Planetary Science Notes

Astronomy is a natural science that studies celestial bodies and phenomena originating outside Earth's atmosphere.
The Universe encompasses everything around us, including all matter and energy, planets, galaxies, and stars.

Ancient people could predict seasons by observing the orientation of the crescent moon illuminated by the Sun.

Solar eclipses were recorded as early as 1375 BC.
Babylonian star calendar combined solar and lunar cycles.
Fragment from the underground library of King Ashurbanipal (668-627 BC) in Nineveh, Iraq.

Thales of Miletus (c. 624 BC – c. 546 BC)
- Focused on timekeeping and changing seasons.
- Proposed that Earth is a flat disc.
- Developed the first known (non-supernatural) model of the Universe.
Plato (427-347 BC)
- Believed heaven is circular in shape.
- Suggested that stars and planets are fixed on a sphere.
- Proposed that all stars and planets rotate around the Earth (geocentric model).
Ptolemy (c. 90 AD-168 AD)
- Wrote "Almagest," covering spherical astronomy, solar, lunar, and planetary theory, eclipses, and fixed stars.
- Believed that fallen objects move towards the center of the Universe.
- Advocated for a geocentric model, stating Earth is the center of the Universe.
- Developed an atlas of the world.
- Cataloged 1022 stars and 48 constellations, giving each star a celestial longitude and latitude.

Nicolaus Copernicus (1473-1543)
- Proposed the heliocentric model, placing the Sun at the center of the Solar System.
- Suggested that the Universe is immense.
- Argued that star positions are not fixed.
- Explained the correlation between the distance from the Sun and orbit size.
- Explained Earth's and Moon's motions.
- Explained differences in star brightness based on varying distances from Earth.
Giordano Bruno (1548-1600)
- Hypothesized countless suns and Earths rotating around their suns.
- Suggested these countless worlds are inhabited like our Earth.

Galileo Galilee (1564-1642)
- Used a telescope to examine the Solar System.
- Observed mountains on the Moon, sunspots, and Venus going through Moon-like phases.
- Discovered four satellites of Jupiter.
- Observed numerous stars in the Milky Way.
Tycho Brahe (1546-1601) & Johannes Kepler (1571-1630)
- Brahe made precise, systematic measurements of planetary and stellar movements.
- Observed a supernova in 1572, proving that the new star was farther away than the Moon.

I. Law of Ellipses: Each planet's orbit around the Sun is elliptical, with the Sun at one focus.
II. Law of Equal Areas: Each planet orbits the Sun so that a line connecting it to the Sun sweeps over equal areas in equal time intervals.
III. Law of Harmonies: The square of the orbital period of a planet (in years) is proportional to the cube of its distance from the Sun in astronomical units (A.U.).
- $p^2 = a^3$
- p = orbital period in years
- a = distance in A.U.
- Example: Mercury orbits the Sun in 88 days, Neptune in 165 years.

Theoretical framework developed by Isaac Newton (1642-1727).
I. Law of Inertia: A body remains at rest or moves in a straight line at constant speed unless acted upon by a net outside force.
II. Second Law:
- $F = ma$
- F = force, m = mass, a = acceleration
III. Third Law: Whenever one body exerts a force on a second body, the second body exerts an equal and opposite force on the first body.

Developed by Isaac Newton (1642-1727).
$F = G \frac{m<em>1m</em>2}{r^2}$
F = force, $m<em>1$ and $m</em>2$ = masses of two bodies, r = distance between the bodies, G = gravitational constant.
There is an attractive force between any two massive bodies.
Gravity is a universal force.