Download The Moon

The Moon
-Around the Earth and in the Sky
Our goals for learning:
• Why do we see phases of the Moon?
• When do we see the phases of the Moon?
• What causes eclipses?
Why do we see phases of the Moon?
• Lunar phases are
a consequence of
the Moon’s 27.3day orbit around
Earth
Phases of Moon
• Half of Moon is lit
by the Sun and
half is dark
• We see a different
amount of the
bright and dark
faces as Moon
orbits
Phases of the Moon: The View from Earth
new
crescent
first quarter
gibbous
full
gibbous
last quarter
crescent
A 29.5-day cycle
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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.
Moon Rise/Set by Phase
Thought Question
What phase of the Moon is this?
A.
B.
C.
D.
First quarter
Waxing crescent
Waxing gibbous
Third quarter
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
Eclipses
Definition:
When light from an astronomical
body is blocked by another body
passing between it and the observer.
Any object can eclipse another
For example there are binary stars called eclipsing
binaries, because they are detected when the orbiting
star is eclipsed.
What causes eclipses?
• The Astronomical bodies cast shadows.
• When an object passes through another’s
shadow, there is an eclipse.
Most Common Types of Eclipse
Solar Eclipse
(Sun is blocked by
Moon’s shadow)
-occurs at New Moon
Lunar Eclipse
(Moon is blocked by
Earth’s shadow)
-occurs at Full Moon
Types of Eclipse
• Partial - Sun or Moon is partly covered.
• Total - Sun or Moon is completely covered over
and goes dark.
• Annular - center of Sun is covered over, leaving
a ring of light.
• Penumbral Eclipse - Moon is solely in the
penumbral shadow, only
mildly darkened.
Eclipses Every Month?
The Moon’s orbit is tilted 5° to ecliptic plane. Most alignments
occur above or below the ecliptic. As a result we do not get an
eclipse every month.
Eclipses Every Month?
View from Earth
When Do Eclipses Occur?
We have about two eclipse seasons each year, when
the Sun, Earth and Moon are in alignment and the
Moon is on the plane of the ecliptic.
When Do Eclipses Occur?
We then get a solar eclipse at new moon and a lunar
eclipse at full moon.
Solar Eclipses
Total Solar Eclipses
The Sun is 400 times
larger than the Moon.
The Moon is 400 times
closer than the Sun.
Consequently, they appear to be about the same angular size.
Solar Eclipses
Eye Protection Vital!
Total Solar Eclipse, 1999
Eye Safety
• Sunlight can permanently burn out the
receptors in your eyes.
• Retinas have no pain sensors, You won't
know until it's too late!
• Eclipses are especially dangerous, since
people are deliberately looking at the Sun.
Eye Safety
• Welder's Glass- excellent if expensive.
• Telescopes with Sun filters -excellent, but need to
make sure it dies not get scratched.
• Pin hole cameras -You do not look at the Sun (it's
cheap and works, but is not a cool-looking)
• Mylar eclipse glasses - cheap but not recommended.
(They scratch far too easily, high risk of line blindness)
Total Solar Eclipse, Zambia 2001
Start of
Eclipse
Sun
partially
eclipsed
‘Second Contact’: the
Beginning of Totality
Totality !
Zambia at Totality
Close-up of Edge during Totality
(‘Bailey’s Beads’)
‘Diamond Ring’ : End of Totality
Composite of Entire Eclipse,
Zambia 2001
Annular Eclipses
Moon is further away, so its angular size is
slightly smaller than the Sun's angular size.
Annular Eclipse Sequence
Greenland, 2003
Partial Eclipse at Sunset,
WA, July 2000
Partial Eclipse at Sunset sequence,
WA, July 2000
Total Lunar Eclipse
Lunar Eclipse
Moon beginning to be eclipsed (partial)
Moon mostly eclipsed
Moon eclipsed by penumbra
Beginning of Totality
End of Totality
Moon still in penumbra
Moon coming out of shadow
Full Sequence of Total Lunar Eclipse
Predicting Eclipses
• Eclipses recur with the 18 yr, 11 1/3 day
saros cycle, but type (e.g., partial, total)
and location may vary.
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 during the Moon's orbit.
• 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