Tides why twice a day

Based upon ancient observations and correlations, Kepler thought the Moon must cause the tides. Several decades later, Isaac Newton published his famous Principia. The book was most famous for describing the laws of gravity, and these same laws finally explained the tides. When it comes to the effects of gravity, there are three major players we have to consider: the Earth, the Moon and the Sun.

As a result, they end up in orbit around each other. If they were the same mass, the centre of their mutual orbit would be half way between them. But the Earth is 81 times more massive than the Moon, so the centre of their orbit is much closer to the Earth — in fact it lies at a point inside the Earth about three-quarters the distance from the centre to the surface. The pulling very slightly elongates the shape of both spheres.

But the distortion is trivial. On the other hand the Earth is covered by a thin layer of ocean, which is very easy to distort. As the Moon pulls on the Earth, the ocean bulges towards it. But remember the Earth is also spinning on its own axis once a day. So once a day the kids at the beach are under the Moon, the bulging high tide, and the great swell that comes with it.

The lowest-order term in the expansion for g is the steady gravitational pull. This is unobservable or at best uninteresting because we the observers and our frame of reference the earth are both freely falling under the influence of this steady pull. This term scales like. We can keep playing this game. There will be a third term, which we can call the hyper-tide driving term. It scales like. As mentioned above, this is small unless we are considering the case of an asteroid that comes very close to the earth.

You may be wondering whether the tidal stretching lowers the density of the earth, or whether the pinch raises the density of the earth. The answer is neither one. It is easy to show that when averaged over the whole earth, the amount of stretching exactly balances the amount of pinching. Tides tend to re-arrange stuff, with no change in density. The lowest-order contribution to the gravitational field is the spatially-uniform piece:.

The average outward component of this averaged over the whole earth gives zero. This should be obvious by symmetry. The next term in the Taylor series is the tide-producing term. Call it h. The average outward component of this, averaged over the surface of the earth S is:. By the divergence theorem this is can be expressed as an integral over the volume of the earth V , namely:. Usually, we see two high tides and two low tides per day.

However, under some conditions we see that one of the high tides is higher than the other, and indeed in extreme cases there is just one high tide and one low tide per day.

How can this be? This is pretty simple. The tides squash the potential into a prolate ellipsoid. The potential bulges up at the point right under the moon and also bulges up at the antipodal point. As mentioned in section 5. Since the year and the month are long compared to the day, we can say to a good approximation that the bulge in the potential stays in one place dictated by the moon and sun while the earth rotates within this potential. They also tend to forget that not everybody lives on the equator.

The sun is directly overhead. The sun is in the constellation Gemini, right near the Taurus border, near the star cluster M At the very same instant, Joe is standing at the antipodal point, namely Antofagasta, Chile.

It is midnight there, and the full moon is directly overhead, in the constellation Sagittarius. The moon will not be directly overhead. Not even close! It will be in Sagittarius, about 45 degrees south of overhead. The earth has rotated around its tilted axis. Hainan will be directly underneath the double star 95 Herculis. This will not be associated with the bulge in the potential.

It will sit in the more-or-less neutral regime between the bulge and the pinch. So Hainan will get a diurnal component to the potential: large at noon, neutral at midnight. The situation is similar at Antofagasta or any other location at comparable latitudes, north or south. If you find some location where the local geography allows the water to slosh with a resonant frequency near 24 hours, you will have a huge diurnal tide. This is an example, i. The general case is, of course, quite a bit more complicated.

In particular, there will always be a semi-diurnal component to the potential, in addition to and larger than whatever diurnal component there may be. So the usual case is that there will be two high-tide potentials and two low-tide potentials per day, but one of the high-tide potentials will be higher than the other. For the same reason, the moon moves north and south with the seasons. Professional tide forecasters keep track of dozens of different contributions to the tide-producing potential.

A list of some of the most-significant components including period, strength, and conventional name may be found in reference 2. A tutorial, including a discussion of how these components are used, may be found in reference 3. Some discussion of the physical origin of the largest components may be found in reference 4. A well-regarded book on the subject is reference 5. As mentioned before, the way that the water responds to this driving force is very complex, depending on quirks of geography.

Resonances and all that. Finally, there are many non-tidal effects on the water that are as large or larger than the tidal effects. Weather systems produce wind and low pressure that move water around. Earthquakes can move spectacular amounts of water around. Figure 1 : Total Acceleration. Figure 2 : Acceleration versus Average.

High tides and low tides are caused by the moon. The moon's gravitational pull generates something called the tidal force. The tidal force causes Earth—and its water—to bulge out on the side closest to the moon and the side farthest from the moon. These bulges of water are high tides. High tide left and low tide right in the Bay of Fundy in Canada. Image credit: Wikimedia Commons, Tttrung. Photo by Samuel Wantman. As the Earth rotates, your region of Earth passes through both of these bulges each day.

When you're in one of the bulges, you experience a high tide. When you're not in one of the bulges, you experience a low tide. This cycle of two high tides and two low tides occurs most days on most of the coastlines of the world. This animation shows the tidal force in a view of Earth from the North Pole. As regions of Earth pass through the bulges, they can experiences a high tide. Tides are really all about gravity, and when we're talking about the daily tides, it's the moon's gravity that's causing them.

As Earth rotates, the moon's gravity pulls on different parts of our planet. Cackling Australian bird. Toy for a budding engineer. Private conversations on Twitter, for short. Nickname for the Mandalorian's charge. Cleveland is on its shore.

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