lecture_17 - F a c t s C o n c e r n i n g t h e S o l a r...

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Unformatted text preview: F a c t s C o n c e r n i n g t h e S o l a r S y s t e m • All the planets roughly orbit the Sun in a plane. • This plane is roughly the same as the rotational equator of the Sun. • Planetary orbits are, for the most part, circular. • The planets all revolve in the same direction about the Sun. • The Sun and the planets, with the exception of Venus and Uranus, rotate on their axis in the same direction. With the exception of Uranus, the tilt between the equator and the orbital plane of the Sun and planets is small. • The planets differ in composition: the planets nearest the Sun tend to be small, dense and metal-rich, whereas the planets farthest from the Sun tend to be large, light and hydrogen-rich. • Meteorites differ in chemical and geologic properties from planetary and lunar rocks, but may be similar in composition to asteroids and small moons. • Planets and most asteroids rotate with similar periods, about 5 to 10 hours, unless obvious tidal forces slow them, as in the case of the Earth and Venus. • The distances of the planets from the Sun generally obey Bode’s Law, which is a simple geometric progression. • Planet-satellite systems resemble miniature solar systems. • Cometary orbits, as a group, define a large, almost spherical, cloud around the Solar System. • The Sun contains 99% of the mass in the Solar System, but the planets, not the Sun, contain nearly all the angular momentum of the Solar System. Lattimer, AST 248, Lecture 17 – p.1/22 A B r i e f H i s t o r y o f S o l a r S y s t e m D i s c o v e r i e s 300 BC Aristotle – argued the Earth is round because of the circular shadow it cast during lunar eclipses, and that some stars are not visible from northern locations 270 BC Aristarchus – Earth revolves around Sun; Moon revolves around Earth; estimated size of the Moon from lunar eclipse; measured relative distances and sizes of Sun & Moon; concluded Sun was much bigger than the Earth 250 BC Eratosthenes – Measured shape & size of Earth 135 BC Hipparchus – Discovers precession of the equinoxes; provides better estimate of the Moon’s distance; rejects notion that Earth orbited Sun. 100 AD Ptolemy – Proposed Earth to be at center of Universe (Aristotle) 1473–1542 Copernicus – Earth and planets orbit Sun 1546–1604 Tycho Brahe – Measured detailed planetary positions 1571–1630 Kepler – Analyzed Brahe data and deduced 3 Laws of Motion: • Each planet moves in an ellipse with Sun at one focus (Wrong: focus is at center-of-masss) • A line between Sun and a planet sweeps out equal areas in equal times • Orbital period squared is proportional to semi-major axis cubed; 1–2–3 Law as corrected by Newton: G ( M ⊙ + m planet ) 1 P 2 = 4 π 2 a 3 1564–1642 Galileo – Laws of motion; telescopic observations proved Copernican theory 1642–1727 Newton – Law of gravity and derivation of Kepler’s 3 laws 1656–1742 Halley – First good estimate of size of solar system from Venus transits;...
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lecture_17 - F a c t s C o n c e r n i n g t h e S o l a r...

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