Slide 1Days, Seasons & Lunar PhasesSlide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34PYTS/ASTR 206 – Days, Seasons & Lunar Phases1AnnouncementsD2L site “up soon” – will only have link to class webpageThe order of the next few lectures has changed a little – see class website for new scheduleTiming of homeworks, exams etc is unaffectedObservationsCraters…Bright/white surfaceTrenchesPYTS/ASTR 206 – Days, Seasons & Lunar Phases2PTYS/ASTR 206 – The Golden Age of Planetary ExplorationShane Byrne – [email protected], Seasons & Lunar PhasesPYTS/ASTR 206 – Days, Seasons & Lunar Phases3What makes planetary astronomy different from other kinds of astronomy?PYTS/ASTR 206 – Days, Seasons & Lunar Phases4What makes planetary astronomy different from other astronomy?Most light is reflected lightComing soonWe’re part of the dynamical system we’re trying to observeToday’s lecturePYTS/ASTR 206 – Days, Seasons & Lunar Phases5Our spinning solar systemRotationDay vs. NightSeasonsObliquity, incidence angle Lunar PhasesSynchronous rotationEclipsesSolar and LunarIn this lecture…In this lecture…PYTS/ASTR 206 – Days, Seasons & Lunar Phases6The solar system is shaped like a flat diskLooking down from ‘above’ it spins anti-clockwise‘above’ = north Inclinations are lowEverything around us is spinning – and we are tooEverything around us is spinning – and we are tooPlanetary InclinationsMercury 7°Venus 3.4°Earth 0°The EclipticMars 1.9°Jupiter 1.3°Saturn 2.5°Uranus 0.8°Neptune 1.8°PYTS/ASTR 206 – Days, Seasons & Lunar Phases7Earth also rotates in this counter-clockwise senseAs do most of the other planetsVenus spins backwardsVery slowly…Uranus spins sidewaysWood, The New Solar System.PYTS/ASTR 206 – Days, Seasons & Lunar Phases8Common spin directionRotation of Sun, Earth, MoonOrbit of Earth and Moon+++PYTS/ASTR 206 – Days, Seasons & Lunar Phases9Earth’s polar axis points to the celestial north poleNowadays it’s conveniently marked by the star PolarisEarth’s equator projects onto the sky to form the celestial equatorRight-ascension and declination measured like longitude and latitude on this celestial spherePYTS/ASTR 206 – Days, Seasons & Lunar Phases10Astronomical objects appear to rotate around celestial north poleCounterclockwise, every 23hrs 56minutes – a sidereal daySun appears to rotate every 24 hrs – a solar dayObjects rise in the East and set in the WestSome stars never setPYTS/ASTR 206 – Days, Seasons & Lunar Phases11Where’s the celestial pole?At the north pole – it’s straight overheadAt the equator – it’s on the northern horizonAt a latitude X°, it’s X° above the horizonPYTS/ASTR 206 – Days, Seasons & Lunar Phases12Visible constellations depend on seasonWe can only see stars at nightThe part of the sky that the night-side of the Earth faces changes with seasonThe sun appears to move through the background starse.g. Orion – winter & Cygnus - summerSeasonsSeasonsPYTS/ASTR 206 – Days, Seasons & Lunar Phases13Earth’s orbital plane is not the same as the celestial equatorOrbital plane is called the eclipticThere is a difference of 23½° - obliquityEarth doesn’t wobble (much), the pole always points at the same spot in the skyNorthern hemisphere tilts towards the sun in the summerVice-versa in the winterPYTS/ASTR 206 – Days, Seasons & Lunar Phases14The Sun and all other planets appear to move around the Earth on the EclipticThe stars stay (mostly) fixed on the celestial sphereThe Sun and planets movePYTS/ASTR 206 – Days, Seasons & Lunar Phases15At (northern) summer solsticeSun is overhead at Tropic of CancerNorthern hemisphere days are longPermanent daylight north or arctic circleAt equinoxesSun is overhead at the equatorLength of day is the same everywhereNo permanent daylight anywhereAt (northern) winter solsticeSun is overhead at Tropic of CapricornNorthern hemisphere days are shortPermanent darkness north or arctic circleTropics: 23½° South to 23½° North. The region where the Sun is directly overhead at some season.Mid-latitudes: 23½° to 66½° North and South. The Sun is never directly overhead. The sun sets and rises every day.Polar regions: 66½° to 90° North and South. The Sun never sets in the summer and never rises in the winter.PYTS/ASTR 206 – Days, Seasons & Lunar Phases16Tilt of ground towards the sun determines how much the solar energy is spread outDepends on latitude and seasoniiSolar power at Earth~1370 W/m2Incidence angle (i)i = 90° – solar elevationPower reduced by cos(i)i=0°, Sun directly overheadI=90°, Sun on the horizon•Zero solar powerPYTS/ASTR 206 – Days, Seasons & Lunar Phases17How about Tucson (32° North latitude), winter vs. summer?NoonWinter SolsticeNoonSummer SolsticeNoonEquinoxSolar elevation 34½° 81½° 58°Incidence Angle 55½° 8½° 32°Solar powercos(i)*1370 W/m2776 W/m21355 W/m21162 W/m2A big difference!iiStraight upStraight up23½°32°Equator23½°December 21st June 21stPYTS/ASTR 206 – Days, Seasons & Lunar Phases18What about at the equator?Is the sun highest in the sky in:A) JuneB) DecemberC) March or SeptemberD) It’s the same all yearPYTS/ASTR 206 – Days, Seasons & Lunar Phases19What about at the equator?Is the sun highest in the sky in:A) JuneB) DecemberC) March or SeptemberD) It’s the same all yearEquatorEquatorPYTS/ASTR 206 – Days, Seasons & Lunar Phases20Every planet/moon is half-illuminatedExcept during eclipsesOur point of view makes planetary objects have ‘phases’Phases of the Moon are easiest to seeThe phase of an object can be expressed as a ‘phase angle’ – more on this laterTerminatorPhases of MarsPhases of VenusPhasesPhasesPYTS/ASTR 206 – Days, Seasons & Lunar Phases21The Moon shows the most dramatic phase changes as it’s nearbyOrbits Earth every 27.3 days……but new-moon to new-moon period is 29.5 daysAny ideas why?PYTS/ASTR 206 – Days, Seasons & Lunar
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