from the moon. Initially, that doesn’t seem to make much sense, right?
While the moon pulls all the water on Earth towards it, it also pulls the Earth slightly towards the moon. Since the water furthest from the moon experiences little lunar gravity, it is being pulled less than the rest of the Earth. This means that, because the water farthest from the moon gets pulled the least, the water is higher compared to the Earth.
Additionally, as the Earth and moon spin around each other, the water furthest from the moon experiences centripetal force. This means that the water furthest from the moon is being spun as the Earth travels around the moon with more force than the water closest to the moon. Together, the centripetal force and the difference in gravity between the sides of the Earth facing the moon create a tidal bulge on the opposite side of Earth from the moon.
The areas of the ocean halfway between the side of the Earth closest to the moon and the side furthest away experience low tides. These low tides occur because neither the lunar gravity is particularly strong, nor the effect from the centrifugal force. Low tides occur halfway between high tides, or about every six hours.
This is how the moon tides work on a day-long period.
Moon tides also depend on a month-long time scale. When the moon is full and new, Earth experiences spring tides. When the moon is half full (a waxing or waning gibbous), the Earth experiences neap tides. Neap tides are weak, meaning the difference between the high and low tides is small. Spring tides, on the other hand, are strong. Spring tides and neap tides, while correlated with the moon, are technically not moon tides; they are solar tides.
The sun also exerts gravity on Earth and contributes about half as much to the tides as the moon. The effect of the sun on tides is called solar tides. Acting together, the moon and sun create large tidal forces.
When the moon is new, it is between the Earth and the sun. This means the lined-up forces from the moon and the sun add together to create an extra strong pull. When the moon is full, it is on the opposite side of the Earth from the sun. Since the sun and moon each pull on an opposite side of Earth, this also creates spring tides.
Bonus fun fact: When three celestial bodies are lined up they are in ‘syzygy.’ Can you believe a word like that exists?
When the moon is half full, the moon and sun make a right angle, with the Earth in the middle. In this position, the moon tides and sun tides interfere with each other, creating small neap tides.
It takes the moon 29.5 days to completely orbit the Earth or to go from full moon to full moon. This is called the moon’s “synodic” or lunar month. Kind of like how the hour and second hand on a clock move at different rates, the moon slightly outpaces the Earth’s 24-hour rotation. This means that the moon will be in slightly further along in the sky once the Earth does its complete 24-hour rotation. If you divide 24 hours by 29.5 days (the moon’s orbit around Earth), you get about 50 minutes. This is why lunar tides are 24 hours 50 minutes and why the time of the high tide gets later by 50 minutes every day.
Not only does the moon orbit around Earth, but the moon also moves through its own, elliptical orbit. When the moon is closest to Earth in its ellipse, it exerts more gravity and, therefore, higher tides. This position is called the perigee. In contrast, when the moon is further away from Earth in its elliptical orbit, tides are unusually small. This position is called the apogee.
This elliptical orbit does not match the moon’s orbit around the Earth. As mentioned above, the moon’s orbit around the Earth, or synodic month, is 29.5 days. The elliptical orbit, or anomalistic month, is 27.5 days. This means that each time the moon is full, it will be in a slightly different place within its elliptical orbit.
When the moon is full or new and in its perigee, the moon is a supermoon. The gravity of the combined sun and moon, plus the closer-than-usual position of the moon make for exceptionally high tides.
Interestingly, the Earth also has a slightly elliptical orbit around the sun. Similarly to the moon’s orbit, the tides are larger when the Earth is closest to the sun, around the New Year. The tides are smaller in early July, when the Earth is furthest from the sun.
The moon also has a wobble in its orbit that changes the magnitude of tides on 18.5 year scale. This wobble will make tides higher than usual in the 2030s.
Not all tides are created equal. In the Bay of Fundy in Canada, the difference between high and low tide is 40-50 feet. Many coastal areas only have a few feet of difference between high and low tide.
While not the scope of this article, the shape of Earth’s oceans, coastline, and ocean floor all contribute to the vast differences between tides in different coastal areas.
The highest tide would happen when many of these factors occurred at the same time. First, the moon must be a supermoon, where the moon is new and at its perigee. If the super moon this happened around the New Year, when Earth is closest to the sun, it would be a massive tide. If this combined with a storm, heightened sea levels from climate change, and the moon’s 18.5 year wobble, it would likely be the highest water level possible. Tides such as this pose great issues to low-lying coastal areas, which are prone to devastating tidal flooding.
Moon tides have lead to the evolution of special tidal critters and all sorts of interesting animal behavior. Next time you are at the beach make sure to pay attention to the moon, how the time and height of the tides change from day to day, and how the birds, and fish use the tides to their advantage.
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