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 for m. Stars form w hen parts of nebulae crushed by own gravity.
Solar sys.-Earth is in it, and it consists of the Sun & objects t hat 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
bor n.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 f ields 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 hor izon to
nor th horizon is called meridian.
Angular size-the angle it appears to span your f ield 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 nor th celestial pole never set or rise but make counterclockwise circles.
Stars near the south celestial pole never r ise above horizon. All other stars r ise in east and set in west.
Latitude-measures north-south pos ition, 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 bot h hemisphere.
Solstices&Equinoxes-4 moments define seasons. Summer solstice-N.hemisphere tipped most to sun.etc
precession-wobble tha t 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.