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Summary of Key Ideas

Science: Key to Comprehending the Cosmos

The ancient Greeks laid the groundwork for progress in science by stating that the universe is comprehensible.
The scientific method is a procedure for formulating theories that correctly predict how the universe behaves.
A scientific theory must be testable, that is, capable of being disproved.
Theories are tested and verified by observation or experimentation and result in a process that often leads to their refinement or replacement and to the progress of science.
Observations of the cosmos have led astronomers to discover some fundamental physical laws of the universe.

Origins of a Sun-Centered Universe

Common sense (for example, Earth doesn’t appear to be moving) led early natural philosophers to devise a geocentric cosmology, which placed Earth at the center of the universe.
Kepler modified Copernicus’s heliocentric (Sun-centered) theory by showing that orbits are elliptical, thereby creating a simplified explanation of planetary motions compared to the geocentric theory.
The heliocentric cosmology refers to motion of planets and smaller debris orbiting the Sun. Other stars do not orbit the Sun.
The sidereal orbital period of a planet is measured with respect to the stars and determines the length of the planet’s year. A planet’s synodic period is measured with respect to the Sun as seen from the moving Earth (for example, from one opposition to the next).

Kepler’s and Newton’s Laws

Ellipses describe the paths of the planets around the Sun much more accurately than do the circles used in previous theories. Kepler’s three laws give important details about elliptical orbits.
The invention of the telescope led Galileo to new discoveries, such as the phases of Venus and the moons of Jupiter, that supported a heliocentric view of the universe.
Newton based his explanation of the universe on three assumptions, now called Newton’s laws of motion. These laws and his law of universal gravitation can be used to deduce Kepler’s laws and to describe most planetary motions with extreme accuracy.
The mass of an object is a measure of the amount of matter in it; weight is a measure of the force with which the gravity of a world pulls on an object’s mass when the two objects are at rest with respect to each other (or, equivalently, how much the object pushes down on a scale).
The path of one astronomical object around another, such as that of a comet around the Sun, is an ellipse, a parabola, or a hyperbola. Ellipses are bound orbits, while objects with parabolic and hyperbolic orbits fly away, never to return.

What makes a theory scientific? A theory is an idea or set of ideas proposed to explain something about the natural world. A theory is scientific if it makes predictions that can be objectively tested and potentially disproved.
What is the shape of Earth’s orbit around the Sun? All planets have elliptical orbits around the Sun.
Do the planets orbit the Sun at constant speeds? No. The closer a planet is to the Sun in its elliptical orbit, the faster it is moving. The planet moves fastest at perihelion and slowest at aphelion.
Do all of the planets orbit the Sun at the same speed? No. A planet’s speed depends on its average distance from the Sun. The closest planet moves fastest, while the most distant planet moves slowest.
How much force does it take to keep an object moving in a straight line at a constant speed? Unless an object is subject to an unbalanced external force, like friction, it takes no force at all to keep it moving in a straight line at a constant speed.
How does an object’s mass differ when measured on Earth and on the Moon? Assuming the object doesn’t shed or collect pieces, its mass is the same on Earth and on the Moon. Its weight, however, is less on the Moon.
Do astronauts orbiting Earth feel the force of gravity from our planet? Yes. They are continually pulled earthward by gravity, but they continually miss it because of their motion around it.