Observing the Solar System A History

Geocentric Model Early astronomers believed that Earth was actually the center of the universe. As early as 6000 years ago, recorded movement of the stars Noticed some stars moved and called them planets Noticed some planets would move eastward and then periodically move back west — Retrograde Motion

Ptolemy’s Geocentric Model Greek astronomer, Ptolemy (2 nd century), was puzzled by retrograde motion Put planets on circular orbits called epicycles to explain retrograde motion (click solar system picture on left for example— scan down on website to see it) Though there were inconsistencies, this model was used until the 16 th century 90 AD – 168 AD

Copernicus’s Heliocentric Model Polish astronomer, Nicolaus Copernicus, proposed a sun centered model of the solar system. Retrograde motion is explained because Earth orbits the Sun faster than a planet like Mars. (click solar system picture on right for example) When earth passes Mars, it appears as though Mars goes backwards

Galileo Galileo ( ) Father of Modern Science Physicist, Astronomer, Inventor When a teen, he discovered swinging motion of pendulums were the same despite their weight or height swinging motion of pendulums Proved that Copernicus’ heliocentric solar system was correct Proved Suggested that gravity caused all objects, no matter their weight or size, to fall at the same rate   Proven by astronauts on the moonastronauts The Telescope: Improved on a Dutch invention that allowed far away objects to be seen up close The Telescope: Improved on a Dutch invention that allowed far away objects to be seen up close The Telescope The Telescope  Made detailed drawings of the moon  Discovered 4 of Jupiter’s Moons Jupiter’s MoonsJupiter’s Moons

Tycho Brahe 16 th century Danish nobleman and astronomer Studied the movements of the moon and planets throughout their orbits using his own instruments   noticed unexpected occurences Most precise observations made before the invention of a telescope   Designed and built new instruments

Johannes Kepler and Planetary Motion An assistant to Tycho Brahe (though Brahe did not trust Kepler) Continued to build on Brahe observations after his death Discovered three laws based on his observations of Brahe’s unexpected occurences

Kepler’s 1st Law The orbit of a planet/comet about the Sun is an ellipse with the Sun's center of mass at one focus

Kepler’s 2nd Law A line joining a planet/comet and the Sun sweeps out equal areas in equal intervals of time **In other words, planets, comets and asteroids go faster the closer they are to the sun

Kepler’s Third Law The farther a planet is from the sun, the longer it’s period of revolution P 2 = D 3 This formula can be used to find the distance between the sun and a planet P = Period or time it takes a planet to travel one orbit around the sun

Issac Newton and the Law of Gravitation An English physicist, mathematician, astronomer and inventor Developed three laws of motion and the law of gravity

Newton’s 1st Law of Motion An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force Newton said that what kept the planets in orbit was a force he named GRAVITY

Newton’s 2 nd Law of Motion Acceleration is produced when a force acts on a mass. The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object).

Newton’s 3rd Law of Motion For every action, there is an equal and opposite reaction The rocket's action is to push down on the ground with the force of its powerful engines, and the reaction is that the ground pushes the rocket upwards with an equal force.

Law of Gravitation Every mass exerts a force of attraction on every other mass and…

Gravity—the Effects of Mass … … the strength of that force is proportional to the mass … (Meaning, the larger the mass, the greater the gravity pull and vice versa)

Gravity—the Effects of Distance … and inversely proportional to the distance between them. (Meaning, the bigger the distance, the smaller the gravitational pull and vise versa)