Chapter 4 Flashcards
When were the first observatories built and by whom?
The first observatories were built by the Mesopotamians and were built 6000 years ago.
Greek Astronomy: What were the first principles and beliefs?
Models were generally wrong because they were based on wrong “first principles”, believed to be “obvious” and not questioned:
- Geocentric Universe: Earth at the Center of the Universe.
- “Perfect Heavens”: Motions of all celestial bodies described by motions involving objects of “perfect” shape, i.e., spheres and circles
What were Aristotle’s(384-322 B.C.E) discoveries in astronomy?
Aristotle proved the Earth was a sphere because:
1. The Earth produced curved shadows on the moon during a lunar eclipse.
2. As you move N to S, you see different stars in the sky.
3. All objects fall straight down
.On a flat Earth, most objects would fall at an angle.
What were Aristarchus’(ca. 310-230 B.C.E.) discoveries in astronomy?
He was able to obtain a value for the relative sizes for the Earth and Moon – and the relative distances for the Moon and the Sun.
.He also was the first to deduce that the Earth
must orbit the Sun. This idea was not accepted.
1. The relative sizes of the Earth and the Moon determined from Lunar eclipse:
.D(Earth) = 3 D(moon)
What did Aristarchus measure as the relative distance to the sun and moon from the Earth?
Aristarchus found this, A, to be 87 degrees.
1. That puts the Sun 19 times farther from the Earth
then the Moon is from the Earth.
2. The true value for A is 89.83 degrees which
places the Sun 337 times farther from Earth than the
Moon is from the Earth.
What were Eratosthenes’(c. 276 - c. 195 B.C.E.) discoveries in astronomy?
The Sun is at its highest point during the solstice. However, even during noon, the Sun casts a shadow (in Alexandria, Egypt). In Syene (500 stadia to the south), the sun casts no shadow.
The angular distance between Syene and
Alexandria is approximately 7 degrees.
The linear distance between Syene and
Alexandria:
Approximately 5,000 stadia.
Earth Radius is approximately 40,000
stadia(probably about 14 percent too large, but
it's
hard to how much the stadia was measured as.) -
better than any previous radius estimate before
him.
What were Hipparchus’(134 B.C.E.) discoveries in astronomy?
He Improved on many of the observations of Aristarchus.
.Hipparchus found that the positions of the stars
had changed over a 170 year period: precession
Precession:
.The north celestial pole moves in a circle among
the stars
1. Currently, Polaris is the star closest to the
north celestial pole: the North Star.
.The entire period of precession takes 26,000 years
What were Tolemy’s(127-151 C.E.) discoveries in astronomy?
Planets ordered out from Earth by sidereal period
1. Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn.
Geocentric:
1. Deferent: Major circular path
2. Epicycles: minor circular path about deferent
3. Geocentric model must explain retrograde motion
.Retrograde motion is the apparent backward
motion of the plenty with respect to the fixed
starts (E to W).
The ptolemic system was considered the “standard model” of the Universe until the Copernican Revolution. It was introduced to explain retrograde.
What were Copernicus’(1473-1543) discoveries in astronomy?
Advocate of the heliocentric model of the Universe
1. Correctly ordered planets about sun
.Inferior planets: Mercury, Venus
.Superior planets: Mars, Jupiter, Saturn
2. Thought the heliocentric model could explain
retrograde motion without epicycles.
What is Retrograde Motion?
It is the apparent backward motion (E to W) of a planet with respect to the background stars, caused by the difference in speed of the planets as they orbit the sun. Planets do not actually go backward, it just appears that way. The speed causes an optical illusion that makes planets appear to slow down or reverse in the sky.
What were Tycho Brahe’s(1546-1601) discoveries in astronomy?
- He kept daily observations of astronomy for 25 years.
- He accurately predicted positions of planets to roughly 1 arc minute(about the angular size of a quarter)
- He hired Johannes Kepler (1600)
What were Johannes Kepler’s(1571-1630) discoveries in astronomy?
- He was aware of the Copernican model and thought it was close to true.
- He was aware of Tycho’s data on the heliocentric model and believed that it was a little bit off.
- He took Tycho and Copernicus’ data and refined it to a better version of the heliocentric model utilizing ellipses rather than circles.
- Made Kepler’s three laws of planetary motion.
What is Kepler’s First Law?
The planets orbit the Sun in elliptical orbits, with the sun at one focus. (notice this doesn’t mean that Kepler is saying an orbit pattern can’t be a circle. Since a circle is a specific type of ellipse, he is saying it could be a circle or perhaps a very stretched-out ellipse.)
What is Kepler’s Second Law?
The law of areas;
- The line joining planet and sun sweeps equal areas in equal time units.
- A planet’s speed varies inversely with distance.(the closer to the sun a planet is, the faster it travels, while the further away it is, the slower it travels.)
What is Kepler’s Third Law?
Harmonic Law;
Psquared = acubed P = sidereal (orbital) period in years a = semimajor axis in AU (distance from the sun to Earth)
In other words, the farther out a planet is from the SUn, the longer it takes to complete one orbit.
