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PHY134 Introductory Astronomy

PHY134 Introductory Astronomy . The Sun and the Moon. So Far. Stars fixed on a large celestial sphere that rotates daily East to West about an axis through Earth’s poles Declination is celestial longitude. Observer’s zenith is at Declination = Latitude

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PHY134 Introductory Astronomy

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  1. PHY134Introductory Astronomy The Sun and the Moon

  2. So Far • Stars fixed on a large celestial sphere that rotates daily East to West about an axis through Earth’s poles • Declination is celestial longitude. Observer’s zenith is at Declination = Latitude • Right Ascension is celestial longitude. Observer’s zenith is at RA = Sidereal Time • Sidereal time changes by about 1h/h • Sun moves to East relative to stars once a year

  3. Sky Charts N Zenith Pole Fixed Azimuth Fixed RA Horizon Equator E W Fixed Altitude Fixed Decl. S

  4. Clocks • Time from noon to noon is a bit (1/365 of a day or about 4min) longer than time it takes Earth to turn 360° • A (mean) solar day is longer than a sidereal day • Our clocks (LT) keep solar time so run slower than sidereal clock (ST) 24 sidereal hours = 23h 56m 4s

  5. Finding Sidereal Time • By convention ST ≅LT on September 21 • D days later (earlier) ST ≅ LT +/- D×4m • This is approximate. In any event ignores time zones and Daylight Savings Time • On December/March/June21 ST ≅ LT + 6/12/18h

  6. It Tilts • Earth’s axis is tilted 23.5° from perpendicular to orbit • Celestial equator tilted 23.5° from plane of orbit – ecliptic • Sun’s orbit along Celestial sphere – ecliptic - tilted 23.5° from Celestial equator • Sun’s Declination changes between 23.5 and -23.5 • Ecliptic meets equator at Vernal/Autumnal equinox at 0h/12h RA

  7. Seasons • When Sun North/South of equator • Days longer in North/South • Sun higher in the sky in North/South • Climate warming in North/South cooling in South/North • Inside Arctic circle Sun becomes circumpolar/never rises (reverse for Antarctic circle) • At equinox day/night equal everywhere • Between tropics Sunis at Zenith once a year

  8. How High is Sun at Noon? • We areat Latitude 36°N • At equinox • At summer solstice • At winter solstice

  9. Why Mean? • 24h is an average Solar day • Sun’s RA increases over the year but not uniformly • Sun moves around ecliptic almost uniformly but ecliptic is tilted near equinoxes and parallel to equator near solstices. So Eastward motion fastest near solstices. • Almost… Earth very slightly nearer Sun in January

  10. It Also Wobbles • The Earth’s axis wobbles like a spinning top – precession • Celestial axis wobbles. • North pole moves to the West in a circle of radius 23.5° every 26,000 years relative to stars • So does celestial equator hence precession of the equinoxes • Coordinates of stars change too –epoch J2000 • Age of Pisces gives way to age of Aquarius ca. 2600

  11. Moon Moves Too • Like Sun, Moon moves around celestial sphere as it orbits Earth West to East • Moon is faster:orbits in a sidereal month (27.32 days) • RA increases by 48min per day • Spin locked to orbit – same side always faces Earth • Moon moves relative to Sun by 44min per day • Full rotation relative to Sun in synodicmonth (29.53 days) • Position relative to Sun controls rise/set times as well as phases

  12. Moon’s Declination • Moon’s orbit inclined 5° to ecliptic about line of nodes • Like Sun, Moon higher in Summer • Twice a year line of nodes aligns with Sun: Eclipse Season • Tilt precessesto the West every 18.6 years so twice an eclipse year of 346.6 days • At New/Full Moon during eclipse season have Solar/Lunar eclipse

  13. Solar Eclipse • Moon almost same angular size as Sun • With near perfect alignment can completely obscure Sun – from up to 250kmshadow – total eclipse • More common – partial eclipse • When Moon farthest from Earth – annular eclipse

  14. Lunar Eclipse • Moon enters Earth shadow from West • Eclipse can be total or partial. Penumbral eclipse when Moon in partial shadow – dims slightly • During totality Moon illuminated through atmosphere looks red

  15. Fun with the Moon • Moon appears larger near horizon • This is a psychological illusion not shared by optical instruments • Various theories as to mechanism • Can see dark part of crescent Moon – “old moon in new moon’s arms” • This is physical viewing dark part by reflected Earthlight

  16. Signs of the Times • Astronomy and timekeeping are always closely related – we want our time to match what happens. • Our 24-hour days are adjusted to mean solar day. • Our months are approximately lunar. • Our years match orbit – 365.2564 days is a sidereal orbit. • Tropical orbit is 365.2422days (precession). • Julius Caesar got 365.25 so invented leap years. • Pope Gregory XIII (1582) corrected for the .0078

  17. Summary • Our cosmos now has moving parts • Sun moves around Celestial Sphere to the East, completes one revolution in a year. The ecliptic tilted relative to celestial equator by 23.5° about equinoxes and precessesWest every 26,000 years • Moon moves around Celestial Sphere to the East, completes one revolution in a month. Moon’s orbit tilted relative to ecliptic by 5° about line of nodes and precessesWest every 18.6 years • The model now explains day/night, lunar phases, eclipses • What else moves?

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