Northeastern Illinois University Lunar Phases, Eclipses, Ancient Astronomy Greg Anderson Department of Physics & Astronomy Northeastern Illinois University Winter-Spring 2020 c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 1 / 66 Northeastern Illinois Overview University The Moon Lunar Phases Months Eclipses The Ancient Roots of Science Review c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 2 / 66 Northeastern Illinois University The Moon Quarter Moon Earth and Moon Earth and Moon Lunar Phases Months Eclipses The Ancient Roots of Science The Moon Review c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 3 / 66 Last Quarter Moon in the Lower Canons, c G. Anderson Earth and Moon (NASA’s Mars Reconnaissance Orbiter) Earth and Moon from Change’s 5-T1 Northeastern Illinois University The Moon Lunar Phases Fig: Phases Phases Label the Eight Lunar Phases Question Months Eclipses The Ancient Lunar Phases Roots of Science Review c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 7 / 66 Northeastern Illinois Fig: Moon Phases University c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 8 / 66 Northeastern Illinois Phases of the Moon University The Moon always keeps the same face towards the Earth. Rotation and Revolution are synchronous. Moon Phase: Fraction of the sunlight side visible to us. • Waxing: increasing illumination – Waxing Crescent: just after New Moon – Waxing Gibbous: just before Full Moon • Waning: decreasing illumination – Waning Gibbous: just after Full Moon – Waning Crescent: just before New Moon c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 9 / 66 Northeastern Illinois Label the Eight Lunar Phases University sunlight Earth c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 10 / 66 Northeastern Illinois Label the Eight Lunar Phases University First Quarter Waxing Gibbous Waxing Crescent sunset noon sunlight midnight Full Moon sunrise New Moon Waning Gibbous Waning Crescent Last Quarter c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 10 / 66 Northeastern Illinois Q: What phase is it? University At 6 A.M. you look up in the sky and see a moon with half its face bright and half dark near the Upper Meridian. What phase is it? A) first quarter B) waxing gibbous C) third quarter (last quarter) D) half moon c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 11 / 66 Northeastern Illinois Q: What phase is it? University At 6 A.M. you look up in the sky and see a moon with half its face bright and half dark near the Upper Meridian. What phase is it? A) first quarter B) waxing gibbous C) third quarter (last quarter) D) half moon c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 11 / 66 Northeastern Illinois University The Moon Lunar Phases Months Lunar Months Sidereal & Synodic Lunar Position Metonic Cycle Analogy Metonic Cycle Metonic Cycle II Months Blue Moon Earth and Moon Perigee & Apogee Peri & Apogee Precession Anomalistic Inclination Draconic Month Libration Phases Eclipses The Ancient Roots of Science Review c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 12 / 66 Northeastern Illinois Lunar Months University The Moon keeps the same face towards the Earth. Rotation and Revolution are synchronous. One “day” on the moon = 29.5 Earth days. • Lunar Sidereal Period (Sidereal Month): From the Latin sidus for star. The time it takes the Moon to complete one orbit with respect to to the stars. Tsidereal = 27.3 days • Lunar Synodic Period (Synodic Month): From the Greek synodos coming together. The time between successive New Moons. Tsynodic = 29.5 days c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 13 / 66 Northeastern Illinois Sidereal & Synodic Months University T = 29.5d = Synodic T = 27.3d = Sidereal New Moon T =0d New Moon c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 14 / 66 Northeastern Illinois Lunar Movement on Celestial Sphere University Relative to the stars, the Moon moves 360◦ in 27.3 days or 360◦/27.3=13.2◦/day, which is just over half a degree per hour (approximately equal to its apparent size). Eastward on Celestial Sphere one day later 13.2◦ Not to scale c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 15 / 66 Northeastern Illinois Metonic Cycle Analogy University Imagine two runners jogging around a track. Runner A circles the track once every 3 minutes and runner B circles the track once every 4 minutes. How often does the scene repeat itself? A) every 3 minutes b b B) every 4 minutes C) every 12 minutes D) every 20 minutes c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 16 / 66 Northeastern Illinois Metonic Cycle Analogy University Imagine two runners jogging around a track. Runner A circles the track once every 3 minutes and runner B circles the track once every 4 minutes. How often does the scene repeat itself? A) every 3 minutes b b B) every 4 minutes C) every 12 minutes D) every 20 minutes 4 × (3 minutes)=3 × (4 minutes) = 12 minutes c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 16 / 66 Northeastern Illinois The Metonic Cycle University 1 year = 365.24 days While twelve synodic months are only: 12 × (29.53) = 354.36 days • Difference: 365.24 − 354.36 = 10.87 days per year. • After 19 years: 19 × 10.87 ≈ 206.5 days. • Coincidence: 206.5 days ≈ 7 synodic months. To keep a 12-month “lunar year” in sync with the solar year, add an intercalary 13th month on seven occasions during the nineteen-year period. 19 years ≈ 235 synodic months = (19 × 12)+7 ≈ 254 sidereal months c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 17 / 66 Northeastern Illinois The Metonic Cycle II University • Named for Greek astronomer Meton (Mǫτων) of Athens. In 432 BCE, Meton noted: 19 years ≈ 235 synodic months • Meton popularized this cycle, it was already known and used: – Babylonian calendar 499 BCE – Ancient Chinese calendar systems 484 BCE The position and phase of the Moon tonight will repeat in 19 years from today. c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 18 / 66 Northeastern Illinois Blue Moon University • The phrase “Once in a blue moon” means very rarely. It has nothing to do with the color of the moon, instead it refers to an extra full Moon. • In current usage, a blue moon often refers to the second full moon in a calendar month. Since the synodic month is 29.5 days long, “blue moons” are rare. • Historically a blue moon referred to the thirteenth full moon in a year which happens seven times in the nineteen year Metonic cycle. An average of once every 2.7 years. c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 19 / 66 Northeastern Illinois Earth and Moon University ǫ =0.0549 Moon To relative scale Earth orbit around Sun Earth Earth-Moon distances To Sun: semi-major axis = 384,748 km 150 million km perigee = 362,600 km apogee = 405,400 km c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 20 / 66 Northeastern Illinois Perigee & Apogee University Not to scale Perigee Apogee 362,600 km 404,400 km (closest) (farthest) c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 21 / 66 Northeastern Illinois Moon at Perigee & Apogee University c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 22 / 66 Northeastern Illinois Precession of the Moon’s Perigee University Precession of perigee of Moon: Once in ≈ 9 years. Caused by Solar tidal forces The closest point to the Earth in the Moon’s orbit is called perigee. The furthest point is called apogee. c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 23 / 66 Northeastern Illinois Anomalistic month University Because the elliptical orbit of the Moon precesses every nine years, the time the Moon spends between two successive perigees (or apogees) is slightly longer than a sidereal month. Tsidereal = 27.32 days Tanomalistic = 27.55 days c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 24 / 66 Northeastern Illinois Inclination of Moon Orbit University The moons orbit is tilted by about 5◦ with respect to the ecliptic. ascending node ecliptic 5◦ descending node The plane of the Moon’s orbit precesses over a full circle in about 18.6 years. c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 25 / 66 Northeastern Illinois Draconic or Nodical Month University Because the plane of the Moon’s orbit precesses westward once per 18.6 years, the time between two successive transits of the moon through its ascending node is slightly shorter than a sideral month. Tsidereal = 27.32 days Tdraconic = 27.21 days ecliptic ascending node5◦ descending node c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 26 / 66 Northeastern Illinois Libration University Simplified Picture • Tidal locking: One hemisphere of the Moon faces Earth. • Our first view of the far side of the Moon resulted from lunar exploration in the 1960s. Over time, 59% of the Moon’s surface is seen from Earth due to libration: the slow rocking back and forth of the Moon as viewed from Earth. Libration is caused by: inclination of the Moon’s axis, eccentricity of the Moon’s orbit, and the Earth’s rotation. • Libration Video c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 27 / 66 Northeastern Illinois Moon Phases: NASA University c 2012-2020 G. Anderson Universe: Past, Present & Future – slide 28 / 66 Northeastern Illinois University The Moon Lunar Phases Months Eclipses Eclipses Umbra, Penumbra & Antumbra Umbra of Earth Lunar Eclipse Eclipses Descending Node Lunar Eclipse Total Lunar Eclipse Saros 137 Solar Eclipses Fig 2-24 Conditions for Syzygy Fig 2-23 Eclipse Season Solar Eclipse Intervals Predicting Eclipses Saros Cycle Saros Cycle Saros 136 The c 2012-2020 Ancient G.