Upload
others
View
4
Download
0
Embed Size (px)
Citation preview
1
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]
2
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?
3
• 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!!!
4
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!