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Astronomy 311: Lecture 0 - Intro Physics
• Diurnal or daily motion: Earth rotates eastward, sky rotates westwards.
• Ecliptic: apparent motion of the Sun through the sky during the course of
the year, ecliptic plane.
• Earth’s spin axis tilted at 23.5 degrees to ecliptic plane.
• Seasons created by tilted spin axis.
• Earth’s orbit is slightly elliptical: perhilion/aphelion are the points in orbit
nearest/furthest the Sun.
• Early Ideas on Planetary Motion.
– Geocentric model, Greeks (Plato, Aristotle Ptolemy), circular motion.
Retrograde motion, epicycles.
– Copernicus ”started” heliocentric models but still circles. Complictaed
and didnt predict position of Mars to 4 moon widths.
– Galileo observed phases of Venus - ie it orbits the Sun not the Earth.
– Brahe: accurate naked eye observations of planets and stars.
– Kepler: discovery of elliptical motion using Brahe’s observations - better predicitions of Martian motion.
– Ellipse: 2 foci, fig. 3.18, p. 72.
– The eccentricity of the ellipse measures how flat it is:
e = c/a,
c is the distance between the foci and a is the ”semi-major” axis.
– For a circle, e = 0.
– Perihilion distance = a(1-e).
– Aphelion distance = a(1+e).
– Kepler’s laws of motion, planets orbit the Sun in ellipses with the Sun
at one focus, equal areas are swept out in equal times, P 2 = a3 , where
P is in Earth years and a is in AU. More accurate version of Kepler’s
third law:
(P/2π)2 = a3 /(G(M + m),
where G is the gravitational constant.
• Newton’s laws of motion.
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• Newton’s law of gravity:
F = GMm/r 2 ,
where G = 6.67 × 10−11 Nm2 /kg 2 .
• Surface gravity (units are m/s2 ):
g = GM/r 2 .
• Weight on planet = surface gravity of planet × mass.
• Calculate your weight on Mercury, Venus, Mars and Jupiter using data
from your textbook.
• Velocity ofqan object moving in a circular or nearly circular orbit (eg a
planet) is GM/r, where M is the mass of the Sun and r is the distance
of the planet from the Sun. It doesnt depend on the mass of the planet.
• Escape Velocity: velocity needed to escape the gravitational pull of a planet,
or minimum velocity need to reach r = ∞:
Vesc =
q
2GM/r,
• Electromagnetic Spectrum
– Light described by wavelength and frequency such that the speed of
light, c is its wavelength (λ) times frequency (ν): c = λν.
– The Electromagnetic Spectrum is the complete range of light’s wavelengths and frequencies.
– Gamma rays, X rays, UV, visible, IR, Microwave, Radio.
– Photons: particles of light. Wavelength of a photon increases as its
energy goes up.
– A blackbody radiator is a body that absorbs all radiation falling on it
and re-radiates such that the spectrum of the radiation has a specific
form that depends only on the temperature of the black body T .
– Hot objects radiate more strongly at blue wavelengths than cool objects:
λmax × T = 3000000.
This is Wien’s law.
– Stars peak in UV (hot stars) to short IR (coolest stars).
– Planets/moons tend to be black bodies with a temperature of about
700-60K which peaks in the long IR.
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