Musings from an Amateur Cosmologist

Moon and Sun relationship

Recently, for a minute or two, a few fortunate Americans found themselves to be in the sixty five to seventy mile wide shadow of the moon during the August 21, 2017, total solar eclipse.

However, most of us do not stop to consider that, essentially, we experience a more frequent and much longer solar “eclipse” by being caught in Earth’s shadow as the planet continuously eclipses the sun. This occurs for several hours each day between sunset and sunrise when half of the planet is always in the earth’s shadow (we call this nighttime). The only exception, other than the cosmonauts aboard the Space Station, are the folks who live in the “land of the midnight sun” at that time of the year when, for them, the sun never sets.

By the way, since it is the Earth’s rotation that gives us the illusion of the rising and setting sun, we should really call these events “Earthset” and “Earthrise” respectively, as in “What a beautiful Earthrise this evening”.telescope-e1504807705987.jpg

When it comes to eclipses, one particular historical event stands out. During a total solar eclipse over the South Atlantic Ocean in May, 1919, the British astronomer, Sir Arthur Eddington, was able to confirm one of Albert Einstein’s most famous predictions.

In 1916, Einstein had predicted, as part of his new General Theory of Relativity, that light on its way to us from distant stars would follow a curved path around a massive body, like the sun, because of the “warping” of space-time in the vicinity of that body (sort of like a marble rolling across a trampoline with a bowling ball nestled in it’s center).

By photographing the position of the Hyades star cluster when the sun was nowhere near the cluster’s line of sight to Earth and again, during the eclipse, when the darkened sun was almost directly in the cluster’s line of sight, Eddington found that not only did the stars appear to shift their positions relative to background stars, but by the exact amount predicted by Einstein. This was in conflict with Newton’s theory of gravity which predicted a much smaller, if any, path-bending effect for light. Star light has no mass and, according to Newton, should not be affected by a gravitational field, but Einstein/Edington found that it indeed is.

We now know that gravity is not some mysterious “force acting at a distance” as Newton believed, but is actually the mass-induced curvature of space-time that causes moving objects to “fall” toward each other along curved paths. This means, for example, that our “small” Earth is continuously falling toward the much larger sun, but the planet’s rate of fall is (fortunately, for us) balanced by its orbital speed and momentum in its curved path around the sun.

Interesting, but this is probably not the same thing as saying, “I think I am falling for you”.

Howard Horton – OLLI Member and Book Group Coordinator


Telescope photo source – shutterstock wikicommons

Moonphase photo Source – Abraren.wikispace

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