Astronomy
On Monday, millions of people in North America will marvel at the rare spectacle of a total solar eclipse. The fact that something like this occurs at all on Earth is thanks to another rarity: our exceptionally large moon.
During a total solar eclipse we do not actually see the sun, but the moon – we are in its shadow. During the so-called totality phase, which lasts only a few minutes, the moon’s disk completely covers the sun’s disk in the sky. Although some sunlight still passes the moon, more specifically from the sun’s halo (the corona), which is only visible during a total eclipse.
A total eclipse is rare: one occurs about every 18 months somewhere in the world. In our country this last happened on August 11, 1999, although the solar eclipse was only total in the southern part of the province of Luxembourg. We will have to wait until September 23, 2090 for a reissue of that special event. Perhaps that is why many Belgians traveled to the United States, where a total eclipse can be admired on Monday, from Texas to Maine.
Total eclipses are special not only because they are so rare. They become even more special when we realize what it takes to make the moon’s disk fit almost exactly on the sun’s disk, allowing the corona to emerge in all its glory. Even though the sun as a celestial body is much larger than the moon (four hundred times), they still appear the same size in our sky. How does that happen? The sun is also four hundred times further away from the earth than the moon. As if our planet and moon were created to generate total solar eclipses.
The Earth is also unique in our solar system in that respect. Mars, for example, has not one, but two moons. However, they are both far too small to completely cover the solar disk in the Martian sky. Several Martian robots have already taken photos and videos of a partial solar eclipse on Mars. But for a total eclipse you have to be on Earth. We mainly owe this to our moon, which is exceptionally large compared to other moons in the solar system.
But Earth won’t remain a total eclipse observatory forever. Every year the moon’s orbit shifts a few centimeters away from Earth. This means that during an eclipse, when the moon is between the sun and the Earth, it moves closer and closer to the first and further and further from the second. As a result, the moon in our sky appears to become smaller, and over time it will no longer be able to cover the entire disk of the sun. In the distant future, several million years from now, solar eclipses will at best only be annular, leaving an outer ring of the solar disk uncovered. That ring will radiate too much to be able to see the corona.
Even in the distant past, the corona was not visible during a total eclipse. Millions of years ago, the moon was much closer, apparently larger than the sun’s disk and more than covering it. The dinosaurs may have experienced solar eclipses, but the way we see them today, even if only occasionally, is unique.
