Brace Yourself for the Great American Eclipse

On April 8, millions of North Americans will experience an awesome astronomical phenomenon that has so inspired and humbled humans throughout the ages. It is a total solar eclipse, a cosmic confluence caused by a delicate "dance" between the Moon, the Sun, and the Earth. Lasting for nearly 4.5 minutes in some areas, the eclipse will move northeast at more than 1,500 miles per hour along a narrow path (110 to 120 miles wide) from Mazatlán in Mexico, cutting diagonally across the US and into easternmost Canada. The next total solar eclipse will sweep across the US on August 3, 2044, but it won't have the broad reach as the upcoming eclipse.

As totality approaches, the sky will become dark, mid-day will turn into dusk, and stars will twinkle in the sky. Wispy strings of light from the Sun's extremely hot outer atmosphere, called corona, will suddenly appear. Just before totality and right before it ends, the bright spot of sunlight called the "diamond" will become visible. A few seconds after the diamond ring forms, the dazzling jewel of sunlight will break up into points of light known as Bailey's beads.

But what causes solar eclipses? And how can we predict their occurrence with great accuracy thousands of years into the future?

The Moon's orbital plane is tilted by about five degrees to the Earth's ecliptic (orbital plane). The points where it crosses the ecliptic are known as nodes. Because of the tilt, the Moon spends half the lunar month (which is 29.53 days) above and the other half below the ecliptic. Solar eclipses can occur only when the Moon is between the Sun and the Earth at or near a node.

Although the Moon crosses the nodes twice each lunar month, episodes of total solar eclipses, with the Sun completely obscured by the Moon, are rare. They occur only when the phase of the Moon at a node on the line of nodes between the Earth and the Sun is New. Furthermore, the Moon must be relatively closer to Earth. If these conditions are not met, the long, narrow shadows of the Earth and Moon will miss their marks and no eclipses will occur.

When the New Moon is farther from Earth but still at a node, it will appear too small to completely shield the Sun, resulting in an annular solar eclipse, in which the outer edge of the Sun remains visible as a ring of sunlight. An eclipse will be partial if the Moon, the Sun, and the Earth do not align in a perfectly straight line. In that case, only a part of the Sun will appear dark, giving it a crescent shape.

So how can the Moon, which is much smaller than the Sun, completely block it out from our view?

It is a happy coincidence that the Moon and the Sun appear the same size in the sky because their angular diameters, or the angle subtended by them at the point of observation on Earth, have the same value of half a degree. Therefore, when their orbital planes intersect and the distances align favourably, the New Moon can completely block the Sun.

People also often wonder how astronomers can predict the occurrence of solar eclipses. Even before the advent of sophisticated computers or even a working theory of the solar system, ancient civilisations predicted solar eclipses with great accuracy. The earliest written record of predicting solar eclipses dates back two and a half millennia ago, when a Babylonian astronomer found that near-identical eclipses recur in a cycle of 6,585.32 days. This interval of time is called the Saros, which is Greek for "repetition."

A solar eclipse repeats with surprising accuracy after one Saros cycle. However, because of the extra one-third of a day in the cycle, it will not be visible from the same place as the previous one and might not be of the same type. In fact, the eclipse could be total, partial, or annular. Also, the extra fraction of a day means that the Earth will rotate an additional one-third of a turn farther east, shifting the region of visibility west.

Eclipses separated by the Saros interval form the eclipse series. Each Saros series starts off with an eclipse near the north or south pole, after which every successive eclipse shifts south or north, respectively ending at the other pole. Once the occurrence of the first eclipse is found, predicting future eclipses is a straightforward task.

A Saros series typically lasts 12 or 13 centuries and contains 71 to 73 eclipses. The total eclipse of April 8, 2024 will be the 30th member of Saros 139, which began on May 17, 1501 and will conclude on July 3, 2763, with subsequent eclipses separated by a Saros.

Sure enough, the immediate forerunner of the April 8 eclipse occurred one Saros earlier, on March 29, 2006. Looking ahead, the next eclipse belonging to Saros 139 will occur one Saros later, on April 20, 2042.

To date, there are 181 Saros series numbered from zero to 180. Thus, total solar eclipses occur, on average, every 18 months somewhere around the world. Though they belong to different Saros series, a given location will experience totality once in about more than 100 years. For example, the last total solar eclipse in Bangladesh was on July 22, 2009. The next one will be on June 3, 2114.

Thanks to the Babylonians, the Saros series is a clever and powerful tool for predicting solar eclipses thousands of years into the future. It is fair to say that Saros' predictions are so precise that, if there is a deviation, it is very small and we will most likely not notice it. But ultimately, modern astronomers do not rely on the Saros to predict eclipses. They use sophisticated mathematical models to predict solar eclipses. Nevertheless, the Saros remains a simple and useful tool to understand the recurrence of solar eclipses.

Dr Quamrul Haider is professor emeritus at Fordham University in New York, US.