astro1 Flashcards
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.
Kuiper Belt
Comets are ice-rich, & many are found in the region beyond Neptune.
Asteroid Belt
Region between Mars & Jupiter where asteroids, made of metal & rock, orbit.
what is mercury mostly made of
Made of metal (mostly Fe) & rock: most metal-rich of all planets!
describe the atmosphere of mercury
No atmosphere:
No wind, no rain
stars can be seen even in daytime!
Very hot (during the day) & very cold (at night)
what is the rotation of mercury
it
rotates exactly 3 times for every 2 of its orbits of the Sun
what is the length of a day in mercy
176 earth days
what is the volcanic and tectonic activity in mercury
No current volcanic or tectonic activity. However, there is evidence
of past geological activity:
are there lava plains in mercury
lains created by ancient lava flows
what planet has the most massive atmosphere of the terrestrial planets
vennus
describe the atmosphere of venus
Atmosphere has no O 2 or liquid H2O, 96% CO 2 & pressure = 92pEarth intense greenhouse effect temperature of 470°C day & night Hellish conditions!
what hides the surface of venus from view
by dense clouds of H
2SO
what are the winds like in venus, if I wanted to go kite surfing
High speed winds keep the thick clouds in constant motion: they zip around
the top of the planet’s atmosphere every 4 days! However, close to the
surface wind speeds drop to only a few km/h.
what is the length of a day on venus
A long solar
day of 117 days!
where does the sun rise on venus
west
name a planet that doesn’t have a magnetic field
venus
what is a sidereal year
Time for Earth to complete one orbit around
the Sun
what is a tropical year
Time for Earth to complete one full cycle of
seasons = the time from one spring equinox to the next one.
what is a sidereal period
Time for a planet to complete one orbit around the
Sun.
what is a synodic period
Time between the moments when a planet is lined
up with the Sun in our sky at one time and the next similar
alignment.
what is a planet’s line up with the sun called
called a conjunction;
what do we call an “opposition”
When the planet appears in a position exactly opposite to that of
the Sun (i.e., if you were on that planet you would see Earth
‘eclipsing’ the Sun)
Which planets can never have an opposition
inner planets (closer from the Sun than Earth: Mercury, Venus):
Which planets will have two conjunctions
inner planets (closer from the Sun than Earth: Mercury, Venus):
what is a superior conjunction
when the planet appears behind the Sun (i.e. is eclipsed by the Sun)
what is the diameter of an eclipse
within
umbra (an area ~270 km in
diameter on Earth’s surface)
what is a partial solar eclipse
within
penumbra (an area ~7,000 km
in diameter on Earth’s surface)
what is an annular solar eclipse
Moon
is relatively further away & its
umbral shadow does not reach
Earth
what are the two types of eclipses
Lunar Eclipse = Earth is between Sun & Moon.
Solar Eclipse = Moon is between Sun & Eart
what are the conditions for an eclipse to occur
The phase of the Moon must be full (for lunar eclipse) or new (for solar
eclipse)
The new or full moon must occur during one of the periods hen the nodes
of the Moon’s orbit are aligned with the Sun and Earth
what is the saros cycle
18 years 11
⅓ days
what makes eclipses recur in a cycle of 18 years 11
⅓ days
The combination of changing dates of eclipse seasons and
the 29.5 days cycle of lunar phases
What causes the phases of the moon
The amount of the illuminated half visible from Earth depends on the Moon’s position on its orbit
what is the precession of earth
The direction of Earth’s rotation axis is not
fixed in space but executes a slow
precession (like a top) with a period of
26,000 years
→ Precession = a gradual
wobble that alters the orientation of
Earth’s axis, which thus sweeps a circle
at a slow rate
what is the north star
polaris
where is earth’s axis pointed at
polaris
where will earth’s axis be pointing in 13000 years
axis will point to Vega (within
a few degrees).
does earth’s precession change due to precession
It does NOT change the axis tilt which
stays close to 23.5
Do the Positions of solstices & equinoxes in Earth’s
orbit gradually shift with cycle of precession.
yes
where does earth get most of it’s direct sunlight
on the equinoxes, & its
least direct sunlight on the solstices.
where are the seasonal variations around the time of solstices more extreme
high latitudes
when do equinoxes occur
2 days of year on which the Sun rises precisely due E and sets precisely due W
Why doesn’t the orbital distance affect our seasons
Earth is only 3% farther from the Sun at aphelion than at
perihelion.
Small variation overwhelmed by effects of axis tilt
what are the four special moments in the year
Summer Solstice (21 June)
The Nern hemisphere receives its most direct sunlight
Winter Solstice (21 December)
The Nern hemisphere receives its least direct sunlight
Spring Equinox (21 March)
The Nern hemisphere just starts to tip towards the Sun
Fall Equinox (22 September)
The Nern hemisphere just starts to tip away from the Sun
how many months does the sun spend north & south of the celestial equator in a year
6 months
What constellations can one see?
It depends on your latitude & time of the year The altitude of the celestial pole in your sky is equal to your latitude The constellations you see depend on your latitude but not longitude.
Because latitude affects the locations of the horizon & zenith relative to
the celestial sphere
What are circumpolar stars
Circumpolar stars always reside above the horizon, and for that reason, never rise or set. All the stars at the Earth’s North and South Poles are circumpolar. Meanwhile, no star is circumpolar at the equator.
how long was the entire process of accretion when the planets were forming
The entire process took less than ~50
m years
Known from age of the oldest rocks in the solar system
what is the most reliable method to measure a rock’s age
radiometric dating
