Solar Eclipse

Exciting event happening in Northern Australia!  =D  In the early morning of November 14 local time, (November 13 east of the International Date Line), there will be a total eclipse of the sun.  =)  So let's learn about it.  ;)

To understand eclipses, we need to keep the following in mind.  Simply put,

Image from: http://www.earthview.com/tutorial/patterns.htm

• The Earth follows a path around the Sun, while the Moon follows a path around the Earth.  
• Along its path as the Moon orbits, it moves between the Earth and the Sun.  Among the phases of the moon, this corresponds to the New Moon (or dark moon) wherein the Moon is not visible to us because it is the far side that is illuminated by sunlight.  During this time, it casts its shadow towards the Earth and a solar eclipse can occur.  
• Conversely, the Moon moves behind the Earth further along in its orbit.  This time, the Earth becomes positioned between the Sun and the Moon, and we see the moon as full.  During this time, the Moon may enter the Earth's shadow and a lunar eclipse can occur.  
• These phases of the moon cycle roughly every month, but solar and lunar eclipses do not happen that often because the plane of the Moon's orbit around the Earth is tilted about five degrees in relation to the plane of the Earth's orbit around the Sun.
• Most of the time, therefore, the Moon or its shadow passes above or below the Earth and its shadow, and an eclipse does not occur.  

• A solar eclipse may only occur when the Moon is near one of its nodes (i.e., the points along its orbit where it intersects the plane of Earth's orbit) during its New Moon phase.  
• One more interesting thing to note: during this New Moon phase, the Moon is actually in the sky during the daytime, moving along with the Sun.  We just don't see it because, as mentioned earlier, the illuminated side is the far side, plus it gets lost in the bright glare of sunlight.  The only time we notice it is during an eclipse when it obscures part of or the whole disk of the Sun.  
• Now, when the conditions are met and a solar eclipse occurs, it can be any one of the four types mentioned below.

Types of solar eclipses:

A. Total eclipse, B. Annular eclipse, C. Partial eclipse
Image from: http://en.wikipedia.org/wiki/Solar_eclipse 

1. Total solar eclipse 
• This occurs when the part of the Moon's shadow called the umbra reaches the surface of the Earth.  The umbra tracks a narrow path called the Path of Totality, which covers less than 1% of the Earth's surface.  This is the reason why total eclipses seem to be a rare event, even though it occurs rather frequently (about one total eclipse every one or two years).  You have to be at the right place at the right time to view it.  Also, the location where this shadow path hits the Earth will be different with every successive total eclipse.  The interval between two total eclipses occurring in one place can be very long.  
• It is very interesting to note that at present, the Sun's diameter is actually about 400 times that of the Moon's. But because the Sun is also about 400 times farther from the Earth than the Moon, their apparent sizes in the sky are about the same.  So when you are in the Path of Totality when the Moon moves directly in between the Earth and the Sun, the disk of the Moon will just about cover the disk of the Sun from your point of view.  (More on the phases and effects seen during a total eclipse below.)
2. Annular solar eclipse 
• A total eclipse does not always occur when the Moon moves directly in between the Earth and the Sun.  This is because the orbits are elliptical so that the distance between the Earth and the Moon (as well as between the Earth and the Sun) varies.  An eclipse will be total if it occurs when the Moon is nearer, and its apparent size is enough to cover the apparent size of the Sun. If the eclipse occurs when the Moon is farther, its apparent size will be smaller and may not be enough to cover the disk of the Sun.  If this is the case, the eclipse will be annular.  
• It is also noted that in this case, the tip of the umbra falls short and does not reach the surface of the Earth.  It is the antumbral shadow that reaches the surface that, similar to the above, tracks a narrow path on the surface, this time called the Path of Annularity.
• This type of solar eclipse is more a subset of partial eclipses, and you will not see the effects seen during a total eclipse.  The rim of the Sun is still seen around the dark disk of the moon, and this is known as the "Ring of Fire".  
3. Hybrid solar eclipse 
• This is a rare type also known as annular/total eclipse.  This occurs when, because of curvature of the Earth, the tip of the umbra reaches the surface at some points of the eclipse path, while at other points, it doesn't. The effect of this is that, at the points where the umbra reaches the surface, a total eclipse is observed, while at the other portions, an annular eclipse is observed.  
4. Partial solar eclipse 
• The part of the Moon's shadow called the penumbra covers a wider area.  So those that are outside the narrow paths of totality or annularity described in the above types of solar eclipses will be able to see a partial solar eclipse.  From their point of view, the Sun will only be partially obscured by the disk of the Moon, the degree of which will depend on how far they are from the direct path.  
• Purely partial eclipses can also occur. This happens when the umbra completely misses the Earth, passing either above the Northern pole or below the Southern pole, and only the penumbra touches the surface of the Earth.  

Phases and Effects of a Total Solar Eclipse:

Image from: http://en.wikipedia.org/wiki/Solar_eclipse

1. First contact  
• The edge of the Moon starts to overlap with the Sun's edge.
• The Moon progresses across the Sun covering more and more of it.  
• Daylight fades quickly a few minutes before totality.
• Shadow bands may be seen moving across the ground, caused by the distortion of sunlight refracting through the Earth's atmosphere. 
2. Second contact 
• Nearing the point where the Sun is completely obscured by the Moon, an effect called Baily's beads is seen where light from the Sun shines through along the edge owing to the irregular surface of the Moon.  When only one of these beads is left, it results in a diamond ring effect.  
3. Totality 
• The Sun is totally obscured by the Moon, and the sky gets very dark.   
• For a few seconds, the Sun's chromosphere is visible as a pinkish glow around the edge of the Moon.  
• The white solar corona, visible only during a total eclipse, is seen all around the Moon.
• Gigantic, red loops of gas called solar prominences are seen arching above the edges.  
• Brighter stars and planets become visible in the rest of the sky.
• Animals and plants behave as if it is nighttime.  
4. Third contact 
• The Sun reemerges from behind the Moon.  A diamond ring effect may be seen again.  The events before totality occurs in reverse order.  
• The Moon progresses from the Sun, uncovering more and more of it. Daylight returns. 
5. Fourth contact 
• The edge of the Moon ceases to overlap with the Sun and the eclipse ends.

Unfortunately for most of us, we won't be able to experience and see the total solar eclipse with our own eyes.  But fortunately, with today's technology, we may still be able share in the experience.  =)
http://www.space.com/18444-total-solar-eclipse-webcasts-2012.html
Enjoy!  =D

Eclipsed Earth.
Photo of the August 11, 1999 total solar eclipse taken from the Mir space station.
Image from: http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sun
Credit: Centre National d'Etudes Spatiales

References and Suggested Readings:

• http://csep10.phys.utk.edu/astr161/lect/time/eclipses.html
• http://www.earthview.com/tutorial/tutorial.htm
• http://www.mreclipse.com/Special/SEprimer.html
• http://eclipse.gsfc.nasa.gov/solar.html
• http://www.exploratorium.edu/eclipse/
• http://en.wikipedia.org/wiki/Solar_eclipse
• http://www.space.com/15584-solar-eclipses.html