Eclipses On multiple occasions throughout the year, the Sun, Moon, and Earth will align to create a lunar or solar eclipse With the use of precise timing and knowledge of the orbits of the Earth and Moon, it is possible to predict these events in advance Not all eclipses are the same, and their appearance can change depending on the alignment and the apparent size of the Moon

Apparent Size of the Moon and Sun The size of an object on the sky is expressed as an angle From horizon to horizon is 180o The size of your outstretched fist is about 10o Astronomical objects are usually quite small Remember, 1o is made up of 60 arcminutes 1 arcminute is made up of 60 arcseconds The apparent size of the Sun is about 32 arcminutes The apparent size of the Moon is about 30-32 arcminutes Remember that the Moon is sometimes closer and sometimes farther away, so this number changes over the course of a month The fact that the Sun and the Moon are so close in apparent size creates the spectacular eclipses we see here on Earth

Solar Eclipse For a solar eclipse to occur, the Moon must block out the light from the Sun This means the Moon must be in between the Sun and the Earth Notice that this position corresponds to a New Moon Therefore, a solar eclipse must always occur around the time of New Moon

The moon passes through these nodes at least twice per year. Solar Eclipse If it has to be a New Moon for a solar eclipse, why doesn't one happen every month? Remember that the path of the Moon through the sky is slightly tilted compared to the path of the Sun Only when the paths cross are conditions right for an eclipse The points where the paths of the Moon and the Sun cross are called nodes. The moon passes through these nodes at least twice per year.

Types of Solar Eclipse - Total When the Moon is closer to us (apogee), its apparent size matches the apparent size of the Sun If an eclipse occurs close to apogee, the disk of the Moon will completely block out the light from the Sun When the disk of the Moon is directly over the Sun, a bright halo of light is visible This is an outer layer of the Sun called the corona (more on this later in the semester)

Types of Solar Eclipse - Annular If the Moon is farther away from us, its apparent size will be smaller If an eclipse occurs during this time, the disk of the Moon will not be able to fully cover the disk of the Sun This results in an annular eclipse Instead of completely blocking out the Sun's light, a thin ring (or annulus) of light is still visible

Solar Eclipse The Right Place at the Right Time Everyone on Earth does not experience an eclipse in the same way Only a very few will see the Moon's disk pass directly over the Sun (a total eclipse) Others will just see the Moon's disk graze the Sun (a partial eclipse) Darkest part of the shadow : Umbra Lighter part of the shadow: Penumbra

Partial Solar Eclipse

Lunar Eclipse For a lunar eclipse to occur, the Moon must pass through the Earth's shadow This means the Earth must be in between the Sun and the Moon Notice that this position corresponds to a Full Moon Therefore, a lunar eclipse must always occur around the time of Full Moon

Lunar Eclipse Like solar eclipses, lunar eclipses only occur if the Moon is passing through a node The gray circles at right show the Earth's shadow Darkest part is umbra Lighter part is penumbra Depending on the timing, the Moon will pass through different parts of the shadow, or possibly not at all

Penumbral Lunar Eclipse Penumbral eclipse occurs when the Moon passes through the lighter part of the Earth's shadow These eclipses are barely noticeable and not particularly interesting

Partial Lunar Eclipse Partial lunar eclipse occurs when the Moon passes through the Earth's umbra, but never goes completely into shadow

Total Lunar Eclipse A total lunar eclipse occurs when the Moon passes completely through the Earth's umbra Notice below how even when the Moon is completely in Earth's shadow, it still appears to have a red glow The Earth's atmosphere can act like a lenses and refract light back onto the Moon's surface Depending on the alignment and atmospheric conditions, the color of the Moon can change from orange to red to simply black

Upcoming Lunar Eclipses 2006 Sep 07 Partial 01h33m Europe, Africa, Asia, Aus. 2007 Mar 03 Total 03h42m 01h14m Americas, Europe, Africa, Asia 2007 Aug 28 03h33m 01h31m e Asia, Aus., Pacific, Americas 2008 Feb 21 03h26m 00h51m c Pacific, Americas, Europe, Africa 2008 Aug 16 03h09m S. America, Europe, Africa, Asia, Aus. 2009 Feb 09 Penumbral - e Europe, Asia, Aus., Pacific, w N.A. 2009 Jul 07 Aus., Pacific, Americas 2009 Aug 06 Americas, Europe, Africa, w Asia 2009 Dec 31 01h02m 2010 Jun 26 02h44m e Asia, Aus., Pacific, w Americas 2010 Dec 21 03h29m 01h13m e Asia, Aus., Pacific, Americas, Europe

Other 'Eclipses' in the Solar System Occasionally, we can see planets interior to our orbit (Mercury and Venus) cross the face of the Sun These crossings are called transits Someone on Mars would have the chance to see an Earth transit Mercury transits the Sun

Planet Transits Around Other Stars Telescopes and cameras have become so sensitive that they can even detect planetary transits around other stars Many extrasolar planets have been detected by looking for a dip in the light received from a star

Eclipses Outside the Solar System Most stars come in pairs and are called binary stars Sometime, the alignment of these stars to our line of sight lets us see one star eclipse the other You will learn a lot more about eclipsing binaries and their importance in ASTR1020