Lecture 2The Solar System

The Universal GravitationChapter 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

Constellations

• Patterns of stars seen in the sky

• There are 88 constellations

• About 50 of them were named by ancient Greeks and Romans

Constellation of Orion

Winter triangle

Daily star paths

Seasonal changes in the sky

Annual change of the Sun altitude

Ancient Observations

• Relations of celestial objects to weather and agriculture

• Observations of the Sun and Moon

• Observations of stars and planets

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

Ptolemy (A.D. 100 170)

Ptolemaic Epicycles

The Ptolemaic model along with a catalog of positions of 1028 stars were published in his book Almagest

Nicolaus Copernicus (14731543)

Copernicus is said to be the founder of modern astronomy

Kepler’s Laws

1. 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

The Solar System

• Content:

Sun (the only star)

9 planets

Nearly 100 moonsAsteroids

Comets

Free-flying gas and ``dusty’’ particles

http://solarviews.com/eng/homepage.htm

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

Sir Isaac Newton (1642-1727)

The Universal Law of Gravitation

Every mass attracts every other mass through the force called gravityThe force of attraction is directly proportional to the product of their massesThe force of attraction is inversely proportional to the distance between the objects

Fg = G x M1x M2 / d2

gravity

The Universal Law of Gravitation

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

TidesTides are due to gravitational attraction between the Earth and the Moon

Spring Tides

Neap Tides

Tidal Friction and Synchronous Rotation

The tidal bulges try to stay on the Earth – Moon lineThe Earth’s rotation tries to pull the bulges around

The tidal friction slows down the Earth’s rotationThe 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)

The Discovery of NeptuneIn 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.

Urbain Leverrier (18111877)

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• The Earth is not the center of the universe but instead is a

planet orbiting a rather ordinary star in the Milky Way Galaxy.

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

• Celestial bodies in the gravitational field of each other move according to Kepler’s laws.