Lunar Eclipse
Chapter 3 from book
Objectives
• To be able to describe the phenomenon of
how a lunar eclipse occurs.
• To be able to apply what they already know
about the moons orbit to this “special case.”
• To apply what they know about the
wavelengths of light to the appearance of the
moon during the eclipse.
Starter
• http://www.space.com/22852-fall-equinoxearth-seasons-explained.html
• Fall equinox on September 22 - Sunday
Lunar Eclipse
• http://www.youtube.com/watch?v=wuhNZejH
eBg
• This is a brief video from NASA explaining how
a lunar eclipse works.
Lunar Eclipse
Take note of
the
components
of Earth’s
shadow, the
umbra and
penumbra
and the role
they play in
the lunar
eclipse
Lunar Eclipse
• Moon is tipped only a few degrees from the
plane of Earth’s orbit around the sun.
• Earth’s shadow points directly away from the
sun (also on the plane of Earth’s orbit)\
• Thus a lunar eclipse is when the Moon’s path
is carries it directly through the shadow of
the Earth.
Umbra
• This is the region of total shadow
• From the book:
– If you were drifting in your space suit in the umbra
of Earth’s shadow, the sun would be completely
hidden behind Earth.
Penumbra
• Again, if you are drifting in your space suit:
– In the penumbra, you would see part of the sun
peeking around the edge of Earth.
– You would be in partial shadow.
– Sunlight is dimmed but not “extinguished” like in
the umbra.
At the Height of an Eclipse – in the Umbra
Lunar Eclipse
• Generally most full moons pass north or south
of Earth’s shadow and there is no eclipse.
• There are partial eclipses and total eclipses.
• Partial Eclipse – When the moon passes a bit
too far north or south of Earth, it may only
partially enter the umbra.
– Usually cannot see the coppery glow.
Total Lunar Eclipse
• During a total eclipse, the moon moves into
the penumbra where it dims slightly.
– As it moves, the dimming increases.
• Finally it reaches the umbra where the moon
begins to darken as it slides into the umbra.
• When it is totally eclipsed, the moon does not
disappear.
Total Lunar Eclipse
• The moon does not receive direct sunlight but
light waves in the red spectrum are bent
around Earth.
– They pass through the Earth’s atmosphere.
– Blue light, much shorter wavelength, is scattered
as it hits the atmosphere (which is why the sky is
blue).
– Red light continues on and does reach the moon,
creating its reddish glow.
Total Lunar Eclipse
• An eclipse can last for as much as 1 hour 45
minutes.
• http://www.youtube.com/watch?v=lcRp1jKJm
JU
• You Tube video of a “speeded up” lunar
eclipse.
Review
1. Which lunar phases would be visible in the sky
at dawn? At midnight?
2. Looking back at Earth from the moon, what
phase would Earth have when the moon was
full?
3. Could a solar-powered spacecraft generate any
electricity while passing through Earth’s umbral
shadow?
4. If a lunar eclipse occurred at midnight, where in
the sky would you look to see it?