Lec 7: 14 September 2011 Chapter 3: Orbit and Phases of...

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Lec 7: 14 September 2011 Chapter 3: Orbit and Phases of the Moon Last Time – Lunar Phases •  observed properties of the Moon •  the lunar phase cycle •  phase v time of day TODAY – The Orbit of the Moon & Eclipses •  The Moon’s Orbit •  Lunar Eclipses •  Solar Eclipses MONDAY - Begin Chapter 4 •  Apparent Motion of the Planets •  review/wrap-up part 1 of the course

EXAM #1: Next Wednesday

Brief Review of Moon Phases •  Angle between Sun and Moon produces phases

–  0o : New Moon –  0 to 90o : crescent phases –  90o : 1st and 3rd Quarter –  90 to 180o : gibbous phases –  180o : Full Moon

•  “Wax” from New to Full (~2 weeks) •  “Wane” from Full to New (~2 weeks) •  Earth blocks our view of half the sky-> can see

certain phases only at particular times of day •  Always see same phases in certain part of sky at

same time of day (every phase cycle)

•  Moon orbits the Earth once in 27.3 days •  Orbits and Rotates in “right hand” sense (like

Earth’s rotation and orbit) •  So, which way should it appear to move

(compared to the distant stars)?

Orbital Motion of the Moon

•  Our rotation (360o/24h) is 30 times faster than Moon’s orbit (360o/27.3d), so it appears to move east->west, but not quite as fast as the stars •  Net motion is west->east (about 13o/day) •  Moon rises about 50 minutes (13/15 of an hour) later each day

How Long is a “Month”? Synodic v. Siderial Period

Moon orbits Earth once in 27.3 days (“siderial”)

It takes 29.5 days to go through cycle of phases (“synodic”)

demo: time from one full moon to the next 365.25/27.3 = 13.4

365.25/29.5 = 12.4

So Lunar Calendar drifts compared to our 12 month calendar

•  Moon’s orbit is not a perfect circle –  sometimes a little closer than other times –  its apparent angular size changes a little – Earth is not at the exact center of Moon’s orbit

•  Plane of orbit tipped 5o to ecliptic plane –  not around Earth’s equator! [demo] –  our equator is tipped 23.5o to ecliptic plane

•  If Earth, Moon, and Sun are all in a line, either –  [solar eclipse] the Moon blocks our view of the Sun

or –  [lunar eclipse] Moon passes through the Earth’s

shadow

Moon’s Orbit and the Ecliptic When Do We See an Eclipse? •  The Moon does not orbit Earth in the same plane that

Earth orbits Sun [Moon usually appears north or south of Sun]

•  However, Moon crosses the ecliptic twice every month, and the Sun is always “on” the ecliptic –  [so you might expect to see about 25 eclipses every year]

•  We see an eclipse only if both happen at the same time: –  (1) Moon, Earth, and Sun are all in a line, and –  (2) Moon is crossing (within 1/2o of) the ecliptic

•  When they are all lined up, the Moon’s phase is either New or Full: if Moon on ecliptic, then

Solar Eclipse at New Moon Lunar Eclipse at Full Moon

•  This can only happen roughly twice per year [demo]

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Eclipses occur only when the ���Sun and Moon are both on the “line of nodes”

Which Only Happens About Twice Each Year Eclipses often happen in pairs: (for example, a solar eclipse at new moon followed 2 weeks later by a lunar eclipse at full moon)

Lunar Eclipses Conical shape of shadow -> Earth is spherical

Why does shadow have 2 parts?

In penumbra, you can still see part of the Sun

•  Entire Moon passes through Earth’s shadow during a total lunar eclipse

•  Only part of Moon enters Earth’s shadow during a partial lunar eclipse

•  Must be on night side of Earth to see lunar eclipse. Why?

•  Moon’s phase must be full. Why?

•  Reddish glow during totality caused by sunlight bending around through Earth’s atmosphere (“earthshine”). Don’t always see it.

•  Lunar eclipses last an hour or two. Can be seen from entire hemisphere of Earth, so they are fairly common.

•  It’s always safe to watch a LUNAR eclipse

What’s So Special About a

Total Solar Eclipse?

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•  strange things happen during a solar eclipse: it gets dark, you can see stars in the daytime, animals act goofy, winds pick up, the temperature drops, weird shadows race across ground at the beginning and end of totality

•  it is safe to look at the Sun during the few minutes of a total eclipse

•  throughout history, solar eclipses have inspired awe and fear

•  predicting eclipses was a good route to fame and fortune

•  understanding them doesn’t make them any less awe inspiring!

“Bailey’s Beads”

The “Diamond Ring”

•  During totality, we can see parts of the solar atmosphere that are not usually visible

•  This part of the atmosphere, the “corona” is very hot (10 million degrees); it emits x-rays and ultraviolet light

•  Eclipses give us a way to study the “active Sun”

•  We’ll learn all about solar activity in a few weeks, but it’s very important for life on Earth

•  We’ll also learn how an eclipse observation changed our understanding of gravity

Eclipses Enable Scientific Studies of Solar “Activity”

Solar Eclipses •  Moon’s umbral shadow only falls on a small portion of Earth during a total solar eclipse

•  Moon’s penumbral shadow falls over a larger portion of Earth in a partial solar eclipse

•  Must be along the path of the lunar shadow to see an eclipse

•  It is rare to see a solar eclipse unless you are willing to travel.

•  Must be on day side of Earth to see solar eclipse. Why?

•  Moon’s phase must be New. Moon and Sun both appear to be ~1/2o

corona

•  Total eclipses last only a few minutes, but they are very exciting!

•  Moon moves ~12o/day (W->E), or 1/2o in 1/24th of a day (1 hour), so partial phases last about an hour; moon moves across Sun from West to East [notice the direction in the photo above?]

Annular Eclipse •  Not all solar eclipses are total

•  Many are only “partial” eclipses, in which the Moon covers only part of the Sun

•  The Moon’s orbit isn’t a circle. If an eclipse happens when the Moon is near the farthest part of its orbit (called the “apogee”), we see only an “annular” eclipse

•  Partial and Annular Eclipses of the Sun are still WAY TOO BRIGHT to be viewed without special filters!!!

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Next total solar eclipse visible from near Charleston: August 2017

Earth viewed from Moon ���(in class activity)

•  Viewed from Earth, Moon appears to be 1/2o

•  How large does Earth appear from Moon?

Viewed from Moon, Earth appears to be 2o, Sun appears to be 1/2o

•  Does Earth go through phases?

Yes. On a monthly Cycle:

- New Moon --> Full Earth

- 1st Quarter Moon -> 3rd Quarter Earth

•  How does Earth appear to move across lunar sky?

- Stars go around once/year

- Sun goes around once/month

- Earth just sits there!