Lecture 2 The Solar System The Universal Gravitation

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Lecture 2The Solar SystemThe Universal
Gravitation
Chapter 1.3 ? 1.11
Homework On-line quiz for Chapter 1 (due August
27th)
Outline

• What we see in the sky
• The Ptolemaic system
• The Copernican system, Keplers laws
• Universal gravitation

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Constellations

• Patterns of stars seen in the sky
• There are 88 constellations
• About 50 of them were named by ancient Greeks and
  Romans

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Constellation of Orion
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Winter triangle
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Daily star paths
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Seasonal changes in the sky
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Annual change of the Sun altitude
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Ancient Observations

• Relations of celestial objects to weather and
  agriculture
• Observations of the Sun and Moon
• Observations of stars and planets

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Ptolemaic model of the Universe
Developed by Claudius Ptolemy (A.D. 100-170) The
Earth is in the center The Sun is at the third
orbit from Earth after Mercury and
Venus Epicycles are added to circular orbits of
planets to explain retrograde motion
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Ptolemy (A.D. 100 ? 170)
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Ptolemaic Epicycles
The Ptolemaic model along with a catalog of
positions of 1028 stars were published in his
book Almagest
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Nicolaus Copernicus (1473?1543)
Copernicus is said to be the founder of modern
astronomy
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Keplers Laws

• Planets move on elliptical orbits
• 2. The planets radius-vector sweeps out the same
  areas in equal times
• 3. The squares of the periods of the planets are
  proportional to the cubes of their semi-major axes

Demonstration of Kepler's laws
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The Solar System

• Content
• Sun (the only star)
• 9 planets
• Nearly 100 moons
• Asteroids
• Comets
• Free-flying gas and dusty particles
• http//solarviews.com/eng/homepage.htm

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Planets of the Solar System
Object Diameter Dist. from Sun Dist. in A.U. Revol. Period Earth years
Mercury 4,880 57.9 million km 0.4 0.2
Venus 12,100 108.2 0.7 0.6
Earth 12,760 149.6 (1 A.U.) 1.0 1.0
Mars 6,790 227.9 1.5 1.9
Jupiter 143,000 778.3 5.2 11.9
Saturn 120,000 1,427 9.5 29.5
Uranus 52,000 2,870 19.2 84.0
Neptune 48,400 4,497 30.1 164.8
Pluto 2,260 5,900 39.4 247.7
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Sir Isaac Newton (1642-1727)
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The Universal Law of Gravitation
Every mass attracts every other mass through
the force called gravity The force of attraction
is directly proportional to the product of their
masses The force of attraction is inversely
proportional to the distance between the objects
Fg G x M1x M2 / d2
gravity
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The Universal Law of Gravitation
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Support of Keplers Laws
Newton found that Keplers first two laws apply
not only to planets, but to any object going
around another one under the force of gravity
The orbits do not have to be elliptical They can
also be parabolic or hyperbolic
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Tides
Tides are due to gravitational attraction between
the Earth and the Moon
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Spring Tides
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Neap Tides
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Tidal Friction and Synchronous Rotation
The tidal bulges try to stay on the Earth Moon
line The Earths rotation tries to pull the
bulges around
The tidal friction slows down the Earths
rotation The length of a day gets longer
It makes the Moon move further away from Earth
The Moon is in synchronous rotation with the
Earth (always showing the same face)
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The Discovery of Neptune
In 1781, the planet Uranus was discovered
telescopically from Britain by William Herschel.
In 1845, a Cambridge mathematician, John Couch
Adams, based on the law of gravitation, predicted
the existence of an unseen planet, to account for
the fact that Uranus was being pulled slightly
out of position in its orbit. He sent the
calculations to test to Englands Royal
Astronomer, who set them aside.
Shortly after that, a French mathematician,
Urbain Leverrier, published a similar prediction
and contacted astronomers at Berlin Observatory,
who found the new planet on the night of 23
September 1846.
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Urbain Leverrier (1811?1877)
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Astronomy and Astrology
Astronomy is a science It describes the real
world, sets new problems and solves them, using
methods of itself and other sciences (such as
physics and mathematics)
Astrology is interpreting apparent positions of
the Sun, planets, and stars to predict human
life. It does not set and solve any problem
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Summary

• The Earth is not the center of the universe but
  instead is a planet orbiting a rather ordinary
  star in the Milky Way Galaxy.

• Celestial bodies in the gravitational field of
  each other move according to Keplers laws.

• Newtons discoveries showed that the same
  physical laws we observe on Earth apply
  throughout the Universe.