• Early Western people believed that the Earth was the center of the universe.

• That is called a geocentric system.

• Ptolemy, a Greek astronomer believed that the sun, moon, and, planets circled the Earth.

• He believed that the planets made little circles of their own.

• He was believed to be correct for 1400 years.

A BRIEF HISTORY OF ASTRONOMY

a. The Big Horn Medicine Wheel in Wyoming is aligned with the sun and other starsb. Mayan temple was an observatory

Stonehenge marks the rising sun on the summer solstice

The geocentric theory

• On Earth the stars, sun, and moon seem to move across the horizon.

• Before telescopes we could not tell that the Earth was actually rotating.

OBSERVING THE SOLAR SYSTEM

• The ancients noticed that most of the stars seemed to move together... BUT There were some objects that did not move with the stars.

• The ancients called these objects

planets which means wandering stars.

The lines represent the paths of planets as seen

from Earth

In 1500 Nicolas Copernicus, a Polish astronomer believed that the sun was the

center of the universe.That is called a heliocentric system.He believed that the Earth, stars, and

planets revolved around the sun in spherical (circular) orbits.

However, most astronomers still believed that Ptolemy was correct.

The invention of the telescope paved the way for observations which made it obvious that the earth is not the center of the universe.

Tycho Brahe in hisobservatory. Hehelped inventmany measuringdevices, buttelescopes hadnot yet beeninvented.

In the 1500s Galileo built one of the first telescopes.With it he saw Jupiter’s moons circling it.He also saw that Venus had phases.People began to believe that Copernicus was correct.In 1580 Tycho Brahe, a Danish astronomer made many observations of the planets over twenty years.In 1600 Johannes Kepler, a German mathematician proved that the planets have elliptical (oval shaped) orbits.

We then knew the shape of the orbits but did not know what kept the planets in those orbits.Sir Isaac Newton proved that gravity and inertia keep the planets in their orbits.Inertia keeps a moving object moving and a still object still.Gravity is the force that attracts an object to another.Earth and the sun are attracted to each other by gravity.Therefore, the Earth should fall into the sun. Why doesn’t it?

If the Earth were not moving, it would fall into the sun, but the Earth is moving away from the sun; it has inertia.The combination of gravity and inertia keep the Earth in its orbit around the sun.