August 23, 2017: TOTAL SOLAR ECLIPSE II
To accompany this week’s full eclipse of the sun, PBS had a nice broadcast of Nova, on eclipses. I hope you viewed it. It showed more than you could see by your own direct observation. The “second contact” (that magic moment of total occlusion) is electric. If you didn’t watch the program, the gist of the science is as follows.
If you placed a tiny black disc directly in front of a huge white one and backed off to view them in alignment, how much of the large white disc would be blocked from your view by the tinier one? That question is related to “parallax,” the proper domain of stereography. The answer depends on the relative distances (between you and the small disc, on one hand, and between the two discs, on the other).
Our sun is 1,500 times wider than the 3.5 KM diameter of the moon. So you it would take 1,500 of our moons touching each other to completely cover the diameter of the sun. (In terms of volume, it would take 4 trillion of our moons to stuff the sun full, but let’s ignore such mind bogglers!)
But, quite by chance, the moon is also 400 times closer to the earth than the sun. Perhaps some people will wonder if that’s not the doing of some celestial mathematician! But no, it results from Newton’s law of gravitation which specifies the balance of forces on the moon as it is tugged by the earth and the sun, given the respective masses of the three bodies. That happy coincidence means our two discs, black and white, can seem to overlap exactly (neither one overlapping the other) when in alignment. That’s why our tiny moon can cause a total eclipse of the immense disc of the sun.
But an eclipse need not be “total.” More common are the partial eclipses, especially the “annular” eclipse, which is when the moon appears to be fully inside the disc of the sun, leaving an “annulus” or rim around the moon. These “lesser” types occur because of some minor irregularity of motion: for instance, the plane in which the moon orbits the earth is slightly inclined to that in which the earth orbits the sun.
As Nova pointed out, our full understanding of the types, occurrence, and the when and where of eclipses (predicting its moment of onset down to a second and its area of coverage down to an area the size of a city suburb) is one commonplace example of the predictive power of science. But that knowledge robs us of a great tool we might have used had we lived one millennium ago: We might have called yesterday’s total eclipse “a divine omen” indicating that our king, Trump, must go!