The naturalists of ancient times thought the universe was made of spheres.

Aristotle, Eudoxus and other Earth-bound explorers observed that the stars revolve together slowly once a year. The planets, or “wanderers,” moved among the fixed stars.

How the stars revolved and planets roamed among the constellations called the Zodiac, were serious matters to natural scientists 2,400 years ago. The night sky inspired awe and trepidation in them, as it still does to us, and implied that activity far beyond human experience was taking place. The activity seemed godlike.

Their best explanation connecting the motions of the stars and looping of the planets was a system of ethereal crystalline spheres, which the planets and stars inhabited. Each planet’s sphere turned in tandem with the others, and the planet-spheres turned inside the star-sphere. The interlocked motions cast the planets in slowly twisting paths, sometimes backwards, in the sky.

The idea that the heavens are made of divine spheres endured. About 400 years ago Johannes Kepler, one of the first great modern astronomers, set out to demonstrate the mathematical perfection of the spheres – an idea implied in the phrase “harmony of the spheres.” Kepler worked tenaciously at the task, using the newly suggested idea that the sun (not the Earth) was the center of the universe, and using detailed observations of fellow astronomers, some of whom had a newly invented device called the telescope. His calculations, however, were persistently out of whack. The math that should describe heaven’s perfect circles did not compute.

Eventually he realized the problem: The planets were not moving in circles, but in ellipses. This meant the spheres were imperfectly shaped. How could heaven be imperfect?

Partly as a result of Kepler’s mathematics, the idea of the crystalline spheres dissolved. It turned out the planets themselves were spherical, not crystalline but rock, ice and gas, looping around the sun on gravity and inertia. An armada of spacecraft has proved this theory of celestial reality in the last 50 years.

But the ancient world leaves odd remains. Even though no scientist now thinks crystalline spheres turn inside each other, different zones up there are still called spheres: Around the Earth is the atmosphere, made up of the troposphere, stratosphere and ionosphere, defined by their components and activities. Beyond that is the magnetosphere.

Around the sun is the heliosphere, a bubble where the solar wind – a stream of particles emanating from the sun – pushes toward the measureless oceans of space. The boundary between the sun’s sphere of influence and interstellar space is called the heliopause.

For some time the heliosphere was believed to be roughly spherical. But the two Voyager spacecraft, which departed our atmosphere in 1977 and are now traveling outward like gossamer filaments about 300 million miles a year, have recast the bubble.

Voyager II, now about 7 billion miles away, recently detected atomic particles pushing in toward the sun, which means it’s near the outskirts of the heliosphere. Voyager I, the most distant human-made object, reported inward-pushing particles when it was about 8 billion miles distant. The heliosphere is imperfectly shaped.

But a halo it is, nonetheless, and in a few years the Voyagers will shoot across the heliopause seeking further spheres, to connect them, too.