In astronomy, temp is like motion  low temp = limited motion, high = increased motionKelvin (k) for temp  0k means no motionK = 273 + CelsiusDistance = speed-of-light x timeSpeed of light = 186,000 mi/sec or 3 x 10^5 km/secWe see the light that left millions of years ago bc although speed-of-light is fast, it does take time to reach usAstronomical unit (AU) = distance from earth to sunSolar mass = mass relative to mass of sun1 solar mass = 1.99 x 10^30 kgAssumptions:There exists an objective, knowable Universe that is the same for everyone.The same physical laws apply throughout the Universe.The same physical laws are equally valid at different times in the Universe.Occam’s Razor: When you have two competing explanations that make exactly the same predictions, the simpler one is the betterNo experiment or observations can measure a value with perfect accuracy. So, all measurements have a value and an associated uncertainty. Knowing the measurement uncertainty can be critical to whether you can reject or accept your hypothesis.What Causes the Seasons?Earth’s tilt = 23.5 degreesSeasons are caused by the tilt of the Earth’s axis, which causes the sun’s path through out sky and the intensity of sunlight to vary over the course of each yearEarth rotates, not sunHorizon – boundary between earth and skyZenith – point directly overheadMeridian – imaginary half-circle stretching from horizon due south, through the zenith, to horizon due north (N to S)Sun reaches zenith only in tropicsSEE PICTURE ON PAGE 20Key point: seasons occur because of combo of earth’s axis tilt and its orbit around sun. If earth did not have an axis tilt, we would not have seasons (bc different sides tipped toward sun at different times this way)Solstice = when a hemisphere is tipped most directly towards sunSummer (June), winter (December)Equinox = when hemisphere changes from being toward sun to away from sun (fall, September) and when away from sun to toward sun (spring, March)Day of these are first days of seasonsOpposite for southern hemisphereAt equator, most direct sunlight on equinoxes and least direct on solsticesAt higher latitudes, seasons are more extreme, with most extreme at poles (sun above horizon for 6 months and below for 6 months)Why do constellations depend on time of year?Constellation = region of the sky with well-defined borders; familiar patterns of stars help us locate these constellationsConstellations make up celestial sphere surrounding earth. It is a useful illusion bc it allows us to map the sky as seen from earthNorth/south celestial pole = point directly over polesCelestial equator = projection of earth’s equator around celestial sphereEcliptic = yearly path of Sun around celestial sphere (annual path with respect to constellations, not daily path through local sky)Earth’s west-to-east rotation causes everything on celestial sphere to appear to move around us from east to west (stars would appear motionless if we were to stop rotating bc stars are so far away, we could not see motion)Can only see half of celestial sphere at one timeAngle from your location to equator = angle from you to celestial equator, so celestial sphere appears tiltedLatitude changes your angle, so it changes how your horizon cuts through celestial sphereSynodic day = time it takes to see sun in same spot in sky, 24 hrs longSidereal day = time it takes to see star in same spot, 23 hrs and 56 min long (360 degrees)Sun appears to move gradually around celestial sphere, completing one full orbit in one year bc as we orbit the sun, it appears to move against background of constellationsConstellations along ecliptic are called the zodiacSun location on ecliptic determines which constellations we see at nightWhy do we see phases of the moon?1. half the ball always faces the sun, half away from sun2. as ball orbits your head, you see different combos of bright and dark facesComplete cycle of phases, from one moon to the next, takes about 29.5 days (a month)New Moon: Sun-Moon near each other. Always during day. Moon sets before or soon after the Sun.Waxing Crescent: Moon rises a little after the Sun. 1st Quarter: Moon rises 6 hours after the Sun.Moon high in the sky when the Sun sets. Full Moon: Moon opposite Sun on sky.Moon rises when Sun sets 3rd Quarter: Moon rises 6 hours before SunWaning Crescent: Moon rises a little before SunSEE PICTURE ON PAGE 27We only see one face of the moonWhat causes eclipses?Lunar eclipse = when earth lies directly between the sun and the moon, so that earth’s shadow falls on the moonEarth’s red light goes through and illuminates moon as redTotal lunar eclipse only when moon in umbra (where sun is totally blocked)Partial when moon half in umbraPenumbral when moon in penumbra (where sun is partially blocked)Solar eclipse = when moon lies directly between sun and earth, so that moon’s shadow falls on earth. People only in area covered by moon’s shadow will see the sun blocked from view.Total solar eclipse is only when you are within the umbra and only lasts a few minutesPartial seen by people in penumbral shadowWhy can you not see the new Moon at midnight?Because it is only in the sky during the dayWhy is the Moon always full during a lunar eclipse?Why a total lunar eclipse seen from more than 1⁄2 of the Earth surface but a total solar eclipse is seen only along a narrow band?Why don’t solar and lunar eclipses happen every month?A neutrino is a neutral particle that interacts very weakly with matterRetrograde motion – when planets move backwards (westward rather than eastward) relative to starsStellar parallax – slight apparent shifts in stellar positions over the course of the year that must occur if earth orbits the sunGeocentric ModelGreek model, “heavenly perfection”Spherical earth at center of a great celestial sphereMany perfect spheres around Earth, ordering reflected observed speed on skyDid not explain retrograde motionAristotle the heavens were composed of 55 concentric, crystalline spheres to which the celestial objects were attached and which rotated at different velocities, with the Earth at the center.EpicyclesPtolemaic SystemEpicycles = circles upon circlesEach planet on own circle at own speedThe hypothesis then was that planets went on perfect circles about the “Center of Deferent” which was a point between the Earth and the “Equant” – a point on the opposite side of the Deferent from the Earth.Earth no longer at