PYTS/ASTR 206 – Days, Seasons & Lunar Phases1z AnnouncementsD2L i “ ”ill l h li k l bD2L site “up soon” –will only have link to class webpage The order of the next few lectures has changed a little –see class website for new schedulesee class website for new schedule Timing of homeworks, exams etc is unaffectedObservationszObservations Craters…Bright/whiteBright/white surface TrenchesPYTS/ASTR 206 – Days, Seasons & Lunar Phases2Days, Seasons & Lunar PhasesPTYS/ASTR 206 – The Golden Age of Planetary ExplorationShane Byrne – [email protected]/ASTR 206 – Days, Seasons & Lunar Phases3What makes planetary astronomy differentzWhat makes planetary astronomy different from other kinds of astronomy?PYTS/ASTR 206 – Days, Seasons & Lunar Phases4What makes planetary astronomy differentzWhat makes planetary astronomy different from other astronomy?Mtlihti fltdlihtMost light is reflected lightComing soon We’re part of the dynamical system we’re ti t btrying to observeToday’s lecturePYTS/ASTR 206 – Days, Seasons & Lunar Phases5In this lecture…In this lecture…z Our spinning solar systemz RotationDNihtDay vs. Nightz Seasons Obliquity, incidence angle z Lunar Phasesz Synchronous rotationz Eclipses Solar and LunarPYTS/ASTR 206 – Days, Seasons & Lunar Phases6Everything around us is spinning – and we are tooEverything around us is spinning – and we are tooz The solar system is shaped like a flat disk Looking down from ‘b ’it i ti‘above’ it spins anti-clockwise ‘above’ = north Iliti lzInclinations are lowPlanetary InclinationsMercury 7°Venus 3.4°Earth 0°The EclipticpMars 1.9°Jupiter 1.3°Saturn25°Saturn2.5Uranus 0.8°Neptune 1.8°PYTS/ASTR 206 – Days, Seasons & Lunar Phases7z Earth also rotates in this counter-clockwise sense As do most of the other planetsp Venus spins backwards Very slowly… Uranus spins sidewaysWood, The New Solar System.PYTS/ASTR 206 – Days, Seasons & Lunar Phases8z Common spin direction Rotation of Sun, Earth, Moon Orbit of Earth and Moon+++PYTS/ASTR 206 – Days, Seasons & Lunar Phases9z Earth’s polar axis points to the celestial north poleNowadays it’s conveniently marked by the star PolarisNowadays it s conveniently marked by the star Polarisz Earth’s equator projects onto the sky to form the celestial equator Right-ascension and declination measured like longitude and latitude on this celestial spherethis celestial spherePYTS/ASTR 206 – Days, Seasons & Lunar Phases10z Astronomical objects appear to rotate around celestial north pole Counterclockwise, every 23hrs 56minutes – a sidereal day,yy Sun appears to rotate every 24 hrs – a solar dayz Objects rise in the East and set in the WestSome stars never setSome stars never setPYTS/ASTR 206 – Days, Seasons & Lunar Phases11z Where’s the celestial pole?At th th lit’ t i ht h dAt the north pole –it’s straight overhead At the equator – it’s on the northern horizon At a latitude X°, it’s X° above the horizonPYTS/ASTR 206 – Days, Seasons & Lunar Phases12Vi ibl ll i d dSeasonsSeasonszVisible constellations depend on season We can only see stars at night The part of the sky that the night-side of the Earth faces changes with seasonEarth faces changes with season The sun appears to move through the background starse.g. Orion – winter & Cygnus - summerPYTS/ASTR 206 – Days, Seasons & Lunar Phases13z Earth’s orbital plane is not the same as the celestial equatorOrbital plane is called theeclipticOrbital plane is called the ecliptic There is a difference of 23½° - obliquity Earth doesn’t wobble (much), the pole always points at the same spot in the skyyz Northern hemisphere tilts towards the sun in the summer Vice-versa in the winterPYTS/ASTR 206 – Days, Seasons & Lunar Phases14z The Sun and all other planets appear to move around the Earth on the Eclipticz The stars stay (mostly) fixed on the celestial spherez The Sun and planets movePYTS/ASTR 206 – Days, Seasons & Lunar Phases15zAt (northern) summer solsticezAt (northern) summer solstice Sun is overhead at Tropic of Cancer Northern hemisphere days are longPermanent daylight north or arctic circlePermanent daylight north or arctic circlez At equinoxes Sun is overhead at the equator Length of day is the same everywhere No permanent daylight anywhereyg yz At (northern) winter solsticeSi hdtTifCiTropics: 23½° South to 23½° North.The region where the Sun is directlyoverhead at some season.Midltitd23½°t66½°NthSun is overhead at Tropic of Capricorn Northern hemisphere days are short Permanent darkness north or arctic circleMid-latitudes:23½°to66½°Northand South. The Sun is never directlyoverhead. The sun sets and risesevery day.Polar regions: 66½° to 90° Northand South. The Sun never sets in thesummer and never rises in the winter.PYTS/ASTR 206 – Days, Seasons & Lunar Phases16z Tilt of ground towards the sun determines how much the solar energy is spread out Depends on latitude and seasonSl tE thiizSolar power at Earth ~1370 W/m2z Incidence angle (i)g() i = 90° – solar elevation Power reduced by cos(i)°S i=0°, Sun directly overhead I=90°, Sun on the horizon• Zero solar powerPYTS/ASTR 206 – Days, Seasons & Lunar Phases17zHow about Tucson (32° North latitude), winter vs. summer?Straight upiStraight upStraight up32°23½°i23½°23½°NNNDecember 21stJune 21stNoonWinter SolsticeNoonSummer SolsticeNoonEquinoxSolar elevation 34½° 81½° 58°Incidence Angle 55½° 8½° 32°Solar power776 W/m21355 W/m21162 W/m2Solar powercos(i)*1370 W/m2776 W/m1355 W/m1162 W/mA big difference!PYTS/ASTR 206 – Days, Seasons & Lunar Phases18z What about at the equator?z Is the sun highest in the sky in: A) JuneB) DecemberB) December C) March or September D) It’s the same all yearPYTS/ASTR 206 – Days, Seasons & Lunar Phases19z What about at the equator?z Is the sun highest in the sky in: A) JuneB) DecemberB) December C) March or September D) It’s the same all yearPYTS/ASTR 206 – Days, Seasons & Lunar Phases20PhasesPhasesz Every planet/moon is half-illuminated Except during eclipses Our point of view makes planetary objects h‘h ’natorhave ‘phases’ Phases of the Moon are easiest to see The phase of an object can be expressed as a ‘phase
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