Modern Astronomy Johannes Kepler was the first astronomer to correctly determine the shape of the planets’ orbits. Isaac Newton, the father of modern physics, was able to explain why objects in space moved as they did. The work of Kepler and Newton is the foundation of our understanding of astronomy.

Kepler’s Laws Kepler’s First Law: all planets orbit the sun in the shape of an ______. Kepler’s Second Law: planets move faster when they are ______ to the sun. Kepler’s Third Law: the length of a planet’s ____ is proportional to its average _______ from the sun. ellipse closer orbit distance

Beyond Kepler Kepler’s three laws were based on observations, not physics or mathematical formulas. Kepler didn’t know why his laws worked, he just knew that they did. It was Sir Isaac Newton who finally provided theories to explain why Kepler’s Laws worked.

Early Astronomy  Sir Isaac Newton English scientist Sir Isaac Newton (1642—1727) explained gravity as the force that holds planets in orbit around the Sun.

Gravity’s Influence on Orbits Makes no sense without caption in book

Early Astronomy The Birth of Modern Astronomy  Sir Isaac Newton • Although others had theorized the existence of gravitational force, Newton was the first to formulate and test the law of universal gravitation. The universal law of gravitation, helped explain the motions of planets in the solar system.  Universal Gravitation • Gravitational force decreases with distance. • The greater the mass of an object, the greater is its gravitational force.

Gravity Gravity is the force that holds us to the Earth causes a rock to fall towards the ground causes the Earth to go around the Sun causes the Sun to be pulled towards the center of the Milky Way galaxy Gravity acts between any two objects even if they are far apart. “action at a distance”

Newton’s Laws of Motion 1st Law A body at rest, or in uniform motion, will remain so unless acted upon by an unbalanced force. 2nd Law The change in motion (acceleration) is proportional to the unbalanced force 3rd Law For every action there is an equal and opposite reaction

Newton’s Laws and Astronomy 1st Law: Law of Inertia Inertia: the tendency of an object to keep moving (or stay still) unless another force acts on it How is this important to astronomy? The planets keep moving in their orbits. They would keep going in a straight line due to inertia, but the sun’s gravity pulls them in.

Newton’s Laws and Astronomy 2nd Law: F=ma Usually written as an equation, it shows us that the force of an object is related to its mass and how much it is accelerating How is this important to astronomy? Since force is related to mass and acceleration, Newton’s second law helps explain the orbits of space objects. A comet passing by a star (with a large mass) will actually accelerate—same with planets in the solar system Kepler’s Second Law

Newton’s Laws and Astronomy 3rd Law: Law of Action/Reaction Forces When you push on an object, you feel pressure on your hand because it pushes back on you (with an equal and opposite force) How is this important to astronomy? We wouldn’t be able to accurately calculate measurements for space flight without this law. A rocket launch is a perfect example: the action force is the fuel exiting the rocket, the reaction force is the rocket pushing off of the ignited fuel

Summary Kepler’s and Galileo’s Laws provided Newton with important clues that helped him formulate his laws of motion Newton arrived at 3 laws that govern the motion of objects The law of inertia The law of force The law of action and reaction Newton also arrived at a law of gravity But it seemed to require action at a distance!