Unformatted text preview: Fig 2.7 - Sun's path here in Boulder Reading - Section 2.3 pp 41-48 The Moon, Our Constant Companion What causes eclipses? This week's Lab topic Phases of Moon Moon is illuminated by Sun We see a changing combination of the bright and dark faces as Moon orbits the Earth Phases of the Moon: 29.5-day cycle
new crescent first quarter gibbous full gibbous last quarter crescent } } waxing Moon visible in afternoon/evening. Gets "fuller" and rises later each day. waning Moon visible in late night/morning. Gets "less" and sets later each day. Nearside We see only one side of Moon
Synchronous rotation: the Moon rotates exactly once with each orbit That is why only one side is visible from Earth
Orientale Basin O Farside O Why is the full Moon sometimes appear larger or smaller?
13% variation Moon farthest from Earth Moon closest to Earth Moon closest to Earth Moon farthest from Earth What have we learned?
It's 6 pm. You look up in the sky and on the meridian you see a moon with half its face bright and half dark. When will the moon set? A. B. C. D. E. 6 am Noon 6 pm 9 pm Midnight Why do we see phases of the Moon? Half the Moon is lit by the Sun; half is in shadow, and its appearance to us is determined by the relative positions of Sun, Moon, and Earth ded Gra Class Notices
1. Class meets in Fiske Planetarium on Wednesday 2. Homework 2 due at beginning of class on Friday no more late homework allowed! 3. Seasons/Motion of Sun day-lab starts Tuesday 4. Night lab sessions start tonight - see schedule A. B. C. D. E. Lunar eclipses happen every month Lunar eclipses happen around full moon In a lunar eclipse the Earth is in the Moon's shadow Lunar eclipses occur more often than solar eclipses All of the above are true A. B. C. D. E. Lunar eclipses happen every month Lunar eclipses happen around full moon In a lunar eclipse the Earth is in the Moon's shadow Lunar eclipses occur more often than solar eclipses All of the above are true What causes eclipses? The Earth and Moon cast shadows. When either passes through the other's shadow, we have an eclipse. Lunar Eclipse When can lunar eclipses occur? Lunar eclipses can occur only at full moon. Lunar eclipses can be penumbral, partial, or total. Earth is ~4 times the size of the Moon Makes a big shadow Lunar eclipses more frequent than solar eclipses Solar Eclipse Remarkable fact: Sun and Moon are same angular size viewed from Earth!! Realistic scale is HUGE - The Sun is 400 times bigger then the Moon and 400 times farther away When can solar eclipses occur? Solar eclipses can occur only at new moon. Solar eclipses can be partial, total, or annular.
TOTAL ECLIPSE Why is the full Moon sometimes appear larger or smaller?
13% variation PARTIAL ECLIPSE Moon closest to Earth Moon farthest from Earth ANNULAR ECLIPSE Annular Eclipses
A. Occur when the Moon is closer to the Earth B. Occur when the Moon is farther from the Earth Moon is ~1/4 times the size of the Earth Makes a small shadow Solar eclipses less frequent than lunar eclipses March 2006 Turkey 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 full moon and solar eclipse at new moon. Predicting Eclipses Eclipses recur with the 18 yr, 11.3 day saros cycle, but type (e.g., partial, total) and location may vary. http://sunearth.gsfc.nasa.gov/ eclipse/SEgoogle/SEgoogle20 01/ASE2006google.html http://spaceweather.com/eclipses/07sep 06/Rapavy2.gif 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). What have we learned? Why do we see phases of the Moon? Half the Moon is lit by the Sun; half is in shadow, and its appearance to us is determined by the relative positions of Sun, Moon, and Earth What causes eclipses? Lunar eclipse: Earth's shadow on the Moon Solar eclipse: Moon's shadow on Earth Tilt of Moon's orbit means eclipses occur during two periods each year Class Notices
eHW 1 graded + answers posted pHW2 due beginning of class Wed 12th Sep eHW3 posted - Wed 19th Sep - 8 am Mid-Term Exam - in class - Wednesday Sept 19th Review sheet to be posted Review sessions to be arranged - watch the CULearn Calendar! ...
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- Fall '07