Week 1: History of Planetary Science

Based off notes taken on August 31st.

Rotation

The movement of a body on an axis.

Revolution

The movement of a body completely around another body.

Prograde

When the direction of rotation is the same as its orbit, and vice-versa.

Retrograde

The “backwards” motion of an object, meaning it is traveling the opposite direction as its orbit.

Geocentric

Earth-centered Solar System model.

Heliocentric

Sun-centered Solar System model.

What did the Ancient Greeks do?

• Recognized the sky is 3D.

• Geocentrism.

• No planet beyond Saturn was observed.

Eudoxus of Cnidus

• Founded the first known observatory.

• Identified 7 “wanderers” —> 5 planets, the Sun and the Moon which moved relative to stars.

• Believed that celestial bodies were moved by transparent crystalline spheres.

Aristotle

• Understood that the Earth must be spherical.

• Refined Eudoxus’ system up to 55 spheres

Aristarchus of Samos

• Developed first Heliocentric system.

• Model first rejected due to the observations of a motionless Earth.

Ptolemy

• Wrote “The Almagest" on astronomical knowledge.

• Used the idea of epicycles to explain retrograde motion.

Nicolaus Copernicus

• Rediscovered the heliocentric model.

• Understood that retrograde motion was only apparent.

• Contradicted all previous ideas and went against popular astronomical thought.

• Work was ignored until Galileo’s trial.

Tycho Brahe

• Observed the sky with instruments of his own design.

• Created the Tychonian System.

Tychonian System

• The Earth is too heavy to move and is “lazy.”

• The Earth is made of substances that change and create mass.

• The Heavens are made of “Aether.”

Johannes Kepler

• Used Tycho’s observations to propose an updated heliocentric model.

• Suggested that planets have elliptical orbits.

• Developed the three laws of planetary motion.

Kepler’s Three Laws of Planetary Motion

1. Orbital paths of the planets are elliptical, with the sun at one focus.

2. An imaginary line connecting the Sun to any planet sweeps out equal areas of the ellipse in equal intervals of time.

3. The square of a duration of planet’s revolution around Sun is proportional to the cube of its semi-major axis.

Perihelion

The point on the elliptical path that is closest to the Sun.

Aphelion

The point on the elliptical path that is furthest from the Sun.

Astronomical Unit (AU)

The mean distance between the Earth and the Sun.

Galileo Galilei

• First to make observations of the sky with a telescope.

• Provided indisputable evidence for a heliocentric model of the universe and that planets and moon are not perfect spheres.

• Mapped the moon and discovered its marias.

• Observed the phases of Venus.

• First to discover Jupiter’s planets Io, Europa, Ganymede, and Calisto.

• Observed Saturn but failed to locate the ring system.

• Discovered that the Sun rotated.

Sir Isaac Newton

• Provided the best explanations of Kepler’s Laws.

• Created the 3 laws of motion and law of gravitation.

Newton’s 3 Laws

1. Every body continues in a state of rest or uniform motion in a straight line, unless acted upon by an outside force.

2. F = ma.

3. Every action has an equal and opposite reaction.

Newton’s Law of Gravitation

The magnitude of the force of gravity between 2 objects is dependent upon the masses of the objects and the distance between them.