• Astronomy—the science that studies the universe
• Properties of objects in space and the laws of the universe
• Greeks were the first to study the sun, moon, and stars
• Used geometry and trigonometry to measure sizes and distances
• Aristotle was the first to believe the Earth was round based on the curved shadow on the moon.
• His beliefs were abandoned in the Middle Ages.

• By the 2 nd Century B.C., the Greeks believed the Earth was a motionless sphere in the center of the universe.
• Geocentric Model—(geo-Earth, centric-centered) the moon, sun, and known planets (Mercury, Venus, Mars, & Jupiter) orbit the Earth.
• Orbit—the path of an object as it goes around another object in space
• Beyond the planets was a hollow sphere where the stars travelled daily around the Earth.

• Eratosthenes was the first to attempt to establish the size of Earth.
• By comparing the noontime sun and shadows in 2
Egyptian cities, and measuring the difference in angles between them, he was able to estimate the circumference of the Earth.
• Hipparchus was the first to create a star catalog.
• Divided almost 850 stars into 6 groups according to their brightness.

• Claudius Ptolemy’s model of the universe (141 A.D) predicted the motion of the planets around the Earth.
• Each planet moves slightly eastward among the stars, but periodically each planet appears to stop, change direction for a while, then continue eastward again.
• Retrograde Motion—the apparent westward drift of the planets.
• Ptolemy believed it resulted from planets moving along smaller circles, which then moved around the Earth.
• His theory predicted planetary motion well, and went unchallenged for 13 centuries.

• Ptolemy was WRONG! The planets do not orbit the Earth.
• Aristarchus was the first to propose a heliocentric universe.
• Helio-Sun Centric-Centered
• He used geometry to calculate distances and sizes of the sun and moon (although his calculations were too small)
• Heliocentric Model—Earth and the other planets orbit the sun.

• We now know that the universe is not heliocentric, but our solar system is.
• Nicolaus Copernicus (2000 yrs after Aristarchus) proposed a model of the solar system with the sun at the center.
• It made more sense that Earth, rather than the sky, rotated once a day.
• He used perfect circles to represent the orbits of the planets, even though they strayed from his predicted position.
• This model also explained the retrograde motion of the planets
• The combination of the motion of Earth and the planet’s own motion around the sun.
• Different sized orbits make the planets appear to change speed and direction.

• Danish, born 3 years after Copernicus died.
• Built instruments & angle-measuring devices which he used to measure the locations of the heavenly bodies (Before the telescope was invented)
• His observations, especially of Mars, were far more precise than earlier measurements.