Cycles of the Sky the celestial sphere - vast distances to stars prevent us from sensing 3D arrangement - created by an ancient astronomer - naked eye observations treat all stars as the same distance, on one big celestial sphere with Earth at the center - an old model which does not match physical reality, what ppl saw w/out telescopes constellations - fixed arrangements of stars that resemble animals, objs, & mythological figures - stars in constellations are not physically related - there are 88 constellations - the simplistic, incorrect, celestial sphere - position of stars change very slowly; constellations will look the same 4 thousands of yrs - constellations do not always appear in the same part of the sky due to earth’s annual motion around the sun (revolution) - ancients used this motion to mark the seasons astronomical unit - the distance from the sun to the earth - around 93 million miles light year - the distance a light photon travels in one year - light has a constant velocity in a vacuum of 186,262 mi per second - equals around 5.88 trillion miles parsec - the distance at which an observer sees the max angle between the sun & earth - equals 3.26 light years or 18.6 trillion miles - degrees can be split up into minutes, and those minutes can be split into arcseconds - really small angle - use parallax angle to measure the distance w/out using a ruler astronomical distance ladder - measurement of cosmic distances relies on diff techniques ○ solar system - uses radar ○ nearby stars - measures w/ parallax ○ milky way - use main-sequence fitting to measure ○ nearby galaxies - use cepheids ○ galaxy clusters - must use type 1A supernova explosions (like a road marker) barnard’s star - 6 light years away from the Earth - 10 billion years old, low mass - a runaway star, doesnt move in the galaxy w/ other general stream of stars - is a red dwarf star, has a lot of flares and has a superearth orbiting it - has a proper motion of 10.39 arcseconds per year (very slight motion) gliese 710 - a star 60% the mass of the sun and 5% the luminosity - 64 light years away, coming towards the earth at 9mi/sec - will take at least 1.3 million years to come within 13k AU from the Sun via Oort Cloud daily motion - the motion of the earth around its axis - we see the Sun, Moon, and planets rise in the east and set in the west - this spin of west to east gives the illusion of this movement - ancient astronomers thought all celestial motion was diurnal (daily) - believed that the “celestial sphere” rotated about the celestial poles annual motion - known as yearly motion - due to the earth’s revolution around the sun - sky is not different day-to-day or year-to-year, but is month-to-month w/ moon’s phases the ecliptic - the path of the sun thru the stars on the celestial sphere - a projection of the earth’s orbit onto the celestial sphere & its tipped relative to the celestial equator - inner planets revolve around the sun faster than the outer planets the zodiac - belt-shaped region of the sky surrounding the ecliptic - passes through twelve constellations - a 13th constellation joined the zodiac, Ophiuchus (snake charmer) earth’s seasons - caused by earth’s tilt - its rotation axis is tilted 23.5° from the sun - northern & southern hemispheres alternate receiving the majority of direct light from the sun and longer days (seasons) - almost everything in the solar system rotates counter-clockwise ecliptic’s tilt - explains the difference in the location of our sun in the sky - equinoxes: when days and nights are equal length; 12hrs - autumn (sept 23rd) and spring (march 21st) equinox - solstices: when the sun has stopped and is abt to reverse, unequal length - winter (dec 21st) and summer (june 21st) solstices - during summer, sun sets northwest; during winter, sun sets southwest - sun is high in the sky during summer months, lower during the winter the moon - we see the moon rise in the east and set in the west (like everything else) - it moves from west to east in reality (bc of earth’s rotation) relative to the stars ○ the phase cycle: the phases are caused by the relative positions of sun, earth, & moon - during a period of thirty days, it goes thru a complete set of phases:

- we always see the same side of the moon eclipse - occurs when the sun, earth, & moon are directly in line w/ each other ○ solar eclipses - occurs when the moon passes between the sun and the earth