Early Astronomers UNIT 11 STANDARDS: NCES 1.1.1, 1.1.2, 1.1.3, 1.1.4, 1.1.5, LESSON 1

Lesson Objectives In this lesson, you will learn about: Early astronomy Early astronomers Universal Gravitation

Early Astronomers Astronomer Aristotle Astronomer is a person who studies the stars. One of the first astronomers was a Greek named, Aristotle; he lived in 350 BC. Aristotle believed the universe was geocentric which means earth was the center of the entire universe, including the sun, the moon, and all of the stars and planets.

Early Astronomer: Ptolemy Main Beliefs Ptolemy lived in 120 AD. He accepted and improved upon the ideas of Aristotle. He also believed the universe was geocentric. Here is the way Ptolemy drew a picture of the universe. The earth is the center. The moon and Mercury were the closest. Then came Venus. Mars, Jupiter, and Saturn next followed the sun.

Astronomer: Copernicus Heliocentric Theory Copernicus lived 1500 AD, he was a Polish mathematician. He believed the earth and other planets orbit around the sun, this is called a heliocentric view of the Solar System. Helios means sun. Centric refers to the center. This means that the sun is the center of the Solar System, not the Universe.

Astronomer: Tycho Brahe Tycho Brahe Tycho Brahe, was an astronomer born in Denmark in Tycho Brahe spent several decades carefully recording the positions of the planets each night. He wanted to explain the motion of the planets. He decided that Mercury, Venus, Mars, and Jupiter rotated around the sun in circles. The sun, the moon, and all of the stars rotated around the earth in circles. He was wrong, but his data about daily positions of the planets were the most accurate ever collected.

Astronomer: Johannes Kepler Right spot, right time Johannes Kepler was a German astronomer and mathematician born in Kepler worked as an assistant to Tycho Brahe, so he had access to all of Brahe’s data about the position of the planets. After Brahe died, Kepler analyzed the data and decided all the planets including the earth moved around the sun, but not in circles. Each planet followed an elliptical path.

Renaissance: Galileo Galileo Also in 1600 AD an Italian man named Galileo began to observe the heavens through a telescope. His telescope had lenses made of glass inside it. With his telescope, Galileo could clearly see the moon and the other planets that go around the sun. This is the basis for modern telescopes.

Isaac Newton Gravity 1700 AD a mathematician named Isaac Newton guessed that the force that pulls all the planets toward the sun is the same force that hold all things on earth. GRAVITY Gravity is the force that pulls all things towards each other. Newton understood that the earth pulls everything toward it. The apple was pulled toward the earth too. Newton also realized that there had to be an invisible but powerful force between the earth and the sun to keep the earth from flying off into space. Without gravity, the earth would not keep orbiting the sub.

Modern Universal Gravity Newton Proposed Here is another way to think about what Newton said. Imagine tying a string to a ball. If you were to sing the ball around your head, the ball would go in a circle. If the string broke or you let go, the ball would fly away (at a tangent to its last path). Now think about the earth and the sun. Imagine gravity as an invisible string holding the earth in orbit around the sun. Newton explained that gravity keeps the earth in its place around the sun. The sun is so massive, that the gravity of the sun holds all of the planets that orbit it.

Newton’s Law Mathematically Applied Scientist use Newton’s Law of Universal Gravity as a basis for all space explorations and calculations. The next slides deal with the mathematical formula, some derivations, and practical applications.

Newton’s Law of Universal Gravity Newton guessed that the gravitational force between any two masses (m 1 and m 2 ) is given by: F gravity = Gm 1 m 2 / [r 12 ] 2 Where r 12 is the distance between the centers of m 1 and m 2 and G is a constant. Newton did not know the value of G. Henry Cavendish first measured G in G has been measured to be 6.67 x newton-meters 2 / kilogram 2.

Example of Universal Gravity Let’s pretend for a moment that people are spherical in shape. Suppose a person whose mass is 50 kg stands 40 cm from a person whose mass is 60 kg. What is the gravitational force pulling them together? Step 1: Convert distance into meters: 40 cm x 1 m / 100cm = 0.40 m F = Gm 1 m 2 / [r 12 ] 2 F = [6.67 x newton-meters 2 / kilogram 2 ] x [50 kg x 60 kg] / [0.4m] 2 F = [6.67 x newton-meters 2 / kilogram 2 ] x [3000 kg 2 ] / [0.16m 2 ] F = [2.001 x newton-meters 2 / kilogram 2 ] / [0.16m 2 ] F = x newtons

Finding the Mass of the Earth Given gravity is g = [GM e ] / [R e ] 2 Which means M e = g [R e ] 2 / [G] We have not measure g yet, but you probably know that the accepted value for g = 9.8 m/s 2. R e = 6.38 x 10 6 m G = 6.67 x N-m 2 /kg 2 or 6.67 x m 3 /kg-s 2 Compute M e, the mass of the earth M e = (9.8 m/s 2 ) x [6.38 x 10 6 m] 2 / [6.67 x m 3 /kg-s 2 ]fill in M e = (9.8 m/s 2 ) x [ x m 2 ] / [6.67 x m 3 /kg-s 2 ]solve M e = [ x m 3 /s 2 ] / [6.67 x m 3 /kg-s 2 ] M e = [ x kg]

Light Energy and the Universe Energy Energy in the universe travels in electromagnetic waves. The electromagnetic spectrum is made up of radio, microwave,infrared, visible, ultraviolet, X-rays, and gamma rays. Humans can only see the visible band. ROY G BIV

Electromagnetic Spectrum

Section Review Compare Aristotle’s belief of the universe to that of Copernicus What contribution did Galileo make to astronomy How did Kepler correct the work of Brahe How did Newton change the concept of planetary orbits around the sun