Put This Date on Your Calendar: August 21, 2017

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 Spance 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. Those not in the path will see only a partial eclipse.

For those in the path “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.” (http://www.space.com/33797-total-solar-eclipse-2017-guide.html)

For those of us who are not astronomers or eclipse experts this rare event is a prod to our curiousity about how the universe, in this case the some of the workings of sun, moon, and Earth.

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. A lunar eclipse happens when the moon is behnd Earth during the full moon phase. 

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 sun?” and “Why does the path of the eclipse move from west to east?”

To fix the model, we need to add some important facts.

First, moon’s orbit is not perfectly round 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. Its mean distance from Earth is 239,000 miles.

Second, the moon’s orbit is titled 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 (representing the moon’s as it orbits Earth). If you put 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 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. It it’s at the bottom of the orbit, the shadow will miss low. 

There are two points on the orbit when it crosses what is called the ecliptic, or the plane of Earth’s orbit around the sun called the “lunar nodes.” The line of the nodes align with the Sun 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:

The moon is in the phase called new moon because it is then that it passes between Earth and Sun.

The moon is at the lunar node and thus in the same orbital plane as Earth and Sun.

Moon is relatively closer to Earth (>236,000 miles) because its shadow (umbra) will only extend 236,000 miles. If the moon is too far away, its shadow won’t fall on Earth’s surface.

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 totality 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 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 also moving counterclockwise.

As it moves across Earth, you can see its shadow moving counterclockwise, towards the east from the west.

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

According to NASA the moon orbits toward the east at about 3,400 km/hour while Earth rotates to the east at 1,670 km/hour near the equator. The moon’s shadow is therefore moving at 3,400 – 1,670 km/hr = 1730 km/hr.

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

While our model for understanding a solar eclipse is complete, a 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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