Monthly Archives: August 2017

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.


The Man Who Invented Nature

Ever since Copernicus displaced Earth from its central position in the Universe, scientific investigations have progressively changed how humans have perceived the natural world.

Andrea Wulf’s book  The Invention of Nature: Alexander Von Humboldt’s New World describes the life and work of Alexander Von Humboldt and how that work altered how we understand nature and our relationship to it.

Humboldt in SAThe book’s protagonist is Alexander Von Humboldt who was born in 1769 and who, over a life spanning eighty-nine years, was the participant in revolutions from the political, the American and French, to the biological triggered by Darwin’s 1859 book The Origin of Species.

Humboldt’s great scientific accomplishment was his five year (1799-1804) exploration of the then unknown region including the Orinoco and Amazon rivers as well as the Andes Corderilla.

The scale of the expedition was tiny, just two scientists: Humboldt and the French botanist Aimé Bonpland.  However, though small in scale, Humboldt ensured that its scientific capacity was state of the art for 1799, seeing that it was equipped with forty-one of the most up-to-date scientific instruments including:  sextants, barometers, thermometers, magnetometers (to measure gravity), each device carefully packed in its velvet-lined protective case.

For five years the instruments accompanied Humboldt and Bonpland everywhere, whether in narrow native canoes, down rainforest trails, or up steep, narrow mountain trails as  Humboldt and Bonpland collected, measured and noted everything they saw.

When they returned to Paris in 1804 (after visiting Philadelphia and Washington where they were welcomed by Thomas Jefferson), their baggage contained some 60,000 plant specimens that represented 2,000 previously unknown species as well as dozens of notebooks in which were recorded the measurements and records gathered during their more than 6,000 mile, five-year-long exploration.

Humboldt translated his notebooks into popular lectures as well as thirty-four books published French, German, English, Polish and Russian.

The “new world” of the book’s subtitle is Humboldt’s legacy for us, that we  that we  see

more deeply into nature including our role in it.

A new view means that there was an older way to see nature.

And there was: In the eighteenth century, an educated European or American was confident of two ideas about nature.

First, nature was degenerate. The Comte de Buffon, the director of the leading botanical institution of the day, made certain that everyone knew how degenerate it was: The primeval forest “an horrendous place full of decaying trees, rotting leaves, parasitic plants, stagnant pools and venomous insects.”

Improving nature meant taming nature.  Wilderness would disappear to be related by orchards and plowed fields. ”Every acre tamed added to mankind’s profit.” (Wulf, p. 68)

Buffon’s perspective was from top down. The assumption that nature was “degenerate” leads the observer to find evidence that supports the assumption: rotting trees, venomous insects.

In contrast, Humboldt started at the bottom of things and worked upwards. What were the individual elements; the uncountable organisms large (the boa constrictor that could swallow a horse) to small (a hummingbird balanced on a delicate blossom) to the tiny (the line of ants moving across the forest floor), large, small, tiny caught in a web of relationships.

On the dry llano (plains) the Mauritia palms astonished Humboldt and Bonpland in the number of different organisms and phenomena that were linked to them. The tree’s fruits attracted monkeys and birds while its fronds mitigated the wind with the result that the wind-born dirt to fell to Earth to accumulate behind the palm trunks. The piled dirt sheltered by the palm trunks retained moisture and provided a place for insects and worms to shelter. (Wulf, p.85)

The network of relationships around the Mauritia palm was example of Humboldt’s “new world.” The new world was revealed by precise observation and careful measurement and then envisioned by an act of imagination that was constrained by fact, connotation, and probability. (Barzun, 1989)

Chimborazo Mon 1810When Humboldt ascended Chimborazo, an extinct volcano, in the Andres, he and his party climbed higher than anyone had before. As they climbed upward, Humboldt observed how the types of plants changed with the altitude and he was reminded how he had seen the same kinds of plants when he had climbed in the Alps.

Certain plants grew in certain temperatures and light, attracting certain kinds of insects, drawing certain kinds of birds and animals.

Humboldt’s impressive memory and active imagination put it together.

“As he stood that day on Chimborazo, Humboldt absorbed what lay in front of him while his mind reached back to all the plants, rock formations and measurements that he had seen and taken on the slopes of the Alps, the Pyrenees…Everything he had observed fell into place. Nature, Humboldt realized, was a web of life and a global force.” (Wulf, p. 101.)

It was Humboldt’s new world that inspired the young Darwin before and during his voyage around the world on H.M.S. Beagle. As a student at Cambridge, Darwin recalled that Humboldt’s Personal Narrative “stirred up in him a burning zeal” to follow in Humboldt’s footsteps.  He copied out passages and read them to his Cambridge botany teacher John Stevens Henslow. Of his fellow undergraduates, Darwin said that “I plague them with talking about tropical scenery.”   (Wulf, 259)Darwin young

On the Beagle, his two guides were Humboldt’s Personal Narrative and Charles Lyell’s Geology. Wulf tells us that on the island of Santiago in the Cape Verde Islands, as he “rushed across Santiago, he saw the plants and animals through Humboldt’s eyes and the rocks through Lyell’s.” Later, back on board The Beagle he wrote a letter to his father “announcing that inspired by what he had seen on the island ‘I shall be able to do some original work in Natural History.’” (Wulf, p. 264)


Barzun, Jacques (1991) “Of What Use the Classics Today?” From Begin Here: The Forgotten Conditions of Teaching and Learning. Chicago: The University of Chicago Press.

Portrait of Darwin retrieved from

Image of Chimborazo retrieved from Retrieved from

Wulf, Andrea (2015) The Invention of Nature: Alexander Von Humboldt’s New World. New York: Vintage Books.