The Moon Questions • What is the motion of the Moon ? • What causes phases of the Moon? • Why does the Moon face always look the same? • What causes eclipses ? • What causes tides ? • What does Earth look like from the Moon ?
Reading: Chapters 2.4, 2.5
The Moon What is the motion of the Moon ?
• The Moon is the faithful follower of the Earth. • It is the object closest to the Earth at about 380,000 km. • It orbits the Earth once every 27.32 days. • It accompanies the Earth on its orbit around the sun.
1 The Moon What is the motion of the Moon ? • Due to the rotation of the Earth from west to east, the Moon, like the stars appears to rise in the east and set in the west every night.
The Moon
What is the motion of the Moon ?
• The Sun moves about 1｡ a day (360｡ in 365.25 days) • The Moon follows roughly the same path as the Sun (ignoring its orbital tilt)
• However it moves 360 degrees in 1 month, rather than 1 year.
2 The Moon
• The moon orbits eastwards around Earth once a month.
The Moon
• The moon orbits eastwards around Earth once a month.
3 What causes phases of the Moon?
What causes phases of the Moon?
• Lunar phases are a consequence of the Moon s 27.32-day orbit around Earth
4 What causes phases of the Moon?
• The Moon reflects light from the sun • Half of the Moon is illuminated by Sun and half is dark • We see a changing amount of the bright and dark faces as the Moon orbits
For an animation of moon phases check out http://www.sumanasinc.com/webcontent/anisamples/astronomy/moonphase.html
Moonrise/Moonset
The new Moon occurs when the Moon lies in the same direction as the Sun. The Moon rises close to sunrise and sets close to sunset
5 Moonrise/Moonset
The first quarter Moon occurs when the Moon is roughly 90 degrees east of the sun The Moon thus rises close to noon and sets close to midnight.
Moonrise/Moonset
The full Moon occurs when the Moon is opposite the sun The Moon thus rises close to sunset and sets close to sunrise.
6 Moonrise/Moonset
The third quarter Moon occurs when the Moon is roughly 90 degrees west of the Sun The Moon thus rises close to midnight and sets close to noon.
Phases of the Moon: 29.5-day cycle
• The orbital (siderial) period of the moon around Earth is not the same as the length of a moon phase (synodic) cycle. • This is because the Earth is moving around the sun
7 Phases of the Moon: 29.5-day cycle new crescent first quarter waxing • Moon visible in afternoon/evening. gibbous }! • Gets fuller and rises later each day. full gibbous last quarter waning • Moon visible in late night/morning. crescent }!• Gets less and sets later each day.
Thought Question It s 9 am. You look up in the sky and see a moon with half its face bright and half dark. What phase is it?
A. First quarter B. Waxing gibbous C. Third quarter D. Half moon
8 Question It s 9 am. You look up in the sky and see a moon with half its face bright and half dark. What phase is it?
A. First quarter B. Waxing gibbous C. Third quarter D. Half moon
Man on the Moon
Synchronous rotation: the Moon rotates exactly once with each orbit
That is why only one face is visible from Earth
9 What causes eclipses?
What causes eclipses? • The Earth and Moon cast shadows. • When either passes through the other s shadow, we have an eclipse • The sun is 400 times larger than our moon and, on the average, 390 times farther away • So, the sun and moon have nearly equal angular diameters. • Thus, each is just the right size to cover the disk of the other
• Lunar eclipses can occur only at full moon when the moon is on the other side of the earth. • Lunar eclipses can be partial, or total.
Lunar Eclipse
• If the orbit of the moon carries it through the umbra of Earth s shadow, you see a total lunar eclipse.
11 Lunar Eclipse
• If the moon passes a bit too far north or south of Earth s shadow, it may only partially enter the umbra. • Then, you see a partial lunar eclipse.
Solar Eclipse
• Solar eclipses can occur only at new moon, when the moon is in between the sun and the earth. • Solar eclipses can be partial, total, or annular.
