first midterm notes
DepartmentAstronomy & Astrophysics
Chapter 1 Our place in the Universe
geocentric-ancestors thought we lived in an Earth-centered universe.
Planet-large bodies which orbit stars,may be various mixture of rocky,gaseous,icy.1. Orbits stars directly,2
massive enough to be compress them into spherical shapes,3. Must have cleared their orbits.
(Pluto is a dwarf planet 'cuz its orbit is full of objects like comets)
Asteroid-rocky Comet-icy Moon- orbits a planet
Nebula-a cloud of gas and dust, cud see light emitted by gas & parts where blocked by dust. Place where
stars form. Stars form when parts of nebulae crushed by own gravity.
Solar sys.-Earth is in it, and it consists of the Sun & objects that orbits it-planets, moons, countless small
objects like rocky asteroids and icy comets. The sun is a star in the milky way.
Galaxy-a great island of stars in space, containing fr. few hundred million to a trillion or more stars. Earth
is at more than halfway from galactic center to the edge of the galactic disk.
-some galaxies are isolated, some in groups. Milky way is one of the two largest among 40
galaxies in the local group.Its a huge disk shaped collection of 100 billion stars.
-galaxy cluster-groups of galaxies w/ more than a few dozen members.
-supercluster-region which galaxies/ g. clusters tightly packed. Our supercluster is called Virgo.
Universe expanding-avg. Distance b/w galaxies are increasing w/ time. The beginning of expansion is the
big bang-14 billion yrs ago. But galaxies don't expand.
Nuclear fusion-process which lightweight atomic nuclei smash tog.to make heavier nuclei.stars are
born.gravity compress material in cloud ,the center becomes dense and hot to generate energy
Early universe- only hydrogen& helium.Earth had more-CO2,O2,etc, from stars. We are star stuff.
Light years-light fr. Star takes many yrs to reach us, so measure distance to stars in units called light year
eg.Sirius,8 light yrs away, the appearance we see it today is actually how it looks 8 yrs ago.
Observable universe-portion of entire universe that we can potentially observe.-14 billion light yrs.
Voyage Scale-model solar sys.- sun as a grape fruit, jupiter as a marble, earth as ball point (pen). Earth is
15 m away. Solar sys. Is empty, so 300 football fields model.
Universe is never static-Earth rotates on its axis once every 24 hrs-solar day. Has axis tilt 23.5 degrees.
Earth revolves around or orbits the sun once every 365.24 days. Whole solar sys.
orbits center of Milky way every 230 million yrs.
Ecliptic plane-defined by Earth's orbital path around sun.
1 Astronomical unit (AU)- 1.5* 10^11 m=avg. Distance b/w earth and sun.
Chapter 2 Discovering the Universe for yourself
Constellations-a region of the sky with well-defined borders, it's located by familiar patterns of stars.
Celestial Shere-illusion that stars seem to lie on a great sphere surrounding Earth.
Ecliptic-Sun's apparent annual path around the celestial sphere. Crosses celstial equator 23.5.
Milky way is seen as a band of light, when look out of galactic plane, have clear view of universe, if look
into the galactic plane, we see stars and intersteller clouds that make up milky way.
Local sky-half of celestial sphere that seen any time. The point directly overhead is Zenith. The boundary
b/w Earth and sky defines Horizon. The imaginary half circle stretching from south horizon to
north horizon is called meridian.
Angular size-the angle it appears to span your field of view.Angular distance-of pair of object is angle that
appears to separate them. 1 degree=60 arcminutes 1 arcminutes=60arcseconds.
Circumpolar-stars near north celestial pole never set or rise but make counterclockwise circles.
Stars near the south celestial pole never rise above horizon. All other stars rise in east and set in west.
Latitude-measures north-south position, parallel to the equator. Longitude-measures east-west.
-Stars vary with latitude ,not with longitude. Altitude of celestial pole=latitude.
Season-Earth's axis point in same direction throughout the yr.the northern hemisphere is tipped toward the
sun in June and away in Dec. while reverse for southern hemisphere. In June, sunlight strike N.
hemisphere at steeper angle(concentrated sunlight and longer and higher path through sky).Spring
and fall begin when sunlight falls equally on both hemisphere.
Solstices&Equinoxes-4 moments define seasons. Summer solstice-N.hemisphere tipped most to sun.etc
precession-wobble that alters orientation of Earth's axis in space.axis sweeps a circle. Doesnt change tilt.
Moon-takes 27 1/3 days to orbit Earth. Lunar Phase-depends on position relative to sun as orbits Earth.
Synchronous rotation-moon rotates on its axis in the same amount of time it takes to orbit Earth. So we
always see the same face of the moon.
Eclipse-When sun, moon, earth in a straight line with every new and full moon and nodes of moon is
aligned with sun and Earth. Two regions of shadows- central umbra, and penumbra (partial).
-Lunar Eclipse-Earth b/w sun and moon.total(moon in umbra and is red)/partial(part in
umbra)/penumbra lunar eclipse(in penumbra).
-Solar Eclipse-Total(When moon is close to earth, Moon's umbra touches small area of Earth)/Partial
(part of sun blocked)/Annular(moon far fr. Earth,a ring of sunlight surrounds moon) Solar Eclipse.
Nodes-as moon's orbit inclined to ecliptic plane.The point when crosses is called nodes
Saros cycle-combination of changing dates of eclipse season and 291/2day lunar phases makes eclipse
recur in a cycle of abt 18yrs 11 1/3 days.
Apparent retrograde motion-when planets move westward (reverse)-but not actually change direction,its
just perspective fr. Earth, when sth. Move slower it looks moving backwards
Stellar Parallax-apparent shift in position of nearby star as we look at it fr different places in Earth's orbit
Chapter 3 The Science of Astronomy
Ancient ppl used positions of sun,moon and stars to keep track of time, weather, seasons.They used star
clocks, sun dales, water clocks. They used Stonehenge, Templo Mayor to mark seasons.
Copernicus-sun-centred model. Tycho-accuarate naked-eye observations.but thought Sun orbits earth.
Kepler's Laws-1.the orbit of each planet around the sun is an ellipse w/ sun at one focus. Distance vary
2.as planets moves around orbit, it sweeps out equal areas in equal times. Travels faster
when closer to the sun, slower when farther fr sun.
3.distant planets orbit the sun at slower avg. speeds obeying a precise mathematical relatio-
n. p^2=a^3 (square of orbital period = cube of avg distance fr sun.)
Hallmarks of Science-1, Seek explanations for observed phenomena that rely solely on natural causes.
2. Progress thru creation & testing of models of nature that explain the
observations as simple as possible
3. Make testable predictions abt natural phenomena. If predictions dont agree
w/ observations, model must be revised or abandomed.