When you get into a tight place and everything goes against you, till it seems as though you could not hang on a minute longer, never give up then, for that is just the place and time that the tide will turn. -Harriet Beecher Stowe
Last week, our longtime reader Pamela asked if I could explain how the tides work. As you all know, when the tide comes in at the ocean, the water level appears to rise (and can do so significantly), while at low tide, the water level appears to drop.
This goes in a cycle twice per day, with the ocean level reaching its highest point twice daily (high tide), having the water recede over a period of six hours until it reaches its lowest level (low tide), and then having the water level rise again over a period of another six hours until it reaches the next high tide. Variations in the height of the water level are typically on the order of three meters (maybe ten feet) each day, depending on a couple of factors, which I’ll go into below.
The reason we have any tides at all are twofold: the Earth is pretty big and gravity cares how far away you are. The farther away you are from something, the weaker gravity’s pull is on you. If you were to take a look at our Solar System, and you were to move the Earth out to where Pluto is, you’d find that the force of gravity from the Sun on the Earth would be an astounding 1,600 times weaker than it is today, as Pluto is 40 times as far away as Earth is from the Sun!
If you were to look at everything in our Solar System and ask what affects the Earth the most, gravitationally, you’d think to look at two things: the Moon, because it’s massive and it’s very, very close to us, and the Sun, because it’s extremely massive, even though it’s quite far away. Let’s start by considering the Moon.
The Earth is quite far from the Moon, at an average distance of 384,400 km. When we speak about this distance, however, we are talking about the distance from the center of the Earth to the center of the Moon. But one edge of the Earth will always be closer to the Moon by 6,370 km (the radius of the Earth), and the opposite edge will always be farther from the Moon by the same amount. This means — after a little math — that the force of gravity of the Moon on the far side of the Earth is about 3.2% weaker at the far edge of the Earth than it is at the center of the Earth, and about 3.4% stronger at the edge of Earth nearest the Moon than it is at the center. This difference in forces between the near edge, the center, and the far edge defines what we call tidal forces.
This means the effect of the Moon’s gravity on Earth is to try to flatten it a little bit at the poles and wherever Moonset/Moonrise is occurring, and to stretch it at its nearest point (when the Moon is directly overhead) and its farthest point (exactly 12 hours from the Moon’s apex). This force is weak enough that it wouldn’t be a big deal at all if the Earth were simply a solid ball; the tidal forces from the Moon are unable to stretch rocks and dirt by more than a few millimeters. But the Earth is covered in water, which changes its shape extremely easily!
So while the solid ground of the Earth remains in its roughly spherical shape, the oceans bulge by just a few meters in two spots around the equator: at the point closest to the Moon and at the point farthest from the Moon. As the solid ground rotates, each point on the Earth passes through the side closest to the Moon and the side farthest from the Moon once per day: these are your two high tides.
The two times that correspond to Moonrise and Moonset are your two low tides per day. And the closer to the equator you are, the more severe your tides are, while the closer to the poles you are, the less drastic your tides are!
But the Moon isn’t the only gravitational body in our Solar System affecting the tides on Earth. While none of the other planets, moons, asteroids or comets in the Solar System matter, the Sun does!
The tidal forces from the Sun are weaker than those from the Moon, but are still quite strong, causing tides that are about 30% as strong as the Moon’s. When the Sun and the Moon are lined up, during a New Moon and during a Full Moon, you get the highest high tides and the lowest low tides, known as Spring Tides.
But when the Sun and Moon are at right angles to each other (during the Moon’s first and last quarter, or when it appears half-full), you get the lowest high tides and the highest low tides, known as Neap Tides.
In fact, if you’re meticulous, you can measure the water level over a long period of time, and can see not only the high tides and the low tides, but also where the Spring Tides and Neap Tides occur. Take a look at this data from Bridgeport, Connecticut.
And that’s how tides work! I freely admit that there are small, subtle details that come into play if you want to predict the times and heights of the tides extremely accurately. But just by considering the gravity of the Sun, Earth and Moon, and by calculating the force on the oceans, you can do an incredible job of predicting all of the above about the tides. Thanks to Pamela for a riveting question, and I hope you all enjoyed the answer!