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Astronomy 10 Week 1 Summary
Chapters 1 and 2
1. Our place in the universe and it’s scale: Solar System is about 100 AU in size. Stars are
typically a few lightyears apart. Galaxies are about 100,000 light years in size, and millions
of light years apart. The observable universe is billions of light years across.
Astronimical Unit: 1 AU = The distance from the Earth to the Sun = 1.5 × 1011 m =
93,000,000 miles
Lightyear: 1 ltyr = The distance light travels in a year = 9.5 × 1015 m = 5.9 × 1012 miles
2. Sky Motions: Every day, everything in the sky rises in the east and sets inthe west due to
the Earth rotating. Planets are seen to wander amongst the stars along the ecliptic. Though
they normally wander from west to east relative to the stars, once in a while they exhibit
retrograde motion. Also, the orientation of the sky changes over many thousands of years
due to the precession of the Earth.
Ecliptic: This is the path the sun travels relative to the stars over the course of a year, or
equivalently, it is the plane of the Earth’s orbit around the sun projected into the sky.
Retrograde Motion: This is the apparent backwards loop that other planets are observed to
make in their paths through the sky.
Precession: The changing of the Earth’s axis of rotation over time, much like a wobbling
top.
3. Seasons: The seasons are due to the tilt of the Earth’s rotation axis, causing different parts
of the Earth to recieve more or less sunlight at different times of the year.
4. Tides: Tides are caused by the Moon (and to a slightly lesser extent the Sun) pulling on the
Earth. Water on the side of the Earth towards the moon is pulled stronger than the Earth
itself, and so is raised up. Water on the far side is pulled less strongly than the Earth, and
so the Earth is pulled away from it, causing it to appear to be raised as well. Hence there
are two tides per day and the Earth rotates through these bulges.
5. Moon Phases: The phases of the Moon are simply caused by the relative position of the
Sun and Moon from the point of view of Earth. The side of the Moon facing the Sun is
always lit, and the phase depends on how much of that side we can see. When the Moon is
between the Sun and Earth, we see a New Moon. When the Earth is between the Sun and
Moon, we see a Full Moon. When the Sun and Moon are at right angles to each other, we
see a Half-Moon. At New and Full Moons, if the alignment is just right, we see a solar or
lunar eclipse, respectively.
6. History: A listing of some of the major historically important astronomers:
Aristotle: First explained the moon phases and noted that the Earth was round by watching
it’s shadow during a lunar eclipse.
Eratosthenes: First measured the circumference of the Earth by noting differences in shadows cast by pillars in different places on the same day.
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Hipparchos: First noted precession, measured the length of the year, and made the first
major star catalog.
Ptolemy: Made the first good predictive model for the motions of the planets. His geocentric
model, which included epicycles, was the best theory for over 1500 years!
Copernicus: First proposed a detailed, heliocentric model.
Tycho Brahe: Revolutionized precission of measurements and collected vast data on the
positions of planets, by far the best of its time.
Kepler: Used Brahe’s data to discover his three laws of planetary motion, including that
the orbits were ellipses and not circles.
Galileo: Used telescopes to find sunspots, mountains on the moon, 4 moons of Jupiter, note
the Milky Way was MANY stars, and see phases of Venus, which proved the heliocentric
model correct.
Newton: Formulated the universal law of gravitation, and used it to explain why Kepler’s
Laws worked.
7. : Formulas and Laws: A few important ones to note so far.
Kepler’s First Law: Planets orbit in ellipses with the Sun at one focus.
Kepler’s Second Law: A planet sweeps out equal areas in its orbit in equal times, meaning
that it moves faster while closer to the Sun and slower while farther away.
Kepler’s Third Law: P 2 = a3 where P is the period of the orbit and a is the average distance
between the planet and the sun (the semimajor axis of the ellipse).
m
Gravity: FG = GM
G is a constant, M and m are the masses of the two bodies involved,
r2
and r is the distance between them.
Newton’s First Law: A body at rest stays at rest and a body in motion stays in motion in
a straight line, both unless acted upon by an outside force.
Newton’s Second Law: F = ma, F is the force, m is the mass of the object, and a is its
acceleration.
Newton’s Third Law: For every force, there is an equal and opposite force exerted as well.