1 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

2 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 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. EarthMoon The Earth-Moon system to scale

4 Two Planes Crossing 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! Node

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 Eclipses Solar eclipses happen when the Moon passes between us and the Sun. The eclipse can only be seen where the alignment is perfect. The sky goes dark, the stars come out, and the solar corona becomes visible. It only lasts a few minutes in any given spot. Although there are 1 or 2 per year, any given place has a long wait to see one.

8 Total Solar Eclipse August 11, 1999: Szombately, Hungary

9 ShadowDance Eclipses usually don’t happen, because the Moon’s orbit is tilted. The Moon just happens to have about the same angular size as the Sun. This is a coincidence of the present, the distant past and future will not get perfect solar eclipses because the Moon’s distance is growing.

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11 Eclipse Prediction The cycles of the sun and moon cause eclipses in a predictable pattern. The same eclipse occurs every 18 years - the saros cycle. Next partial solar eclipse in DeKalb is March 19, Next total solar eclipse in the US is August 21, sunearth.gsfc.nasa.gov