Because earth rotate from east to west and the position of the Moon await us on our west since it must stay between Earth and Sun. Eclipse is then observed to progress from western part.
Another Answer
The Moon is in orbit with and around the Earth. It flies in the same direction Earth rotates, but at slower speed. This is why the Moon rises in the East and sets in the West.
Although the Moon tracks is East to West, as does the Sun, the Sun overtakes the Moon as it sets which drags the Moon across the face of the Sun. The full eclipse is only visible to those on the path of the umbra. A partial eclipse is visible to those in the path of the penumbra.
If we imagine a line segment with us at one endpoint, the center of the Sun at the other, and the segment intersecting the center of the Moon, we could watch the full eclipse indefinitely. Of course we would have to be able to stay at this end of the segment and at this distance from the Moon.
During a TOTAL solar eclipse, one can see the corona, the part that extends millions of miles out into space.
There are four different types of solar eclipse, namely Partial eclipse, Annular eclipse, Total Eclipse and Hybrid Eclipse. A partial solar eclipseoccurs when only part of the Sun is covered by the Moon which appears to take a "bite" out of the Sun.
You can draw a diagram with the Sun at one side, the Moon in the middle and the Earth on the other side. A shadow falls on the Earth and anyone on that part of the Earth sees a solar eclipse. Obviously the Moon is not that big, so when there is an eclipse on one part of the Earth, other people in many different places don't see one.
During a solar eclipse, it is the shadow of the moon that travels across part of the Earth as it passes between the Earth and the Sun. This creates the phenomenon where the moon blocks the sunlight from reaching the Earth's surface, causing a temporary darkness known as the eclipse.
A solar eclipse occurs when the moon blocks the sun's light from reaching Earth. This phenomenon can result in either a total solar eclipse, where the sun is completely obscured, or a partial solar eclipse, where only part of the sun is covered.
No part of the solar eclipse of July 21-22, 2009 was visible from anywhere in the US.
The next solar eclipse of which any part will be visiblein Zimbabwe will occur on November 3, 2013.
A solar eclipse
-- During a partial solar eclipse, part of the sun is obscured from our view (by the moon) and the rest of it is still there. -- During a total solar eclipse, the entire disk of the sun is obscured from our view (by the moon).
The corona.
Corona
You will see a partial eclipse if you are in the penumbra.