Eclipses
• What is an eclipse?
– Any time one
astronomical object
casts a shadow on
another we say that an
eclipse is occurring.
Jupiter’s Moon Io is causing a solar
eclipse to occur on part of Jupiter
• The Earth can cast a
shadow on the Moon
• The Moon can cast a
shadow on the Earth.
1
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)
• For eclipses it is important to
Your thumb, held at arm’s length,
note that the angular size of the
appears to be about 2 degrees wide
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 2are
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?
3
The Moon is Far Away
Earth
Moon
The Earth-Moon system to scale
• Compared to their sizes,
the Earth and Moon are
far apart.
• In fact it would take 30
Earths to span the
distance between the
Earth and the Moon!
• And the Moon is only a
third the size of Earth!
• Since the Moon is far and
small it’s easy for its
shadow to miss the Earth
if the two aren’t closely
aligned with the Sun.
4
Two Planes Crossing
Node
Node
• The orbital plane of the
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
the nodes, that is when the
Moon is on the Ecliptic!
5
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
6
Solar and Lunar Eclipses
• When the shadow
of the Moon
strikes the Earth it
is called a Solar
Eclipse
• When the shadow
of the Earth
strikes the Moon it
is called a Lunar
Eclipse
7
Solar and Lunar Eclipses
8
Solar Eclipse
9
Lunar Eclipse
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