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Eclipses• What is an eclipse?

– Any time one astronomical object casts a shadow on another we say that an eclipse is occurring.

• The Earth can cast a shadow on the Moon

• The Moon can cast a shadow on the Earth.

Jupiter’s Moon Io is causing a solar eclipse to occur on part of Jupiter

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• 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 note that the angular size of the Moon and the angular size of the Sun are the same, both are about 1/2 a degree. This does not mean the actual sizes are the same!

Your thumb, held at arm’s length,appears to be about 2 degrees wideor 4 times the width of the Moon. (Try this yourself).The Moon is much larger than your thumb but it is also much farther away!

Angular vs. Actual Size or Which is Bigger Your Thumb or the Moon?

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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 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.

Earth Moon

The Earth-Moon system to scale

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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!

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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• 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

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Solar and Lunar Eclipses

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Solar Eclipse

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Lunar Eclipse