The tide comes in and out twice a day, sometimes a little higher, sometimes a little lower. It’s so much a part of our daily lives that it’s easy to forget that what we are witnessing is gravity in action. The tide is the result of gravitational forces of massive bodies — the Earth, Moon, and Sun — interacting in powerful ways.
Most people know that the tides have something to do with the Moon. Here is a closer look at exactly how they relate.
The Moon orbits our planet because the Earth is larger and more massive than it is — our gravity has captured it. But the Moon has gravity, too, and it tugs back a little on the Earth. The Moon’s gravity is relatively weak: you won’t feel that tug. But its larger-scale effects are noticeable.
Because gravity weakens with distance, the effects of the Moon’s gravity vary depending on where you are in relation to the Moon. For example, if the Moon is high in your sky, its gravity is stronger for you than it is for someone standing on the opposite side of the Earth, 7,900 miles farther from the Moon. That difference is the key to tides.
If you take the Moon’s gravitational force for a given location on Earth and subtract from it Earth’s own gravitational force for the same location, the result is the tidal force for that place. It is a squeezing and pulling force, which causes Earth and everything on it — rock, water, air, people — to be drawn toward the part of the Earth that’s closest to the Moon, forming what’s called a tidal bulge. At the same time, a second tidal bulge forms on the exact opposite side of the Earth, farthest from the Moon.
This may seem counterintuitive. But here is how it works in more familiar terms. As the part of the Earth we’re standing on rotates, and the Moon rises in the sky, the ground, oceans, and even the atmosphere stretch toward it — we’ve entered the tidal bulge. Solid rock isn’t very elastic and so it moves only an inch or so under this force. But the ocean flows along.
As the Moon climbs higher in the sky, the water creeps up beach and pier pilings. When the Moon reaches its zenith, its highest point in the sky for our location, we are in the center of the tidal bulge — high tide. (The timing isn’t perfect. Because of inertia, local shorelines, seafloor topography, and currents, high tide usually lags behind the Moon a little.)
Earth keeps rotating and the Moon sinks towards the horizon. We pass through the center of the tidal bulge and towards its edges; the tide goes out. About six hours after high tide, we’re at low tide. And another six hours after that, the Moon reaches its zenith on the opposite side of the planet from us. The people there are experiencing high tide, and so are we — the second high tide of the day is at the antipodal tidal bulge. And so it goes.
There are solar tides, too. That’s because the sun also exerts a differential gravitational force on Earth. But because the sun is much farther away (93 million miles, as opposed to the Moon, which is only 200,000 miles away), solar tides have a much lower amplitude. We generally don’t notice them.
But when the Moon lines up in front of or behind Earth with respect to the Sun, which happens twice every 27 days, it creates a combined tidal force with the sun that is slightly greater than normal. That’s when we get a spring tide. Spring tides having nothing to do with the seasons. They happen all year round. The origin of the term seems to be in the observation that when the high tide is higher than usual, it seems to spring forth.
While on the subject of variations in the tides, there is also the neap tide to consider. That’s when the Moon is on the other side of the Earth with respect to the Sun, and there is a slight canceling effect. This also happens twice every 27 days. That’s when high tide is lower than usual, and low tide is higher than usual.
The tide is a tangible way to appreciate the sky. And maybe watching the sky will make you wiser to when and where to put your beach towels or build a sandcastle. Until then, clear skies!
