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Chapter 19.1 Part 2

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Planetary Motion The solar system which is now 4.6 billion years old, is not simply a collection of stationary planets and other bodies around the sun. Each one moves according to strict physical laws. The ways the Earth moves, for example, cause seasons and even day and night.

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**Rotation and Revolution**

Rotation: The spinning motion of a body (Earth) on its axis. By rotating on its axis ½ of the earth is facing the sun at different times causing days and nights. In addition to rotating on its axis, the Earth also travels around the sun in a path called an orbit. This motion around the sun along its orbit is called revolution.

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**These questions were answered by scientist in the 1500s and 1600s.**

The amount of time it takes for a single trip around the sun is called a period of revolution. The period for the Earth to revolve around the sun is 365 days. Planetary Orbits Why do the planets continue to revolve around the sun? Does something hold them in their orbit? Why doesn’t gravity pull the planets pull toward the sun or why they don’t fly off into space? These questions were answered by scientist in the 1500s and 1600s.

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**Kepler’s First Law of Motion**

Discovered that planets did not move in a circle around the sun, but in an elongated circle called an ellipse. An ellipse is a closed curved in which the sum of the distances from the edge of the curve to two points (Called foci) inside the ellipse is always the same

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**Major axis: the maximum length of an ellipse.**

Semimajor axis: half of the distance and is used to give the size of an ellipse. The semimajor axis of Earth’s orbit is 150 million kilometers. It represents the average distance between the Earth and the sun and is called one astronomical unit, or one AU.

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**Keplers Second Law of motion**

Kepler also discovered that the planets seem to move faster when they are close to the sun amd slower when they are farther away. To keep the area of A, equal to the area of B, the planet must move farther around its orbit in the same amount of time.

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**Keplers Third Law of Motion**

Compares the period of a planet’s revolution with its semimajor axis. By doing some mathematical calculations kepler was able to calculate a planets distance from the sun from knowing a planets period of revolution The data he used to make these laws came from his former boss Tyco Brahe data.

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**Newton’s Law of Universal Gravitation**

Kepler wondered what caused the planets closest to the sun to move faster than the planets farther away, but never got an answer. This didn’t happen until Sir Isaac Newton was able to do it with his ideas of gravity. Today we do not fully understand gravity. But Newton was able to combine the work of earlier scientist to explain how the force of attraction between matter works

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Law of universal gravitation states that the force of gravity depends on the product of the masses of the objects divided by the square of the distance between them. If two objects are moved twice as far apart, the gravitational attraction between them will decrease by a factor of 2 x 2 = 4. If the objects are moved 10 times as far apart, the gravitational attraction will decrease by a factor of 10x10 = 100

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**Falling Down and Around How Newton explained the orbit of the moon**

Newton explained that the pull of gravity causes the moon to fall toward the Earth while its forward motion tries to have the moon fly off into space. With the two forces somewhat balanced causes the moons resulting path to be curved. This same principle holds true for all bodies in our solar system.

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