Motion of the Moon. Phases of the Moon. Phases of the Moon. Key Concepts: Lecture 5

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1 Key Concepts: Lecture 5 Motion of the Moon Phases of Moon Solar Eclipses Angular Size Lunar Eclipses Motion of the Planets Phases of the Moon Why does the Moon shine? It emits no visible light of its own The Moon shines by reflecting light The half of the Moon that faces the Sun is lit up The lit portion of the Moon s surface which is visible from Earth is called the phase Phases change in a regular sequence over a 29.5 day period (synodic period) Motion of the Moon The Moon rises in the east and sets in the west moving across the sky in an arc The Moon moves slowly eastward against the stars (half a degree per hour) The motion of the Moon relative to the stars causes it to rise 50 minutes later every day The Moon returns to the same position amongst the stars every 27.3 days (its orbital period or sidereal period) Phases of the Moon The changes in the Moon s phase are due to changes in the angle between the Sun, Moon and the Earth

2 Eclipses 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 Question If the Sun sets at 6pm, when does a first quarter Moon rise?

3 The Moon Moves Over the Face of the Sun Angular Size of Sun and Moon The Sun and Moon have very different physical sizes Radius of Sun is 7x10 5 km Radius of Moon is 1.7x10 3 km So the Sun is 400 times bigger than the Moon!! How can they appear to be nearly the same size during an eclipse? Solar Eclipse Video Sketch the sequence of a solar eclipse... Angular Size 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 (measured in radians) = Physical Size Distance or we write it as: Earth Moon θ r s Sun (2π radians = 360 degrees)

4 Question Based on their physical and angular sizes, how much further from us is the Sun than the Moon? Shadows & Eclipses A total solar eclipse can seen from only a small region on the Earth in order to see a total eclipse the Moon must appear to cover the entire disk of the Sun you observe this when you are in the inner shadow (umbra) of the Moon Partial solar eclipses are seen over a larger area to see a partial eclipse, only part of the Sun needs to be covered by the Moon you observe this when you are in the Moon s outer shadow (penumbra) An Annular Solar Eclipse Why does an annular eclipse look different? The Moon s shadow moves over the Earth during a solar eclipse

5 (see also p18 C&M) Text Next solar eclipse visible from USA Aug. 21st 2017 When and How Often Are Solar Eclipses? Solar eclipses occur at new Moon They do not occur every month The Moon s orbit is tilted 5 degrees to the ecliptic They occur when the Moon crosses the ecliptic when it is new Moon A partial, total or annular eclipse can occur some place on Earth between 3 and 5 times each year A total may occur 0 to 3 times a year

6 Lunar Eclipses Question Why is the Moon s surface still visible during a total lunar eclipse? We see a total lunar eclipse when the Moon moves through the Earth s inner shadow (umbra) We see a partial lunar eclipse when the Moon moves through the Earth s outer shadow (penumbra) Lunar eclipses are visible anywhere on the night-time side of the Earth Why does the Moon then have a reddish color? A Lunar Eclipse When & How Often? Lunar eclipses occur at FULL Moon PHASE Lunar eclipses do not occur every month because the Moon s orbital plane is tilted by 5 degrees 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 2-5 times per year

7 Motion of the Planets The bright planets (Venus, Jupiter, Mars, Saturn) are the brightest objects in the night sky (with the exception of the Moon) & they rise in the east and set in the west Planet derived from Greek for 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 Inferior Planets show Phases The Inferior Planets Can you ever see Venus at midnight? Why do inferior planets always appear near the Sun (i.e. in the evening or morning)? Why is it hard to see Mercury? 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. 15 th century). Mercury moves fast - the messenger Venus depicted in the Aztec Codex: Aztec god, Xolotl (evening star)

8 Planetary Transits: The Transit of Venus Motion of the Outer Planets Outer (superior) planets generally move eastward relative to the stars Outer (superior) 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 confused & perplexed people for centuries & led people of many cultures to attribute superior powers to the planets The Superior Planets The naked-eye 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.) Saturn (ca. 18th century) - Arabic illustration showing agricultural activities under direction of Saturn You do not need to recall this slide for HW or Exam Planets, Gods & Days of Week English names for most of the days of the week come from Norse gods Tuesday : Tiwes - god of war (Mars) Wednesday: Woden - god of day & night (Mercury) Thursday: Thor - god of thunder - head god (Jupiter) Friday: Frega - goddess of spring (Venus)