Notes

〔Bill Cipher ; Astronomy ― Slytherin, first grade, Subject: Gravity〕

1. Gravity is a force pulling together all matter (which is anything you can physically touch). The more matter, the more gravity, so things that have a lot of matter such as planets and moons and stars pull more strongly.

Mass is how we measure the amount of matter in something. The more massive something is, the more of a gravitational pull it exerts. As we walk on the surface of the Earth, it pulls on us, and we pull back. But since the Earth is so much more massive than we are, the pull from us is not strong enough to move the Earth, while the pull from the Earth can make us fall flat on our faces.

In addition to depending on the amount of mass, gravity also depends on how far you are from something. This is why we are stuck to the surface of the Earth instead of being pulled off into the Sun, which has many more times the gravity of the Earth.

2. Gravity is one major force that creates tides. In 1687, Sir Isaac Newton explained that ocean tides result from the gravitational attraction of the sun and moon on the oceans of the earth. Tidal forces are based on the gravitational attractive force. With regard to tidal forces on the Earth, the distance between two objects usually is more critical than their masses. Tidal generating forces vary inversely as the cube of the distance from the tide generating object. Gravitational attractive forces only vary inversely to the square of the distance between the objects. The effect of distance on tidal forces is seen in the relationship between the sun, the moon, and the Earth’s waters.

Newton’s law of universal gravitation states that the gravitational attraction between two bodies is directly proportional to the product of their masses, and inversely proportional to the square of the distance between the bodies. Therefore, the greater the mass of the objects and the closer they are to each other, the greater the gravitational attraction between them.

The gravitational attraction between the Earth and the moon is strongest on the side of the Earth that happens to be facing the moon, simply because it is closer. This attraction causes the water on this “near side” of Earth to be pulled toward the moon. As gravitational force acts to draw the water closer to the moon, inertia attempts to keep the water in place. But the gravitational force exceeds it and the water is pulled toward the moon, causing a “bulge” of water on the near side toward the moon.
The moon appears to rotate around the earth each day; however, it is the earth’s rotation that gives this appearance. The moon actually orbits the earth in an elliptical pattern, taking 27.3 days to complete one orbit. The length of time that it takes for the earth to rotate around so that the moon is in the same position is actually a little over a normal 24 hour day. It is 24 hours and 50 minutes or a tidal day. That is why the tidal cycle starts approximately 50 minutes later each day. As the earth rotates, the moon’s gravitational force continually mounds the water and that fluid mound moves around the earth. The actual height of the tide is influenced by the shape of the coastline and depth of the water.
The gravitational pull of the moon tugs on the surface of the ocean until its surface mounds up and outward in the direction of the moon. When the mound of water has reached its highest point it is called high tide. On the side of the earth opposite the moon, the centrifugal force caused by the earth’s rotation produces another mound of water and high tide on the opposite side of the earth. Somewhere in between these two high tides are two flat areas on the surface of the ocean, these are low tides.