The Nighttime Sky

Visible stars on a clear night Unaided human eye – ______ stars At any one time – ______ stars – ___above horizon, ___ below horizon Reality –Suburban: ______ –Inner-city: ______ 6,000 3,000 1/ light pollution and other factors limit visible stars to a few hundred light pollution

Constellations In ancient times, constellations only referred to the brightest stars that appeared to form groups, representing mythological figures.

Orion, the HunterLeo, the Lion Cygnus, the Swan Scorpius, the Scorpion Antlia, the Air Pump

Today, constellations are well-defined regions on the sky, irrespective of the presence or absence of bright stars in those regions.

Constellation names are derived from the myths and legends of antiquity. Belt of Orion

Looking at the Sky Constellations – _____ official constellations Many origins –Greeks, Southern European cultures, Middle East, Asia, etc. Traced back as far as 5,000 years ago Stars define the celestial sphere 88

Constellations Constellations are patterns of stars that the eye picks out. The stars are usually not near each other; they just lie in the same direction.

Ursa Major Big Dipper, the Asterism in Ursa Major Big Dipper 3D animation

The stars of a constellation only appear to be close to one another Usually, this is only a projection effect. The stars of a constellation may be located at very different distances from us.

Stars are named by a Greek letter (  ) according to their relative brightness within a given constellation + the possessive form of the name of the constellation: Betelgeuze =  Orionis, Rigel =  Orionis Betelgeuze Rigel Orion

Observing Focus: Ursa Major (Big Bear, Big Dipper) Alcor, Mizar

Finding stars Polaris, Arcturus, Spica using big dipper “Take arc to Arcturus” “Then ‘spike’ Spica” Pole Star (Polaris)

Winter constellations

Real motion in the solar system Earth is spinning (_______) on its axis Earth is orbiting (________) around the Sun All planets are orbiting around the Sun Rotation Revolution

Paths of Stars Earth’s _______________ rotation –Stars rise in the East, set in the West But some stars never set! –Polaris stays nearly stationary in the sky –Stars near Polaris move slightly, in circular, counterclockwise paths –Some objects are never visible from the north Magellanic clouds (SH) Star positions change from night to night as the Earth revolves about the Sun – rise and set 4 minutes earlier each day. Counter-clockwise

Risings and Settings The spin of the earth causes the stars to appear to move about the celestial pole. Some stars are therefore circumpolar and never set, while others dip below the horizon. Which stars are which depend on where you are on earth. Polaris happens to be very near the North celestial pole.

Standing at the North Pole NCP SCP Star Celestial sphere Earth Celestial equator Rotation of CS

Apparent motion: Earth’s rotation Imagine standing at the North Pole –The North Star is straight overhead –All of the stars move in big ______ along the horizon –Stars never rise and never set! circles

Circumpolar Stars - SH

Circumpolar Stars

What you see depends upon where you are Horizon for observer at north pole NCP Star Earth np Horizon for observer at equator NCP Star Earth np

Apparent motion: Earth’s rotation Imagine standing at the equator –The North Star is on the horizon –All of the stars rise in the East, ___________________, and set in the West pass straight overhead

The Night Sky at the Equator North & South Celestial Poles are on the horizon

Most of us are in between these extremes we see fixed Polaris (N. Celestial Pole star) over the night, stars move WESTWARD about it E W Looking North

Constellation Motion

Apparent Motion Star trailsStar trails

Mid-latitudes

Apparent Motion

Diurnal Motion Each day, the earth rotates once (counter clockwise) on its axis. This causes us to face different directions and see different stars. The stars’ daily (diurnal) motion reflects the earth’s spin.

The Yearly Motion In addition to rotating, the Earth also revolves about the Sun. As the earth revolves the Sun is projected in front of different constellations at different times of year. The path the Sun takes across heavens is called the ecliptic. The constellations which the Sun passes through are zodiac constellations. zodiac constellations Because the Sun is bright, we can only see some constellations at certain times of year.

Apparent motion: Earth’s rotation The celestial sphere appears to rotate once every 24 hours Polaris (the “North Star”) is very close to the North Celestial Pole Everything appears to move in circles around the North Star

Apparent motion: Earth’s orbit: Seasonal stars & constellations To see the stars at night, you must look away from the Sun As Earth moves in its orbit, the stars you can see at night keep changing Constellations and stars are seasonal

Apparent motion: Earth’s orbit: The apparent motion of the Sun The Sun is always ‘in front’ of some stars As Earth moves in its orbit, the Sun’s apparent position in the sky changes The constellations that the Sun passes in front of are known as the zodiac