PLANETARY MOTION

Chapter 13

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The universe is defined as all matter and energy. It is known that the universe is expanding and that all other galaxies are rushing away from us. The universe is made up of many different structures arranged in a fairly well-defined hierarchy.

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Galaxies-are large groups of stars, nebulae, and other celestial objects. There are hundreds of billions of galaxies.

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A light year is the time it takes light to travel in one year, 186,000 miles per second.

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Galaxies are separated by hundreds, thousands or millions of light years.

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B. Stars are huge balls of hydrogen and helium held together by gravity. There are hundreds of billions of stars contained in each galaxy.

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Stars shine because they are producing energy through nuclear fusion. This happens when two atoms of hydrogen combine to form a helium atom and a lot of energy is released in the process.

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A solar system consists of a star and all the celestial objects that orbit that star. Those objects can include: planets, moons, comets and asteroids.

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Our solar system contains a star, (we call it sun) and 8 planets with many other objects orbiting the sun.

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Many of the hundreds of billions of stars in our Milky Way galaxy and other galaxies have planets orbiting them.

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D. Planets- The International Astronomy Union has an official definition for a planet. It states, "A 'planet' is defined as a celestial body that (a) is in orbit around the sun,

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(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,

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and (c) has cleared the neighborhood around its orbit."

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E. Satellites are objects that revolve around objects of greater mass. They can be man-made or natural like our moon. Earth is a satellite of the sun and the sun is a satellite in the Milky Way. 

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II. Ancient astronomers believed the Earth was the center of the Universe. By 1543 Copernicus had figured out the motions of the planets made if they moved around the Sun.

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The Earth was one of them and if the more distant ones moved more slowly, so sometimes the Earth overtakes them, and then they seem to move backwards for a while. The orbits of Venus and Mercury were inside that of the Earth, so they never move far from the sun.

Venus Crossing the Sun. Photo Credit: NASA

III. Planetary orbits according to Kepler’s Laws:

A. Johannes Kepler (1571- 1630) worked as an assistant to Tycho Brahe in an observatory near Copenhagen. Brahe spent his life trying to find the proof of the heliocentric model (sun is in the center of the solar system) by studying the orbit of Mars. After his death, Kepler used Brahe’s data and mathematics to form the 3 laws of planetary orbits.

Johannes KeplerPhoto Credit: etsu.edu

Tycho BrahePhoto Credit: Famousscientists.org

1. First Law- Planets move around the sun in ellipses. Ellipses are not circles but more oval shaped. Also, most of the planets are in or around the same plane. You can think of this as a big pancake with all the planets orbiting the sun in ellipses.

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a. An ellipse happens because there are 2 points called the foci, with the sun being one of the foci and usually nothing in the other foci.

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2. Kepler’s Second Law- The line connecting the Sun to a planet sweeps equal areas in equal time. 

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a. Although the orbit is symmetric, the motion is not. A planet speeds up as it approaches the sun, gets its greatest velocity when passing closest, and then slows down again.

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b. What happens is best understood in terms of energy. As the planet moves away from the sun, it loses energy by overcoming the pull of gravity, and it slows down, like a stone thrown upwards. And like the stone, it regains its energy as it comes back. (Dr. David Stern, an overview for science teachers.)

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3. Kepler’s Third Law- “The Law of Harmonies”. This is the law that Kepler used to measure the distance of planets from the sun. He came up with a mathematical formula that is still used today.

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IV. The Moon orbits around the Earth. Since its size only changes slightly, its distance stays about the same, and hence its orbit must be close to a circle. To keep the Moon moving in that circle--rather than wandering off--the Earth must exert a pull on the Moon, and Newton named that pulling force gravity.

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V Motions of the Earth- The two main motions of Earth are rotation and revolution.

A. Rotation is the spinning of Earth on its axis and the cause of night and day. Each rotation equals 24 hours, and since the Earth is roughly 24,000 miles in circumference, that means we are spinning about 1,000 miles per hour.  

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B. Revolution is the period of time it takes for the Earth to orbit the sun at a speed of 66,000 miles per hour, in one year. It takes Earth 365.25 days to make one complete orbit. That leaves one extra day every four years that we call leap day or year.

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C. Seasons- Earth’s axis rotation is tilted about 23.5 degrees, this is the reason for the seasons.

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D. Precession- This is the change in direction of the axis, but without any change in the tilt. This changes the relative positions of the stars but does not affect the seasons.

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E. Nutation is a wobbling around the precessional axis. This change in the angle---half degree one way or the other. This occurs over an 18 year period and is due to the Moon exclusively. This would very slightly increase or decrease the amount of seasonal effects.

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F. Barycenter is the point between two objects where they balance each other. (For example, it is the center of mass where two or more celestial bodies orbit each other. )

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When a moon orbits a planet, or a planet orbits a star, both bodies are actually orbiting around a point that lies outside the center of the primary (the larger body).  

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For example, the moon does not orbit the exact center of the Earth, but a point on a line between the Earth and the moon approximately 1,710 km or 1062 miles below the surface of the Earth, where their respective masses balance.•  This is the point about which the

Earth and moon orbit as they travel around the sun.

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G. Motions of the sun- Our star (called sun) is not stationary in our solar system. It actually moves as the planets tug on it, causing it to orbit the solar system’s barycenter. The sun never strays too far from the solar system barycenter.

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VI. Tides- What kind of force can move Earth’s oceans back and forth daily? Gravity! It is the gravity exerted on the Earth by the moon and to a lesser extent, by the sun.

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Even though the enormity of the sun should produce more gravity, the sun is only responsible for 46 percent of the gravitational pull on the oceans. This is because the sun is so far away.

VII. Circumference- The Earth like most celestial bodies is spherical because gravity is pulling the matter in from all directions.  

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However, the circumference around the equator is slightly bigger than the circumference around the poles.  The reason for this is that the Earth is rotating on its axis causing it to bulge out at the equator.