August 21! Totality!

A total solar eclipse will be visible in South Carolina on Monday, August 21, 2017.

Neil DeGrasse Tyson, the director of the Hayden Planetarium at the Rose Center for Earth and Space in New York, tells us to put it on our bucket lists because it is both rare and spectacular.

The moon’s shadow will sweep along a 70 mile wide path at 1.5 times the speed of sound, providing witnesses in that path with just about 2 minutes of spectacle.  “It will be like having a 360-degree sunset all around you says NASA’s LIka Guthathakurta.  ‘Stars appear.  The temperature drops. You can actually hear chirping of grasshoppers.  So, animals actually naturally go back to their nocturnal behavior.'” Jay Pasachoff, a Williams College astronomer and eclipse expert, will travel from his home in Massachusetts to a location along the path because “It’s a tremendous opportunity…to see the universe change around you.”

For those of us who are not astronomers or eclipse experts this rare even is a prod to our curiosity about operations of the universe.

The textbook model of a solar eclipse is pretty simple.

A solar eclipse happens when the moon passes between Earth and the sun during the new moon phase.  If the moon is full, it will be Earth’s shadow that will block out the sun darkening the moon in a lunar eclipse.

But the simple explanation leaves two interesting questions unanswered, as “why isn’t there an eclipse of the sun each month when the new moon passes between Earth and the sun?” and “Why does the path of the eclipse move from west to east?”

We can update our model by adding some important facts.

First, moon’s orbit is not perfectly circular which means that at its perigee (or lowest point in the orbit) is about 225,000 miles from Earth and at its apogee (or highest point in the orbit) is 251,900 miles from Earth.  The moon’s mean distance from Earth is 239,000 miles.  The importance of the moon’s distance matters because its shadow (umbra) will extend out only 236,000 miles.  So if the moon is more than the distance from Earth, there will be no shadow on Earth’s surface.

Second, the moon’s orbit is tilted by 5 degrees relative to Earth’s orbital plane.  To visualize this imagine that you have two rings, a larger (representing Earth’s orbital plane as it revolves around the sun) and a smaller ring (representing the moon as it orbits Earth). If you attach the smaller ring onto the larger ring and tilt it slightly (about 5 degrees) you will see that there are only two points on the ring at which the two rings are in the same plane.
​As the moon orbits Earth (following the path of the smaller ring), it will ascend relative to Earth’s orbital plane for half of its orbit and decline for the other half.

If a new moon occurs when the moon is at the top of its orbit, as it moves between Earth and the sun the moon’s shadow will miss Earth high.  If it’s at the bottom of the orbit, the shadow will miss low.

There are two points (called lunar nodes) on the moon’s orbit when it crosses the plane of Earth’s orbit around the sun.  The lunar nodes align with the Earth’s orbital plane every 346.6 days (= an Eclipse Year).

Now, our model is more complete and we can derive from it that the following conditions must be met in order to have a total eclipse of the Sun:

  1. The moon is in the phase called new moon because it is then that it passes between Earth and Sun.
  2. The moon is at the lunar node and thus in the same orbital plane as Earth and Sun.
  3. Moon is fewer than 236,000 miles from Earth so its shadow will fall on Earth.

All three conditions will be met on August 21, 2017, with moon’s shadow falling on the U.S. beginning in Oregon and ending in Charleston, South Carolina.

We are accustomed to thinking of the sun and moon rising in the east and moving westward.  So why does the path of total eclipse move from west to east, from Oregon to South Carolina?

You can work this out for yourself by simply changing your point of view from Earth to imagine that you can travel far enough into space so that you can see the sun, earth, and moon in one view.

You can see the Earth rotating on its axis counterclockwise and the moon orbiting around it, moving above it and also moving counterclockwise.

As it moves across Earth, you can see its shadow moving counterclockwise, towards the east from the west.  You can test this by timing the moon’s rise on successive nights.  Each night it will rise about 50 minutes later than the previous night.

This also helps to explain why the shadow moves so very quickly, giving each observer only about two minutes of spectacle.

According to NASA, the moon orbits toward the east at about 3400 km/hour while Earth rotates to the east at 1670 km/hour near the equator.  The moon’s shadow is therefore moving at 3400 – 1670 km/hr = 1730 km/hr.

According to NASA, you would need to travel at Mach 1.5 to keep up with the moon’s shadow racing across Earth!

A total solar eclipse makes it possible to study another phenomenon that is one of astronomy’s great mysteries.

When the moon’s shadow completely blocks the Sun, the solar corona, a beautiful aura, will surround the moon revealing it for scientific study.  The solar corona is composed of plasma (the fourth state of matter).  Mysteriously, the corona is much hotter than the sun’s surface.  How can something farther from the source of heat be hotter than material at the source?

We will examine this mystery in a future blog.

There are many, many resources for understanding eclipses: The following are just a few.  In addition there are many apps that allow for interactive study of solar and celestial events.


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