Galileo, Newton and the Birth of Astrophysics Galileo Galilei scientist and outspoken supporter of Copernicus’ heliocentric model lived and worked at the time of Brahe and Kepler Galileo Galilei (1564-1642)

Galileo’s Physics Experiments Performed many experiments on the motion of falling bodies Showed that objects of different mass dropped from the same height will fall to Earth at the same time (neglecting air resistance).

Galileo’s Telescope He was among the first to use a telescope to observe the sky and publish his observations. Observed: Mountains and valley on the Moon (like Earth). Spots on the Sun and Solar rotation (imperfect Sun). Phases of Venus Moons of Jupiter Galileo’s drawings of the Moon as seen through his telescope.

The Moons of Jupiter The discovery of these moons proved that at least some things did not go around the Earth. These 4 moons were later named the Galilean satellites in honor of Galileo. A copy of Galileo’s original notes showing Jupiter and four of its moons.

Phases of Venus If Venus went around the Earth and since we only see Venus due to reflected sunlight we should only see crescent phases of Venus. Instead we see almost all of the phases. Venus can’t orbit Earth. Ptolemy’s Earth-centered model. Venus moving along its epicycle. Copernicus’ Sun-centered model. Venus in orbit around the Sun.

Photos of Venus’ Phases What Galileo probably saw may have looked something like this.

Galileo and the end of Ptolemy’s model By the end of Galileo’s life Copernicus’ Sun-centered model of the Solar System and Kepler’s Laws had gained wider acceptance. Galileo died almost 100 years after Copernicus published his model

Issac Newton (1642 - 1727) Newton was born the year Galileo died. It had been believed that there was one set of laws that applied to Earth and another set to the stars, Moon and planets. Newton recognized that there is one set of physical laws that apply everywhere. The same force that causes an apple to fall also keeps the Moon moving around the Earth.

Newton’s Achievements Considered the greatest physicist of all time. Made significant contributions to astronomy, physics and optics Invented the reflecting telescope Developed the branch of mathematics called calculus His most famous contributions are to the study of motion and gravity. He recognized that gravity was the only force involved in keeping the planets moving around the Sun

The Universal Law of Gravitation Every particle in the Universe attracts every other particle. The force is proportional to the product of their masses and inversely proportional to the distance between them

Newton’s Laws 1st Law (Law of Inertia) A body at rest remains at rest unless made to change by forces acting on it A body in uniform motion (in a straight line) remains that way unless made to change by forces acting on it. Uniform motion - both speed and direction unchanged

Mass and Weight Mass - the amount of matter a body contains. (not the same as weight) Weight - a measure of the amount of gravity pulling on an object On the Moon we would weigh less but have the same amount of mass

Newton’s Laws 2nd Law a=F/m The amount of acceleration (a) that a force (F) can produce depends on the mass (m) of the object being accelerated a=F/m

Newton’s Laws 3rd law When two bodies interact, they create equal and opposite forces on each other

Newton’s Version of Kepler’s 3rd Law Newton generalized Kepler’s 3rd Law so that it can be applied anywhere in the Universe not just to planets going around the Sun. (Remember P2(years) = a3(AU) ?) Newton’s version includes the mass of the two objects. Because of this if you know the period of the orbit and distance you can determine the mass. For example, by knowing the period of the orbit of Jupiter’s moons and their distance from Jupiter you can “weigh” Jupiter. We can use gravity to weigh anything (stars, galaxies, etc.)