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PHY134 Introductory Astronomy

PHY134 Introductory Astronomy . The Moon and Planets. 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

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PHY134 Introductory Astronomy

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  1. PHY134Introductory Astronomy The Moon and Planets

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. New Moon in Old Moon’s Arms

  8. 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

  9. 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?

  10. Wanderers • Five wandering stars (planets) also move along paths very near the ecliptic • Rates of motion vary among planets, so each located on its own sphere. We now have seven spheres • Planetary motion less regular than Sun or Moon. Rate changes and sometimes turns retrograde

  11. Ptolemaic Astronomy • Hipparchus (150BC): Planets move on epicycles which move along deferents • Ptolemy (150AD) elaborates model to account for small deviations • Ptolemaic model predicts planetary motions accurately – successful scientific model • Ptolemaic order of spheres: Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn; Fixed Stars

  12. How it works • Venus never too far from Sun • Its deferent circles about once a year • Epicycle rotation accounts for periodic change in elongation • When West/Eastof Sun we see morning/evening star

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