Lecture 2 the solar system the universal gravitation
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Lecture 2 The Solar System The 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, Kepler’s laws Universal gravitation. Patterns of stars seen in the sky

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Lecture 2 the solar system the universal gravitation l.jpg
Lecture 2The Solar SystemThe Universal Gravitation

Chapter 1.3  1.11

Homework: On-line quiz for Chapter 1 (due August 27th)


  • What we see in the sky

  • The Ptolemaic system

  • The Copernican system, Kepler’s laws

  • Universal gravitation

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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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Relations of celestial objects to weather and agriculture

Observations of the Sun and Moon

Observations of stars and planets

Ancient Observations

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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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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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Kepler’s Laws

  • Planets move on elliptical orbits

  • 2. The planet’s 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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Sun (the only star)

9 planets

Nearly 100 moons



Free-flying gas and ``dusty’’ particles


The Solar System

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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


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Support of Kepler’s Laws

Newton found that Kepler’s 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 are due to gravitational attraction between the Earth and the Moon

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Tidal Friction and Synchronous Rotation

The tidal bulges try to stay on the Earth – Moon line

The Earth’s rotation tries to pull the bulges around

The tidal friction slows down the Earth’s 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 England’s 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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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

Summary l.jpg

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 Kepler’s laws.

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