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ASTRONOMY 161 Introduction to Solar System Astronomy

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Tuesday, Jan 22, 2008: full Moon in Cancer. ... Wednesday, Feb 20, 2008: full Moon again (but now it is in Leo) ... eclipses occur at FULL MOON. Three types of ... – PowerPoint PPT presentation

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Title: ASTRONOMY 161 Introduction to Solar System Astronomy


1
ASTRONOMY 161 Introduction to Solar System
Astronomy
Class 4
2
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3
Moon Phases Eclipses Wednesday, September 30
4
Astronomical movies
  • Lunation (Phases of the Moon)
  • http//antwrp.gsfc.nasa.gov/apod/ap051113.html
  • Shadow of the Earth (Lunar eclipse)
  • http//antwrp.gsfc.nasa.gov/apod/image/0311/112003
    lunareclipse_koehn.gif
  • Shadow of the Moon (Solar eclipse)
  • http//antwrp.gsfc.nasa.gov/apod/image/9803/moonsh
    ad_goes_big.gif

5
Moon Phases Eclipses Key Concepts
  • (1) Lunar phases change as we see more or less
    of the Moons sunlit half.
  • (2) The Moon rotates about its axis as it
    revolves around the Earth.
  • (3) The sidereal month27.3 days the synodic
    month29.5 days.
  • (4) A lunar eclipse occurs when the Moon passes
    through the Earths shadow.
  • (5) A solar eclipse occurs when the Earth passes
    through the Moons shadow.

6
(1) The Moons phases change as we see more or
less of the Moons sunlit half.
  • Full
  • Waning Gibbous Waxing Gibbous
  • Last Quarter
    First Quarter
  • Waning Crescent Waxing Crescent
  • New

7
The Moon is a sphere illuminated by the Sun.
  • Half closest to Sun is light.
  • Half furthest from Sun is dark.

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Gibbous and crescent shapes result from
perspective.
10
Example Crescent Moon
  • We see Moon in almost the same direction as Sun.
  • We see only a sliver of Moons sunlit side.
  • We see crescent Moon close to Sun in sky.
  • Horns of crescent point away from Sun.

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(2) The Moon rotates about its axis as it
revolves around the Earth.
  • On Earth, we always see the same side of the Moon
    (near side).
  • The far side is always turned away from us.

13
No rotation of the Moon
  • Observer on distant star always sees same side.
  • Observer on Earth sees all sides.

14
One rotation per revolution
  • Observer on distant star sees all sides.
  • Observer on Earth always sees same side.

15
(3a) The length of the
SIDEREAL month 27.3 days
  • Sidereal month time for one revolution of Moon
    around Earth.
  • The view from Earth
  • Sunday, Oct 4, 2009
    full Moon in Pisces.
  • Saturday, Oct 31, 2009
    Moon in Pisces again (but now it is
    gibbous).

16
(3b) The length of the
SYNODIC month 29.5 days
  • Synodic month time between one full Moon and
    the next (or one new Moon and the next).
  • The view from Earth
  • Sunday, Oct 4, 2009
    full Moon in Pisces.
  • Monday, Nov 2, 2009
    full Moon again (but now it is in
    Aries).

17
Why is SYNODIC month longer than SIDEREAL month?
  • Synodic measured relative to Sun
  • Sidereal measured relative to stars
  • Sun is a moving target 2.3 extra days to catch
    up.

18
For the same reason SYNODIC (solar) day is
longer than SIDEREAL day.
19
Total Solar Eclipse
20
Total Lunar Eclipse
21
The shadow of the Earth (or any other object) has
two parts
  • Umbra inner part of shadow, Sun completely
    hidden.
  • Penumbra outer part of shadow, Sun partially
    hidden.

22
Earths Shadow
  • Earths umbra stretches 1.4 million kilometers
    (3.7 times average Earth-Moon distance).
  • At the Moons orbit, Earths umbra is 9000
    kilometers wide (2.6 times Moons diameter).
  • The Moon can fit inside the Earths umbra, with
    room to spare.

23
(4) A lunar eclipse occurs when the Moon passes
through the Earths shadow.
  • Lunar eclipses occur when Earth is between Sun
    and Moon.
  • Lunar eclipses occur at FULL MOON.

24
Three types of lunar eclipse
  • (1) Penumbral
  • None of Moon enters umbra. Boring.
  • (2) Total
  • All of Moon enters umbra. Totality lasts up to
    1 hour 40 minutes.
  • (3) Partial
  • Part of Moon enters umbra.

25
(5) A solar eclipse occurs when the Earth passes
through the Moons shadow.
  • Moon umbra stretches for 380,000 kilometers.
  • Minimum Earth-Moon distance
  • 363,000 kilometers.
  • Maximum Earth-Moon distance
  • 405,000 kilometers.

26
  • Solar eclipses occur when Moon
    is between Sun and Earth.
  • Solar eclipses occur at NEW MOON.

27
Three types of solar eclipse
  • (1) Total Observer is in Moons umbra Moon
    completely hides Sun.
  • (2) Annular Moon umbra falls short of Earth
    Moon is surrounded by a ring of Sun.
  • (3) Partial Observer is in penumbra, to one side
    of umbra Moon takes bite out of Sun.

28
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29
Moons Shadow
  • The Moons orbital motion makes its umbra sweep
    rapidly over the Earth.
  • Total solar eclipse is visible from a narrow path
    (up to 270 km wide) for a short time (up to 7.5
    min).
  • Total lunar eclipse is visible from half the
    Earth.
  • From Columbus, last total lunar eclipse was Feb
    20, 2008 next total lunar Dec 20/21, 2010
    next total solar Apr 8, 2024

30
Wolfram Alpha
  • http//www.wolframalpha.com/

31
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32
Few closing questions
  • 1) Why are eclipses not frequent?
  • 2) During a total lunar eclipse, what is seen by
    an observer on the surface of the Moon?
  • 3) Are there more solar or lunar eclipses? (hard)
  • 4) For an observer on the Moon, does Earth show
    phases?

33
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36
Few closing questions cont.
  • 5) Could the sidereal month be longer than the
    synodic month? How to arrange that?
  • 6) Could the sidereal (solar) day be longer than
    the synodic day?
  • 7) You just had a solar eclipse. Can you have a
    lunar eclipse a week later? Two weeks later? Can
    you have another solar eclipse month later?
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