The Birth of Modern Astronomy

Earth Science 1st semester

Mr. Riordan

Nicolaus Copernicus

• Became convinced that Earth is a planet just like the others known at the time.

• He proposed a model of the solar system with the Sun at the center.

• He used circles to represent the orbits of the planets.

• This was not ultimately successful as the planets still seemed to stray from their predicted positions, indicating that their path around the Sun was not a perfect circle.

Tycho Brahe

• A Danish astronomer who was noted for his precise measurement of the locations and orbits of celestial bodies.

• He is most noteworthy for his observations of Mars.

Johannes Kepler

• Assistant to Brahe, he kept all of the data, and used it to continue making observations after Brahe’s death.

• Discovered three laws of planetary motion.

• The most important was that the orbit of planets are not circles but ellipses.

• Ellipses are oval in shape; almost like an egg

• There are two points inside an ellipse, each known as a focus, that help determine the shape of the ellipse.

Summary of the Three Laws

• The path of each planet around the sun is an ellipse with the sun at one focus. The other focus is an empty position in space located at the opposite end of the ellipse.

• A line connecting a planet to the sun would move in such a way that equal areas are swept in equal time periods.

• This translates into the planets revolving slower when they are further from the sun and revolving faster when they are closer to the sun.

• There is a mathematical relationship between the time it takes a planet to orbit the sun and that planet’s distance from the sun. Distances from planets to the sun can be calculated when their periods of revolution are known.

Galileo Galilei

• His most important contributions were the descriptions he made of the behavior of moving objects.

• He made his own telescope with which he could view celestial objects in a way not able to be done previously.

• His observations supported Copernicus’s view of a sun-centered universe.

• The discovery of four moons orbiting Jupiter.

• This disproved that the notion that the Earth was the only center of motion in the universe.

• The discovery that planets are circular disks – not points of light.

• This showed that the planets were not stars.

• The discovery that Venus went through phases, just like the moon.

• This meant that Venus must orbit it’s source of light – the Sun.

• The discovery that the moon’s surface was not smooth.

• The discovery that the sun had sunspots, and that it rotated in just under a month.

Sir Isaac Newton• Sir Issac was the first to

formulate and test the law of universal gravitation.

• According to this law every object in the universe attracts every other body with a force directly proportional to their masses and inversely proportional to their distance from one another.

• In other words, the more mass an object has the more gravitational pull it can exert.

• The closer two objects are to each other determines how much gravitational pull they can exert on each other.

• Mass is the measurement of how much matter an object has.

• Weight is a measurement of the force of gravity on an object.

• Weight can vary depending on location (Earth vs Moon) while mass will always remain the same.

• Newton was able to prove that the force of gravity coupled with the tendency of a planet to remain in a straight line motion results in the elliptical orbits that Kepler first discovered.