Section 3

Constellations  Sections of sky with recognizable star patterns  Used by ancient people for navigation  Also used to keep track of time

Constellations (con’t.)  88 constellations  Each constellation borders another constellation (like states in a country)  Every star/galaxy belongs in 1 of the 88 constellations

Constellations & Seasonal Changes  Different constellations are visible in the Northern & Southern Hemispheres  Due to Earth’s revolution around Sun, the locations of constellations appear to change

Finding Stars in the Night Sky Relative to Observer  Astrolabe Device used for measuring height of objects Used for determining altitude of stars Needs 3 points of reference to work ○ Zenith ○ Altitude ○ Horizon

Zenith  Imaginary point in sky  Directly above observer  Altitude is always 90 degrees

Horizon  Line where sky & Earth appear to meet

Altitude  Angle between the object & the horizon

Describing Star Position Relative to Earth  Astronomers use the celestial sphere  An imaginary sphere surrounding Earth

Parts of Celestial Sphere  Celestial equator Made by extending the Earth’s equator into space

Parts of Celestial Sphere  Declination Measures star position north or south of celestial equator Measured in degrees

Parts of Celestial Sphere  Right Ascension Measures star position eastward from vernal equinox Measured in hours

Declination & Right Ascension

Path of Stars Across the Sky  Planets & stars appear to rise & set  Caused by rotation of Earth on its axis

Circumpolar Stars

 Always visible at a certain latitude  Circumpolar stars never set  Appear to circle the celestial poles

Due to Earth’s revolution, different constellations are seen at different times of yr. Summer Constellations in Northern Hemisphere Winter constellations in Northern Hemisphere

The Size & Scale of the Universe  Stars are much farther away than the planets.  Distance between stars varies  New length unit created to measure the distance of the stars: light-years The distance light travels in 1 year About 9.46 trillion kilometers

The Doppler Effect  The effect produced when a source of waves is moving with respect to the observer

Doppler Effect (con’t.)  Happens with all types of waves – water, sound, light  Happens with light sources in the universe

Redshift Galaxies  Are moving quickly away from Earth  Appear red because of the Doppler effect

Blueshift Galaxies  Are moving quickly toward Earth  Appear blue because of the Doppler effect

An Expanding Universe  Discovered by Edwin Hubble  Light from all galaxies except those close to us is affected by redshift  Galaxies are moving away from each other, so the universe is expanding