Tuesday, June 24, 2008

Why are eclipses so rare?

When the moon in its journey around the earth passes directly between the earth and the sun, it casts its shadow on the surface of the earth, and an eclipse of the sun takes place.An eclipse of the sun occurs only when the moon is new, for then the moon is on that side of the earth facing towards the sun. Then why isn't there an eclipse of the sun every time there's a new moon? The reason is that the path of the moon around the earth does not lie directly in line with the orbit of the earth about the sun. In its 29-day trip around the earth, the moon passes sometimes above and sometimes below the path of the earth.An eclipse of the sun can be total, anuular, or partial. If the moon hides the sun completely, the eclipse is total. But the moon is not always the same distance from the earth. Often, it is too far from the earth to hide the sun completely. Then , when an eclipse takes place, the moon is seen as a dark disk which covers the whole sun except a narrow ring around its edge. This thin circle of light is called " the annulus", meaning "ring". This is an annular eclipse. An eclipse is partial whwnever only a part of the disk of the moon comes between the sun and the earth.An eclipse of the moon occurs only when the moon is full, for then it is at the opposite side of the earth from the sun. When the moon comes directly behind the earth, as seen from the sun, it passes gradually into the great shadow-cone cast by the earth and disappears from view. A total eclipse of the moon then occurs. A partial eclipse takes place when the moon enters only partly into the shadow.In some years, no eclipse of the moon occur. In other years, there are from one to three. Every year, there must be at least two solar eclipses, and there may be as many as five. At any one place on the earth's surface, a total solar eclipse will be visible only once in about 360 years.

No comments: