Chapter 26.2: Observing the Solar System

Early views of the organization of Space were much different than ours.

Early astronomers believed that the Earth was the center of our Universe, and that the Earth was surrounded by a ball, called a Celestial Sphere, on which were fixed most of the objects of the heavens.

This interpretation of our solar system is called a Geocentric Model, meaning Earth-centered.

Early astronomers noticed groups of stars that were visible at the same time every year. These were named constellations, and were used as a basis for calendars. The Orion Constellation

Astronomers noticed that some celestial bodies did change position relative to the constellations. They called these “wandering stars” planets. Notice the planet Mars moving across the constellations Gemini and Leo over the course of 11 months.

Astronomers also noted that, periodically, these planets which normally moved eastward, moved backward for a few weeks, then resumed their path eastward. This is referred to as retrograde motion.

In 200 A.D., the Greek astronomer Ptolemy explained this “retrograde’ motion by stating that the planets orbited the Earth in a circle, but also orbited another point in a circle, what he called an epicycle.

In the late 1400’s, the astronomer Copernicus proposed what was to become known as the Heliocentric Model.

Copernicus stated that the Sun was the center of the Solar System, the Earth was a planet, and that it orbited the Sun.

And, here is how he explained retrograde motion.

Mars retrograde motion Venus retrograde motion

In the 16th –century, the astronomer Johannes Kepler developed three laws to describe the way in which planets move through Space.

Kepler’s 1st Law: Planets move through Space in an elliptical orbit, not a circular one. This causes the distance from a planet to the Sun to vary.

Kepler’s 2nd Law: When a planet is closer to the Sun, it moves faster Kepler’s 2nd Law: When a planet is closer to the Sun, it moves faster. (centrifugal force and gravity at work!)

Kepler’s 3rd Law states that the further a planet is from the Sun the longer its period of revolution (its year) will be.

In the 1600’s, Isaac Newton stated that a force called gravity was causing the planets to behave as Kepler had observed.

He stated that all objects with mass exerted a force of attraction on other objects with mass, and that the strength of that force is proportional to the mass of the objects and their distances from one another.

Early Astronomers Astronomers

Ptolemy devised the geocentric model for our solar system believed until 1500's said planets are on small circular orbits, called epicycles developed 1st model to predict the positions of planets had support of Catholic church, despite inaccuracy AD 90-168

challenged Ptolemy's geocentric model and proposed the heliocentric model for our solar system suggested the Earth was a planet, it rotated, and that Earth & other planets revolved around the sun tried to explain retrograde motion (i.e. when 2 bodies move the same direction & one moves faster than the other Copernicus 1473-1543

discovered that planets orbit in ellipses, not circles developed 3 laws of planetary motion "planets travel in elliptical orbits with the sun at one focus & the planet's distance from the sun varying" equal area law= "the speed at which the planet travels around the sun is not constant; planets closer to the sun orbit faster than those further away" harmonic law= "the period of a planet squared is equal to the mean distance cubed“ P squared= D cubed Johannes Kepler 1571-1630

Galileo Galilei 1564-1642 explained why we don't feel the Earth moving first to use a telescope; invented refracting telescope discovered Jupiter's 4 largest moons and proved that they revolved around Jupiter disproving geocentric model Galileo Galilei 1564-1642

Isaac Newton incorporated a mirror into Galileo's telescope(refracating telescope), doubling its power (reflecting telescope) said that gravity keeps the planets in orbit- supported by his 3 laws of motion Newton's 3 laws of motion: an object at rest stays at rest until another force acts on it the greater the mass of an object being accelerated, the more force needed to accelerate the object for every action, there is an equal but opposite reaction 1642-1727

Joseph Von Fraunhofen 1787-1826 invented the spectroscope invented many telescopes and optical instruments to describe stellar parallax Analyzed light (spectroscopy) to determine chemical composition of stars 1787-1826

Albert Einstein 1879-1955 Theory of Relativity: adds the effect of gravity of large masses to light and time (predicts black holes) corrected Newton's theory of gravity by saying that not all objects attract all other objects E= mc2 Albert Einstein 1879-1955

Edwin Hubble 1889-1953 Hubble Telescope In orbit since April 24, 1990 Hubble's Law: all objects in deep space possess Dopplar shift and this shift is proportional to their distance from Earth. demonstrated the existence of other galaxies than our own with Dopplar shift, he proved that our universe is expanding and continuously moving away from us disccovered and analyzed "cosmic background radiation" which supports Big Bang Theory Hubble Telescope In orbit since April 24, 1990 Edwin Hubble 1889-1953