Why is the 2017 eclipse getting so much media attention?  Comparing its theoretical size and length it has been calculated to be one  of the top ten of all time. The last time a total solar eclipse was visible from coast to coast was 100 years earlier, on June 8, 1918, so yes this is a once in a lifetime eclipse.

Defining the various eclipses

Eclipses of the sun only take place during conjunctions of the sun and moon, or when the moon is  between the sun and the earth with the eclipse itself being when the moon conceals the sun from the earth, or to the earth  plunged in the shadow of the moon.

That seems strange as the moon is indeed much smaller than the sun but because the moon is so much nearer to the earth that its apparent diameter, means that the diatremic difference is inconsequential.

Total, annular, and partial

If the centers of the sun and the moon were in a same straight line to the eye of a spectator on the earth, a person would see the sun eclipsed .  It is a total eclipse when the the apparent diameter of the moon surpasses the sun, but based on the angle of site, when the moon’s diameter is at its smallest, the observer instead will see a luminous ring, formed by that part of the sun’s disk which exceeds that of the moon’s, and then the eclipse will in that case be annular.

If the center of the moon is not in the same straight line to the observer and the center of the sun, then eclipse can only he partial, as the moon can only conceal a part of the sun’s disk. This is why there is great variety in the the appearance of the solar eclipses. Another reason is the elevation of the moon above the horizon, which changes its apparent diameter considerably.

The moon’s diameter appears larger when she is near the horizon than when she is elevated far above it — think about sunset when the Moon seems enormous sitting on the horizon.  Her size does not change six hours later when she is fully risen in the sky and is now just a glowing orb, but the altitude of our viewing has and thus she seems smaller than before.  This is also why an eclipse in one longitude is seen differently, if at all, at another.

How many eclipses happen in a year?

In any year, the number of eclipses of both luminaries cannot be less than two, nor more than seven with the most typical number being four; it is rare to have six or more for while the sun passes both the nodes once a-year, unless the first was at the very beginning of the year then it is unlikely he will pass that same node again that year.  That is because  these points move 197 degrees backwards every year, so the sun come to one 173 days after he has passed the other.

When does an eclipse happen?

Eclipses occur when either node is within 17 degrees of the sun at the time of new moon, then the sun will be eclipsed. At the subsequent opposition, the moon will be eclipsed in the other node* and come round to the next conjunction again ere the former node be 17 degrees past the sun, and will therefore eclipse him again, so that makes it definite that there will always be at least two eclipses per annum. Thus when three eclipses fall about either node, the like number generally falls about the opposite.

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