Eclipse News

2017 Total Solar Eclipse: Da Cosmic Mojo

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We already know about the first condition of Da Cosmic Mojo to be met in order for a total solar eclipse to be possible – our moon must be a New Moon.  In this post we will explore the other two requirements that make a total solar eclipse certain, somewhere on our planet, on some particular date.

In coming up with Da Cosmic Mojo, the dawgs do not mean to make light of the complicated and precise science of eclipse prediction.  The mathematics used to describe and predict the geometric dance of sun, moon and earth as they fiddle about their normal business of orbiting each other is beyond the combined brain power of the dawgs.  Scientists involved in such things can tell us about eclipses that occurred 5,ooo years ago or ones that will not occur for another 5,000 years.  These predictions will be accurate down to the second, the degree, the thousandth of a mile, the whatever

 

Of Nodes and Men

Our solar system is like a panini sandwich, pretty flat but it still has some meat and cheese to it.  The sun, moon and planets all appear to move through only 12 of the 88 constellations in our northern and southern skies as viewed from our earthly viewpoint.  We know the 12 constellations as the zodiac.

In the case of the sun and moon, this difference amounts to about 5°.  Twice every month these orbits intersect at points known as lunar nodes

A solar eclipse may occur only when the New Moon and the sun are in close proximity to one of the lunar nodes

So here is our second requirement of Da Cosmic Mojo but let’s take a closer look at a few scenarios to better understand what exactly is up.  If the New Moon is not near a node at all, which is what occurs most months, then no eclipse will occur.

If our New Moon is close to a node, but not close enough to produce a total solar eclipse, then a partial solar eclipse will occur In the middle graphic below you may notice the moon’s penumbral shadow falls upon the earth, but the umbral shadow does not. 

When the New Moon and the sun are in close proximity to a lunar node, then a solar eclipse will occur.  Take note that both the penumbral and the umbral shadows fall on  the earth and a solar eclipse will occur somewhere on our planet. 

In this final scenario a solar eclipse is guaranteed, but not necessarily a total solar eclipse.  For a total solar eclipse to occur we need the cooperation of the third and final piece of Da Cosmic Mojo.

 

Size Matters But Distance Matters More

A total solar eclipse will occur when the New Moon and the sun are in proximity of a lunar node and the moon is near perigee

Uh-oh, we have another scientechy term to define….perigee.  For that let’s enlist the help of the great astronomer and mathematician, Johannes Kepler.

In 1609 Kepler gave the world the first of his three laws of planetary motion, proving that planets do not move in circles around the sun but in egg-shaped ellipses.  Sun and planet (or earth and moon) actually orbit each other around a common center of gravity, but the unequal distribution of mass usually places this point somewhere deep inside the larger of the two objects.  In practical terms for our discussion this means the orbiting planet or moon has a point of closest approach (perigee) and a point of furthest approach (apogee).

In the case of our moon perigee occurs at a distance of about 226,000 miles from us and apogee is a point 26,000 miles further away.  Only when the New Moon is close to perigee is its apparent diameter large enough to completely block the disk of the sun.  If it is closer to apogee then its apparent diameter is not large enough to block the disk of the sun and observers are left with an annular eclipse, the dark disk of the moon sporting a ring of the bright sun, but totality does not occur. 

A New Moon, close to a lunar node, near perigee – Da Cosmic Mojo will be workin’ for millions of Americans and their guests on Monday, August 21, 2017.

 

That mockingbird’s gonna sail away                                                      We’re gonna forget it,                                                                                        That big fat moon is gonna shine like a spoon                                           But we’re gonna let it, you won’t regret it.

Bob Dylan, “I’ll Be Your Baby Tonight”, John Wesley Harding, 1967                            Copyright © 1968 by Dwarf Music; renewed 1996 by Dwarf Music

 

 

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