astro1 Flashcards
(72 cards)
How was time estimated at night for ancient egyptians
At night, time was estimated from the
position & phase of the Moon
How does Newton’s law of gravity extend Kepler’s laws?
1 Objects orbit their common center of mass (the point at
which 2 objects would balance if they were connected)
2 Allows the mass of a distant (massive) object to be
calculated IF you can observe another object orbiting it
What is the speed needed to orbit the earth in freefall
28000KMH
who saw sunspots on Sun (imperfections), mountains & valleys on the Moon (not
a perfect sphere)
Galileo
what is Stefan-Boltzmann Law
Stefan-Boltzmann Law explains the growth in height of
Planck’s radiation curve asT↑.
The Doppler effect refers to the
change in frequency of a
wave for an observer moving relative to the source of the wave.
The amount of shift is proportional to the relative velocit
Due to the Doppler effect:
The light (spectrum) of an object moving towards us is blueshifted The light (spectrum) of an object moving away from us is redshifted
Doppler’s shift does NOT tell us
the tangential component of the object’s movement = how fast the object is moving across our line of sight
Long wavelength radio wave telescopes have to be
e very large to achieve reasonable angular resolution. Angular resolution is proportional to wavelength λ
why are Ground-based telescopes at wavelengths outside
the transmissionwindows are completely useless!
Only certain wavelengths can be transmitted thru Earth’s
atmosphere:
Adaptive optics are used to
cancel out turbulences: the shape(s) of
the mirror(s) are computer-controlled to rapidly change so as to
compensate in real-time for the atmospheric distortions.
For this purpose the image of a bright star (near the object of study) is
observed, OR an artificial one can be created by shining a powerful
laser beam into the sky
A basic lunar calendar has
r = 12 months, with some of 29 days and others
of 30 days
→ the final average has to agree with the approx. 29.5 day
lunar cycle
A 12-month lunar calendar has only 354 or 355 days!
To avoid having their months cycle thru the seasons, some cultures used what
the fact that lunar phases repeat on the same
solar dates about every 19 years (because 19 solar years = 235 lunar
months) = Metonic cycle
1st recorded by Greek astronomer Meton in 432 B.C., though
Babilonians almost certainly knew about it)
Lunar calendars that follow the Metonic cycle add a 13th month to 7
of every 19 years
Early Greek philosophers, like Plato & Aristotle, adopted what model of the universe
geocentric model of the Universe:Earth at center of a celestial sphere with stars & planets orbiting
around it
Heavens must be “perfect”: objects moving on perfect spheres or in
perfect circles
what are the limitations of the geocentric model
Hard for the geocentric model to explain the
apparent retrograde motion of planets.
Why did the greeks reject the idea of a sun centered model by aristarchus
KEY REASON: their
inability to detect the stellar parallax
they believed all stars lie
on the same spher
what was the copernican revolution
His Heliocentric Model: Sun is at the center of the universe. Earth orbits like any other planets. Inferior planets have smaller orbits. Retrograde motion occurs when Earth “laps” Mars & the other superior planets.
why was the copernican model incomplete
The model had the right general ideas but its predictions were not
much better than those of Ptolemy’s Earth-centered model
because:
Copernicus held fast to the belief that heavenly motion must occur in perfect
circles
Still needed to use epicycles to make decent predictions
As complex as Ptolemaic model but still not more accurate!
what did tycho brahe do
: performed many accurate & comprehensive astronomical & planetary observations
what did johannes kepler do
used Tycho’s observations to discover the truth about planetary motion and eventually deduced what now are known as Kepler’s Laws of Planetary Motion which finally revealed the underlying truth of planetary motion & became accepted as a general model of Natural laws.
Kepler’s First Law
The orbit of each planet around the Sun is an ellipse with the Sun at one focus. Nothing at the other focus
Kepler’s Second Law
As a planet moves around its orbit, it sweeps out equal areas in equal times. A planet travels faster when it is nearer to the Sun & slower when it is farther from the Sun It is actually an embodiment of the law of conservation of angular momentum L for the orbiting object of mass m
Kepler’s Third Law
More distant planets
orbit the Sun at slower
average speeds, obeying
the relationship:
p2 =a3 p = Orbital period, in[years]
a= Average distance from Sun, in [AU] …
Oort Cloud
Even more comets orbit the Sun in a distant, spherical region about a light-year away.
