When the sky darkens for a partial or total solar eclipse, the air is still and filled with mystery. Nocturnal creatures are confused by the sudden respite from the harsh light of daytime. Twilight falls and the stars shine, breaking the normal daily rhythm. And people gaze with wonder at the celestial show — their eyes protected, if they dare look at the sun, with special glasses that block its harmful radiation.

On Monday, much of the continental United States will witness its first total solar eclipse in almost 40 years. The Philadelphia region will be hundreds of miles off the direct swath of totality, and therefore observe only a partial eclipse with about 80 percent of the sun covered by the moon. Nevertheless, it will surely prove a mesmerizing spectacle, especially for young people who have never experienced a solar eclipse before.

COMING MONDAY: Derrick Pitts, the Franklin Institute's chief astronomer, will take your solar eclipse questions live on Philly.com's Facebook page at 3 p.m., just after the eclipse's peak in Philadelphia.

The fact that total solar eclipses happen with regularity has to do with an auspicious feature of the Earth-moon-sun system. While the sun is extraordinarily larger than the moon, from Earth's vantage point their discs are roughly the same size. Therefore, if the three objects are lined up just right, the moon seems to block all or part of the sun, leaving visible only its outer envelope, the solar corona. If the moon were much smaller or farther away, the effect wouldn't happen. So we are lucky that it takes place.

The temporary reduction in direct light leaves a notable shadow on the part of Earth where that alignment takes place. Of course, as the Earth rotates, different regions are exposed to that shadow. Temporarily, in shadowed regions, temperatures drop and skies darken. Close to the shadow, where there is only partial blocking, the effect is somewhat less dramatic.

Solar eclipses are not just thrilling, they are also of utmost scientific importance. They offer rare opportunities to study the solar corona, the effect of reduced light on Earth's vegetal and animal life, and the properties of ionization sources in the ionosphere. (Ionization is the tearing apart of atoms into electrons and positively charged ions due to intense radiation, such as that produced by cosmic rays, solar flares, and many other astral mechanisms.)

Gravitational physicists studying the work of Albert Einstein have valued solar eclipses for more than a century. In 1915, Einstein completed his magnum opus, the general theory of relativity, which models gravitation through the warping of spacetime. According to his theory, the sun "weighs down" our region of spacetime, rendering it curved instead of flat. Consequently, everything in the sun's vicinity must naturally travel along curved paths, rather than straight lines. Earth, Mars, Venus, and the other planets are thereby forced to engage in elliptical orbits around the sun, like cyclists circling around a tilted velodrome.

Even light is affected by the sun's warping of spacetime. Einstein predicted that the sun's gravitation would bend starlight by a notable angle. Therefore, a star located in the celestial dome near the sun would appear to have a slightly different position. Because obviously we can't normally see the stars and the sun at the same time, a total solar eclipse, Einstein realized, offers the perfect opportunity to measure the relativistic light-bending effect.

The first total solar eclipse after World War I (when it was hard to organize an expedition — one team tried and was captured by the Russian army!) took place in 1919. It was centered in the Southern Hemisphere over parts of South America and Africa. Two teams of British astronomers, led by Arthur Eddington and others, set out to the coasts of Brazil and West Africa, respectively, to take measurements. When Eddington correlated the data, British scientists announced success. They found a rough agreement with Einstein's predictions — which would be borne out again and again in subsequent eclipse measurements (most famously in 1922.

So on Monday, take suitable precautions and enjoy the sky show. But also think about the immense scientific value of solar eclipses, and the lucky coincidence that they happen here on Earth.

Paul Halpern, a physics professor at University of the Sciences, is the author a more than a dozen books, including The Quantum Labyrinth: How Richard Feynman and John Wheeler Revolutionized Time and Realityp.halper@usciences.edu