# Eclipses

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```					               Lunar Phase Review

Lunar phases
repeat
through the
Lunar cycle-
6am
certain
12am     12pm
phases are
visible only                6pm
at certain
times of the
day (or
night).
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Lunar Phase Review

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Lunar Phase Review

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Eclipses
• What is an eclipse?
– Any time one
astronomical object
another we say that an
eclipse is occurring.
• The Earth can cast a
• The Moon can cast a
Jupiter’s Moon Io is causing a solar
eclipse to occur on part of Jupiter
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Angular vs. Actual Size or
Which is Bigger Your Thumb or the Moon?
• In astronomy we often talk
about the angular size of an
object.
• Angular sizes are measured in
degrees or fractions of a
degree. (1 arcminute = 1/60 of
a degree, 1 arcsecond = 1/60 of
an arcminute)
Your thumb, held at arm’s length,       • For eclipses it is important to
appears to be about 2 degrees wide        note that the angular size of the
or 4 times the width of the Moon. (Try    Moon and the angular size of
this yourself).                           the Sun are the same, both are
The Moon is much larger than your thumb   about 1/2 a degree. This does
but it is also much farther away!         not mean the actual sizes are
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the same!
What do you need for an Eclipse?
• You need the Sun,
Earth and Moon (in
some order) to line
up.
• During a New Moon
the Sun, Moon and
Earth are lined up.
• During a Full Moon
they are lined up too.
• But are they really
lined up?
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The Moon is Far Away
• Compared to their sizes,
the Earth and Moon are
far apart.
• In fact it would take 30
Earth                          Moon     Earths to span the
distance between the
Earth and the Moon!
The Earth-Moon system to scale
• And the Moon is only a
third the size of Earth!
• Since the Moon is far and
small it’s easy for its
if the two aren’t closely
aligned with the Sun.
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The Moon’s Orbital Plane is Tipped
• Compared to their sizes,
the Earth and Moon are
far apart.
• In fact it would take 30
Earth                          Moon     Earths to span the
distance between the
Earth and the Moon!
The Earth-Moon system to scale
• And the Moon is only a
third the size of Earth!
• Since the Moon is far and
small it’s easy for its
if the two aren’t closely
aligned with the Sun.
8
Two Planes Crossing
• The orbital plane of the
Node            Earth around the Sun (the
Ecliptic) and the orbital
plane of the Moon around
the Earth are tilted by 5
degrees.
• The two planes cross at
two points called nodes.
• Eclipses can only occur
when the Moon is at one of
Node     the nodes, that is when the
Moon is on the Ecliptic!
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So What Do You Need for an Eclipse?
The Moon must
be either at
– Full Moon or
– New Moon
and
– be crossing
the Ecliptic
(be on one of
the nodes)
This happens at
most twice a
year
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Solar and Lunar Eclipses
of the Moon
strikes the Earth it
is called a Solar
Eclipse
of the Earth
strikes the Moon it
is called a Lunar
Eclipse

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Solar and Lunar Eclipses

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Solar Eclipse

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Lunar Eclipse

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Eclipse Seasons

• Eclipse seasons are roughly 6 months apart
– May have two eclipses (1 lunar, 1 solar) 2 weeks apart
– Eclipses may not be total
• Solar: total, annular, partial
• Lunar: total, partial, penumbral

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Lunar Node Precession
• Lunar orbit precession
• Lunar nodes precession period is 18.6 years
• Eclipse seasons come about three weeks
earlier in subsequent years

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Annular Eclipse
15 Jan 2010

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