Astronomy- the study of the universe Year- the time required for the Earth to

orbit once around the sun Month- a division of the year that is

based on the orbit of the moon around the Earth

Day- the time required for Earth to rotate once on its axis

In 140 CE a Greek astronomer named Ptolemy wrote a book on all the ancient knowledge on astronomy and expanded many theories; this was called the Ptolemaic theory.

Ptolemy thought that the Earth was the center of the Universe.

Because his theory explained the motion of the planets so well it was a popular theory for over 1,500 years in Europe.

Copernicus was a Polish astronomer. In 1543 Copernicus came out with a

new theory that the Erath and all of the other planets rotated around the sun.

Eventually when the scientific community accepted his theory major changes to science and society took place.

This change was called the Copernican revolution.

Tycho Brahe was a Danish astronomer. In the late 1500s he used several large

tool to make very accurate and detailed astronomical observations.

Although Brahe believed the theory thatt everything rotated around earth he also believed that the sun and the moon rotated around the Earth while the other planets rotated around the sun.

Johannes Kepler was Brahe’s assistant. After Brahe died Kepler reviewed all of

his important observations and concluded that all of the planets orbit around the sun although the sun is not the exact center of their orbits.

Kepler also stated three laws of planetary motion that are still used today.

In 1609 Galileo Galilei became one of the first people to use a telescope to view things from space.

Galileo discovered=• Craters and mountains on the Earth’s moon.• Four of Jupiter’s moons.• Sunspots on the Sun.• The phases of Venus. These discoveries showed that the planets

were not “wandering stars” but physical bodies like Earth.

In 1687 Sir Isaac Newton, a scientist, showed that all of the objects in the Universe attract each other through gravitational force.

Newton discovered that the force of gravity depends on the mass of the objects and the distance between them, therefore Newton explained that the planets rotate around the sun because it is the most massive object in the universe.

In 1924 Edwin Hubble proved that there were more galaxies in space than just the Milky Way.

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