3Need for Astronomy Predicting seasons enables Survival MigrationStore food (like squirrels)Plant crops in spring (after last frost?)Predict seasonal floodingKnowing time of day enables SurvivalHide in cave at night (lions, tigers & bears!)What else is there to do at night without a light?
4Astronomy tells time of year Which stars are up at nightStar patterns = ConstellationsOrion high in winterCygnus high in summerAltitude of sun at noonHigh in summerLow in winterLocation of sunrise/sunsetNE/NW in summerSE/SW in winterE/W on 1st day of spring/fall
6Constellations 88 Official Constellations Asterism Examples: Ursa Major, Taurus, ...Often drawn as “stick figures” (stick figures not official; may vary)Official Constellations are regions of the sky (like states)Asterisma popular name for a group of stars that is not an official constellationExamples: The Big Dipper, The Pleiades
11Locations on Celestial Sphere Zenithstraight up (overhead)Nadirstraight down (beneath your feet)Horizonlowest place you see the skyShape: a circle (you are at the center)Location: halfway between Zenith and NadirNorth Celestial Pole
12Directions in the Sky Azimuth = direction (N, S, SSW, etc) you face Measured in degrees along horizon turning eastward from NExamples: 45 º azimuth = NE; 90º = E; 270º = WAltitude = how high in skyMeasured in degrees above horizon
13Motion of Objects in the Sky What do you know about the motion of:the Sun?the Stars?
14high in the southern sky You observe a star rising due east. When this star reaches its highest position above the horizon, where will it be?high in the northern skyhigh in the eastern skyhigh in the southern skyhigh in the western skydirectly overheadhigh in the southern sky
15In-class exercise #1 - “Position” Pages 1-2 in workbook
161) How much of the celestial sphere can an Earth observer see at one time? a) less than halfb) exactly halfc) more than halfb) exactly half
17The Spinning Celestial Sphere Looking NorthLooking South
18Why do the stars move? The Earth Rotates (from W to E) Path of Stars appears to us as if the sky (the Celestial Sphere) rotates (from E to W)Path of StarsStars “attached” to celestial spherePath is a circle (like latitude circle)Called diurnal circle (diurnal = daily)
20Locations on Celestial Sphere Projection of Earth’s rotation axisNorth Celestial Pole / South Celestial PoleProjection of Earth’s EquatorCelestial EquatorShape: circleLocation: halfway between the Celestial Poles
21Rise / Set / Transit Rise - move above horizon (appear) Set - move below horizon (disappear)Objects rise “in east” and set “in west”Transit - moving past highest point in pathESW
22CircumpolarSome stars never rise or setThese stars are circumpolar
23Rotation at Different Latitudes Altitude of Pole = Latitude of ObserverCircumpolar zone depends on latitude
24Imagine you are standing at the North Pole Imagine you are standing at the North Pole. Of the stars that you can see, roughly how many of these stars are circumpolar?Noneless than halfmore than halfallall
25Imagine you are standing on the Equator Imagine you are standing on the Equator. Of the stars that you can see, roughly how many of these stars are circumpolar?Noneless than halfmore than halfallNone
26In-class exercise #2 - “Motion” Pages 3-6 in workbook
27You are looking toward the north and see the Big Dipper to the right of Polaris. Fifteen minutes later, the Big Dipper will appear to have moved in roughly what direction?a) east (to your right)b) west (to your left)c) up (away from the horizon)d) down (closer to the horizon)?
28At what time will star B appear highest in the sky? 6amNoonMidnightAt what time will star B appear highest in the sky?a) early in the morningb) around noonc) in the afternoond) in the eveninge) around midnight
29When star A is just above the eastern horizon, in what direction is star A moving? a) up and to the northb) westc) up and to the southd) south
30a) high in the northern sky 4) You are observing the sky from your southern hemisphere location in Australia. You see a star rising directly to the east. When this star reaches its highest position above the horizon, where will it be?a) high in the northern skyb) high in the eastern skyc) high in the southern skyd) high in the western skye) directly overheada) high in the northern sky
31Motion of the Sun Diurnal (daily) motion like stars Sunrise “in east” Transits “high” in south = NoonSunset “in west”Altitude at noon depends on time of year
32Time of DayMeridian:circle halfway between east and westStars, etc. are highest when they Transit the meridianTime of day = solar position w.r.t transit (Noon)am = ante meridianpm = post meridian1 pm11 am2 pm10 amMeridianS(6am)EW(6pm)
33Motion of the Sun Annual (yearly) motion Earth orbits Sun once per yearSun seen in front of different constellations throughout year
35Motion of the Sun Annual (yearly) motion From day to day, Sun “slips” a little bit on Celestial SphereAppears to shift all the way around the Celestial Sphere once per yearAppears to move “from W to E” relative to the background of starsSo from day to day, any given star rises earlier
37Annual Path of Sun Ecliptic Zodiac path of sun around celestial sphere shape: circleZodiacSet of 12 Constellations containing EclipticSun in each constellation for about one month(solar) signs of the zodiac
38For today, you should have done: Homework: “Seasonal Stars” Exercise in workbook (p. 7-10)Let’s go over the workbook exercise…
39One evening at midnight, you observe Leo high in the southern sky at midnight. Virgo is to the east of Leo and Cancer is to the west. One month earlier, which of these constellations was high in the southern sky in at midnight?a) Leob) Virgoc) Cancer
40You go out tonight and see the brightest star in the constellation Orion just rising above your eastern horizon at 10 PM. One week later at 10 PM this same star will bea) slightly higher in the sky.b) at the same height as before.c) below your horizon.d) setting on your western horizon.
41One night, you see the star Sirius rise at exactly 7:36pm One night, you see the star Sirius rise at exactly 7:36pm. The following night it will risea) slightly earlier.b) at the same time.c) slightly later.
42The Day1 day = time for object to return to same point on sky (e.g. transit to transit)Solar (Sun) dayTime from noon until next noonSidereal (star) dayTime for a star to return to same point.Solar day is ~4 min longer than Sidereal day
43In-class exercise“Solar vs. Sidereal Day” Pages 11-12 in workbook
44What component of Earth’s motion causes the stars to rise earlier on successive nights? a) its rotation about its axisb) its orbit around the Sunc) the tilt of its rotation axis
45Which takes longer to complete? a) one solar day b) one sidereal day c) Both take the same amount of time.Solar Day = 24 hours Sidereal Day = 23 hrs 56 min
46Ecliptic on Celestial Sphere Earth’s axis tilted 23º with respect to orbit
47Locations on Ecliptic Solstice: sun stops (moving N or S) Summer SolsticeJun 21=1st day of summerSun farthest N (from celestial equator)Longest day of yearWinter SolsticeDec 21=1st day of winterSun farthest S (from celestial equator)Shortest day of year
48Locations on Ecliptic Equinox: equal night and day Vernal Equinox ~March 21 = 1st day of springSun on equator (crossing from S to N)Autumnal Equinox~Sept 21 = 1st day of fall (autumn)Sun on equator (crossing from N to S)Equinoxes are intersection points of Ecliptic and Celestial Equator
49Diurnal Path of Sun Revisited Summer: Sun above Celestial EquatorSun high in south at noonDays are longWinter: Sun below Celestial EquatorSun low in south at noonDays are shortZenithWSNE
50For today, you should have done: Homework: “Path of the Sun” Exercise in workbook (p )Let’s go over the workbook exercise…