Chapter 3: Motion of Astronomical Bodies

Retrograde Motion

Backward motion observed when an interior planet appears to move backwards as it zooms past.

Epicycles

Circles within a circle used to explain planetary motion, introduced by Ptolemy in 150 CE.

Heliocentric Model

Model proposed by Copernicus where the sun is at the center of the universe.

Superior Planets

Planets outside Earth that are best seen during opposition.

• conjunction

• opposition

Inferior Planets

Planets that are always visible close to sunrise or sunset.

• superior and inferior conjunction

Synodic Period (S)

The time it takes for a planet to realign with the Sun and Earth.

Sidereal Period (P)

The time taken for a planet to complete one full orbit around the sun with respect to the stars.

Tycho Brahe

• he created geoheliocentric model with other planets orbiting the sun, but with the sun orbiting earth

• begged Kepler to find a model

Johannes Kepler

• worked as an assistant to Tycho

• Empirical rules to describe planetary orbits in a heliocentric system

• these empirical rules are known as Kepler’s laws

Kepler's First Law

Planets orbit the sun in ellipses, with the sun at one of the foci.

• the average distances between the Sun and the planet is the semimajor axis

Kepler's Second Law

Planets sweep out equal areas in equal times, moving faster when closer to the sun.

Kepler's Third Law

Relates the orbital period of a planet to the size of its orbit.

a³=p²

a- semimajor axis (A)

p = orbital period of planet: years (P)

Isaac Newton

physical laws explain how things work, while empirical laws only describe nature

Newton's First Law of Motion

An object will remain at rest or in uniform motion unless acted on by a net force.

Galilleo’s Law of Inertia

“Constant” motion means at a constant speed and in a constant direction: This is an inertial frame of reference

Newton's Second Law of Motion

The acceleration of an object depends on the net force acting upon it and its mass (F = ma).

Newton's Third Law of Motion

For every action, there is an equal and opposite reaction.