- august 11 -
The Sun eclipses
After the Larousse dictionary, an eclipse means the temporary dissapearance of a body due to it's passing through the umbra or the penumbra shadow of another body.
An occultation represents the disappearance of a body behind another body with a bigger apparent diameter.
The image below shows very clearly that a Sun eclipse is in fact an occultation and not a real eclipse.
There are three kinds of Sun eclipses: the partial eclipses, the annular eclipses and the total ones. A partial eclipse takes place when the Moon is very close to the line of nodes, but not as close as needed for it's shadow to reach the Earth. A certain part of the Earth surface will be covered by the Moon's penumbra shadow, but the umbra will pass near the Earth without reaching it's surface.
Due to the fact that the Moon's orbit is elliptical, and not perfectly circular, it's distance from the Earth is variable and it's aparent diameter seen from Earth is also variable, from 33'29'' when the Moon is at perigee (the minimal distance from Earth) to 29'23'' when the Moon is at apogee (it's maximal distance from Earth).
The same thing happens to the apparent diameter of the Sun; it's variation goes from 31'27'' when the Earth is at aphelion (the maximal distance from Sun to Earth) to 32'31'' at perihelion. It is very clear that when the Moon's apparent diameter is smaller than the Sun's, it's disk won't completely cover the Sun's disk, and the eclipse will appear as annular.
When the Moon is close to the line of nodes and close enough to the Earth so that it's apparent diameter is almost equal to the Sun's, a total Sun eclipse will be produced.
The total eclipses are, from all the Sun's eclipses, the most interesting ones. During the phase of totality some solarelements can be observed: the cromosphere, the corona and the solar proeminences, all of them very interesting for the amateur observer, but mostly for scientific researches which cannot be accomplished in other conditiones.