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Motion of the Moon Line of Nodes East to west in arc across sky Eastward w.r.t. stars - half degree per hour Sidereal Period - 27.32166 days Moon rises ~ 50 minutes later each day Phases of the Moon Why does the Moon Shine? Reflected sunlight How much of the Moon is lit up by Sun? Half of the Moon is always lit up Fraction of lit surface we see from Earth - phase Synodic Period - 29.530588 days Sidereal & Synodic Periods Phases of the Moon • Sidereal Period: 27.3 days The changes in the Moon’s phase are due to changes in the angle between the Sun, Moon and the Earth Synodic Period: 29.5 days – 0 degrees : New Moon – 90 degrees: Quarter Moon (First or Third) – 180 degrees: Full 1 Question • If the Sun sets at 6pm, when does a first quarter Moon rise? Eclipses A First Quarter Moon rises at noon The Moon Moves Over the Face of the Sun • • A solar eclipse occurs when the Moon moves between the Earth and the Sun • A lunar eclipse occurs when the Earth moves between the Sun and the Moon and the Moon moves through the Earth’s shadow • In ancient cultures eclipses were bad omens Nearly Covered The Sun is Nearly Covered • 2 The Diamond Ring Angular Size of Sun and Moon • The Sun and Moon have very different physical sizes – Radius of Sun is 7x105 km – Radius of Moon is 1.7x103 km – So the Sun is 400 times bigger than the Moon!! • How can they appear to be nearly the same size during an eclipse? Angular Size An Annular Solar Eclipse • Answer: the Moon and Sun, coincidentally, have nearly the same angular size • Angular size of an object depends on two things – The physical size of the object – The distance to the object Angular size (radians) Size = Physical Distance 3 Shadows & Eclipses Question • Why does an annular eclipse look different? • The Moon’s shadow moves over the Earth during a solar eclipse Next solar eclipse visible from USA Aug. 21st 2017 • A total solar eclipse can seen from only a small region on the Earth – entire disk of the Sun covered – In umbra or inner shadow the Moon • Partial solar eclipses are seen over a larger area – only part of the Sun is covered by the Moon – in the penumbraor Moon’s outer shadow Paths of Solar Eclipses When and How Often Are Eclipses Solar eclipses occur at new moon New moon crosses ecliptic partial, total or annular eclipse - 3 to 5 times each year total eclipse 0 to 3 times a year http://sunearth.gsfc.nasa.gov/eclipse/eclipse.html 4 The Lunar Eclipse A Lunar Eclipse • total lunar eclipse - the Moon moves completely through the Earth’s umbra • partial lunar eclipse - the Moon moves partially through the Earth’s umbra • Penumbral eclipse – the Moon passes through the penumbra but does not come in contact with umbra • Lunar eclipses are visible anywhere on the night-time side of the Earth When & How Often? Question • • Why is the Moon’s surface still visible during a total lunar eclipse? • Lunar eclipses occur at FULL Moon PHASE Lunar eclipses do not occur every month because the Moon’s orbital plane is tilted with respect to the ecliptic – a total lunar eclipse occurs when the Moon crosses the ecliptic at full Moon – since the Earth’s shadow is much bigger than the Moon, total lunar eclipses occur more often than solar eclipses Lunar eclipses occur 2 to 5 times per year Motion of the Planets • • • Motion of Planets The planets are the brightest objects in the night sky (with the exception of the Moon) Rise in the east and set in the west Planet means “Wanderer” – The planets move slowly among the stars staying near the ecliptic – Different planets move at different speeds relative to the stars (of the visible planets, Mercury is the fastest, Saturn is the slowest) – They move in complex patterns changing their direction of motion 5 Motion of the Planets • • • • • • • • • • Inferior Planets Planets generally move eastward relative to the stars Planets undergo retrograde motion – motion relative to stars: • • slows reverses moves westward relative to stars slows again reverses again resumes its general eastward motion • • This motion is unique among all astronomical objects This confused & perplexed people for centuries & led people of many cultures to attribute superior powers to the planets Superior Planets • The visible superior planets are Mars, Jupiter and Saturn • Can appear far from the Sun but remain near the ecliptic Mars & Venus Fresco from Pompeii (ca. 1 A.D.) Venus and Mercury Stay near the Sun on the sky Are visible only near sunrise and sunset (“morning & evening stars”) Move away from the Sun and then move toward it Mercury - (ca. 15th century) Mercury is holding a bag for commerce and twin snakes for healing powers. Reigns over Virgo & Gemini Venus depicted in the Aztec Codex: Aztec god, Xolotl (evening star) at the crossroads of fate. Later became twin of Quetzalcoatl (morning star & supreme god) Can you ever see Venus at midnight? Why do inferior planets always appear near the Sun (i.e. in the evening or morning)? Mars is the war-god & Venus, the god of love. Saturn (ca. 18th century) - Arabic illustration showing agricultural activities under direction of Saturn Planets, Gods & Days of Week Planets show Phases 15th century engraving of days of the week and their astrological counterparts • English names for most of the days of the week come from Norse gods • Tuesday : Tiwes - god of war • Wednesday: Woden god of day & night • Thursday: Thor - god of thunder - head god • Friday: Frega - goddess of spring 6 Ancient Astronomy • • Since pre-historic times, ancient people have observed and recorded the regular & cyclic patterns in the sky To explain these motions, a dual development in human thought began: – search for natural & unchanging laws – creation of mythology The Earliest Calendars • During the stone age (50,000 years ago) people first began to leave a record in carvings and paintings. These included: – Pictures of constellations – The first calendars 14 Phases of the moon seen in cave paintings in Lascaux, France (18,000 BC) 25,000 year old Ishango bone (Congo, Africa) with possible lunar phase calendar 7