• Standing in the umbral zone, you would be in total shadow —unable to see any part of the sun s surface. This is called a total eclipse. • If you moved into the penumbra, you would be in partial shadow, but could also see part of the sun peeking around the edge of the moon. This is called a partial eclipse.
Solar Eclipse
• Due to the moon s orbital motion and Earth s rotation, the moon s shadow sweeps rapidly across Earth in a long, narrow path of totality.
• If you want to see a total solar eclipse, you must be in the path of totality.
13 Solar Eclipse
• Sometimes, when the moon crosses in front of the sun, it is too small to fully cover the sun. • Then, you would witness an annular eclipse. • The size of the Moon changes due to the elliptical orbits of the Moon around the Earth and the Earth around the sun.
Frequency of eclipses
Why don t we have an eclipse at every new and full moon?
14 Frequency of eclipses
Why don t we have an eclipse at every new and full moon? – The Moon s orbit is tilted 5° to ecliptic plane… – So we have about two eclipse seasons each year, with a lunar eclipse at new moon and solar eclipse at full moon.
Frequency of eclipses
• The orientation of the moon s orbit in space varies slowly. • As a result, solar and lunar eclipses repeat in a pattern called the Saros cycle lasting 18 years 11 1/3 days.
15 Summary: Two conditions must be met to have an eclipse: 1. It must be full moon (for a lunar eclipse) or new moon (for a solar eclipse).! ! ! !!AND! 2. The Moon must be at or near one of the two points in its orbit where it crosses the ecliptic plane (its nodes). ! !
Tidal Forces
The differential gravitational pull of the Moon on the near and far sides of the Earth causes high and low tides twice a day:!
Spring and Neap tides depend on whether the Sun is aligned parallel or perpendicular to the Moon
16 Earthrise
July 20, 1969 marks the first time a human being (Neil Armstrong) stepped onto another celestial body - the Moon.! ! For the first time, we got to see the Earth rising over the Moon !! ! This is still viewed by many as the single greatest technological achievement of all time.! ! Movie about Lunar landing:! https://www.youtube.com/watch?v=pCp2jTtay0w
Summary • What is the motion of the Moon ? It orbits the Earth once every 27.3 days. Its monthly path in the sky is roughly the same as the yearly path of the Sun (ignoring its orbital tilt). • What causes phases of the Moon? Half the Moon is lit by the Sun; half is in shadow, as the Moon orbits the Earth we see different amounts of the lit and dark sides. • Why does the Moon face always look the same? The Moon rotates exactly once with each orbit, so only one face is visible from Earth
17 Summary • What causes tides ? !The differential gravitational pull of the Moon on the near and far sides of the Earth causes high and low tides twice a day • What causes eclipses? – Lunar eclipse: Earth s shadow on the Moon – Solar eclipse: Moon s shadow on Earth – Tilt of Moon s orbit relative to the plane pf the elliptic means eclipses occur during two periods each year
The Moon Illusion
Why does the Moon look bigger close to the horizon and smaller when it is higher up ?
18 The Moon Illusion Why does the Moon look bigger close to the horizon and smaller when it is higher up ? Its an illusion !
A time-lapse sequence of the moon rising over Seattle. To the camera, the moon appears to be the same size no matter what its location on the sky. Credit and copyright: Shay Stephens.
The Moon Illusion The explanation of the Moon illusion may be quite complicated.
A similar illusion: Ponzo s illusion
Both yellow lines are the same size!
BUT airline pilots flying at very high altitudes sometimes experience the Moon Illusion without any objects in the foreground. What tricks their eyes?!
19 The Moon Illusion The explanation of the Moon illusion may be quite complicated.
A possible explanation: Flattened sky effect
Humans perceive the sky as a flattened dome, with the zenith nearby and the horizon far away. When the moon is near the horizon, your brain miscalculates the moon's true distance and size.!
For other explanations visit http://facstaff.uww.edu/mccreadd/
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