There will be a total solar eclipse across much of Norther New England today as the Moon’s shadow sweeps across North America. In the center of this shadow band, the sun will be partially obscured for about 2½ hours, and totally blocked for 3-4 minutes. After posting this essay, we will be on our way out to see it!
There are at least two things about this phenomenon that amaze me.
The Coincidence of Size, Distance and Orbit
First is the coincident location and movement of our Sun, Moon, and planet. There are other solar systems with dual stars, and other planets with many moons. We have only one of each, and they are just the right size and have just the right orbits to make a total solar eclipse a rare and somewhat mystical sight. When these three line up perfectly, it becomes a special occasion that people will travel miles, sometimes lots of miles, to see.
The relative distance and relative size of the Sun and Moon allow rare and brief instances when they appear to us on Earth to be of the same size with one completely blocking the other – amazing! And as I understand it, the fact that the Moon’s orbit is tilted 5 degrees relative to Earth’s orbit is what makes this such a rare occasion. If they were in exactly the same plane, we would have a solar eclipse every month. What would be the fun in that!
And here is another result of this amazing coincidence. We routinely see images like this one of the Sun’s corona – the outermost region of its atmosphere - that is so clearly visible only when the orbiting pieces line up perfectly for those few minutes of total darkness. Even minor differences in orbit or size would rule out such a perfect alignment. This is like holding a cosmic piece of cardboard up to block the Sun, but that cardboard cutout is about 239,000 miles away, and the sun is 93,000,000 miles distant - the ratios of sizes and distances is what give us this incredible view of the violent chemistry of the Sun.
You can find many long lists of important historical events triggered or altered by solar eclipses. Heads have rolled, empires have fallen, wars have started or ended because of eclipses, especially when one side knew it was coming and the other didn’t.
We can add this rare coincidence to a longer list of characteristics of the Sun-Earth relationship that have allowed life to become what it is on this planet. Any number of things could have easily gone wrong!
The Precision of Prediction
The second thing that amazes me about solar eclipses is just how precisely we can predict where and when they are going to occur. You might guess from the recent essays using basic data sets rather than complex models to predict our climate future that I would be intrigued that both theoretical models and accurate, long-term observations have been used to predict eclipses.
The modern, first-principles-physics approach uses computations of the gravitational interactions of Earth, Sun and Moon based on extremely precise determinations of mass, shape, distance, and speed of movement. Those computations allow predictions that are precise to within a minute over a span of hundreds of years – going either forward or backward in time.
The observational or data-rich approach is based on what are called Saros Cycles. Perhaps as much as 2,800 years ago, Babylonian astronomers, who kept careful records of such events, noted with amazing precision that eclipses occur in patterns that now bear this name.
According to NASA:
The Saros Series is a period of 223 lunar months that has been used to predict eclipses for thousands of years. In a Saros Series, exactly 9 years, 5.5 days after any lunar eclipse, a solar eclipse will occur, and vice versa. Approximately 6585.3211 days, or 18 years, 11 days, and 8 hours after one eclipse, the Sun, Earth, and Moon return to about the same relative geometry, and a nearly identical eclipse occurs. These similar eclipses are part of the same Saros Cycle, and the time between the two eclipses is called a saros.
Accurate observations often precede general theories, but here is an exception. One particular solar eclipse played a role in testing a then new fundamental law of physics – Einstein’s theory of relativity.
That theory, and some preceding ideas, predicted that light would be deflected as it passed around a massive object. During a solar eclipse in 1919, simultaneous observations of a distant star were made in England, South America and Africa confirming Einstein’s theory.
According to Wikipedia:
The result was considered spectacular news and made the front page of most major newspapers. It made Einstein and his theory of general relativity world-famous. When asked by his assistant what his reaction would have been if general relativity had not been confirmed by Eddington and Dyson in 1919, Einstein famously made the quip: "Then I would feel sorry for the dear Lord. The theory is correct anyway."
When we Trust the Science and When Some Choose Not To
There is one conundrum that also comes to mind in thinking about how well we can predict eclipses. No one disputes the science behind the predictions, and the evidence is plainly in favor of all the theoretical calculations each time an eclipse occurs as predicted. The same can be said for other physical processes like phases of the moon or the timing and height of ocean tides.
The physical laws governing the flight of an airplane or the functioning of an automobile are commonly taken for granted, although I think both are little short of miraculous. Electricity, and all we can do with it, is another modern miracle in my eyes, and also underappreciated because it is so common. Among many other amazing things, it allows us to turn cold air into warm, and vice versa.
Yet some equally well-established physical principles, like the fact that liquids expand as they get warmer, or that certain gases absorb long-wave radiation, seem to remain beyond the pale in the public dialog about climate change.
Many of the physical relationships and processes that control weather and climate have been known for a century or more, and the observed outcomes support the basic science. Increases in carbon dioxide in the atmosphere are directly correlated with increases in global average temperature, as they should be. Sea level rise is strongly related to the amount of heat stored in the ocean, as it should be, although this is only part of the story. About half of the global rise in sea level is related to melting of land-based ice, especially in Greenland and Antarctica. And of course we have known for some time that ice melts if it gets too warm.
Maybe soon the scope and focus of media reports on recurring climate disasters will begin to note that an increase in the frequency of such events is the direct result of the simple physical properties of gases and water. We should not be surprised by that increase, and maybe we should be doing more to counteract it.
But enough of that for now – it’s time to go out and watch the very predictable yet awe-inspiring coincidence of a total solar eclipse!
Sources
General sources on solar eclipses include:
https://eclipse2017.nasa.gov/eclipse-history
https://www.astronomy.com/science/the-10-most-important-eclipses-in-history-picked-by-an-expert/
https://www.cnet.com/science/solar-eclipse-2020-a-history-of-eclipses-and-bizarre-responses-to-them/
https://en.wikipedia.org/wiki/Lists_of_solar_eclipses
https://en.wikipedia.org/wiki/Solar_eclipse
The image of the path of the 2024 eclipse is from:
https://science.nasa.gov/eclipses/future-eclipses/eclipse-2024/where-when
The diagram of how an eclipse happens is from:
https://eclipse2017.nasa.gov/how-eclipses-work
The corona image is from:
https://spaceplace.nasa.gov/sun-corona/en/
The NASA quote is from:
https://science.nasa.gov/eclipses/faq/
And another source on Saros cycles is:
https://en.wikipedia.org/wiki/Saros_(astronomy)
Sources on the test of general relativity include:
https://eclipse2017.nasa.gov/testing-general-relativity
The Wikipedia quote is from:
https://en.wikipedia.org/wiki/Tests_of_general_relativity#Deflection_of_light_by_the_Sun