Copernicus in His Prime
The Solar System Viewed by Moonlight
We draw life from the glorious, incandescent sun. It rises daily in the east, until by nightfall it hides behind some horizon, whether land, sea or cloud, diving unseen to reappear at dawn in a different part of the starry background. The glittering stars move as a whole; each year the sun returns to a backdrop of bright points very near the one it left. The moon shows us a disk as wide as the sun's; its changing details are bright but cold, never a hot blaze. In about one month any viewer stationed on Earth can see the moon pass across the entire Zodiac. The outline of the bright, inconstant moon attends strictly to the position of the sun. When the sun lies behind any moon viewer, the moon is full face. When the moon lies right before us and the sun close to the same direction, we have a new moon. That new moon is unseen, for the moon, a cold and lightless rock, glows only under the sun's rays, and is lost to us whenever it is masked within the sun's power of brightening a skyful of blue air.
The moon waxes from new to sickle, crescent, half-full and disk, and wanes back again as the relative positions of sun and moon change. As the moon slowly spins in unison with its round of Earth, we can never see its averted face. Recall that our own planet shows up too against the night sky; each essential bounding horizon we see is in fact made of Earth's lands, seas or mountaintops, unless fogs of cloud block most views of the surrounding world.
How can we judge the full spatial relationships of these easily seen moving lights? An infant may raise its tiny hand before its eyes to block the entire moon. It cannot be size alone that matters. The easiest judgment of distance is that the farther object can be solidly hidden by a closer one. The moon sets to or rises from its hiding place at the edge of Earth, at any horizon under the sky, never beyond sun or star. The moving edge of its disk, whether lit or not, can be followed as from time to time it encroaches to hide sun, stars and planets. All night the moon eludes you, steady traveler, never coming close, never on the near side of any tree or hill. It can hide any part of star-space it enters; rarely, it even eclipses the sun, the new moon eating away its dazzle to a brief blackness. Never does moon conceal behind it even the worst of Earth-bound weather, or any auroral glow.
These days, having famously traveled beyond our Earth to reach it, we can easily accept the moon as a rocky, dry, airless, globelike satellite of our planet. This closest of sky luminaries, a sphere as well as the Earth, casts its own shadows on its surface, tracing the shape that is then turned away from the sun. Earth, too, is a bright sphere by day as it encircles the sun with its moon daughter. Day and night span 24 hours. Night is the name we give to the sun-cast shadow of our globe Earth upon itself, the time when the sun leaves our local view to hide behind the horizon, and day is the time when the sun rises again into view from behind and over another horizon spun into place.
One test of distance is the time between round trips to a sky position marked by the stars. Stars move only together as an entire sky pattern. We call them fixed, even over millennia; they do move relative to one another, but only to the magnified view of telescopes. The moon traverses the star-sky about once a month as seen from Earth. The planets wander in a complex ballet, attentive to the sun's position. The innermost sun-circler is speedy, elusive Mercury, making the loop in less than three months; a 30-year period brings stately Saturn back against the star scene, making it the outermost planet known before lenses. Saturn's distance from the sun is about tenfold that of the Earth's orbit. In due course, the moon's quickness signals that it is close to Earth; the planets parade more slowly among the stars the farther away they are from the sun.
The dance patterns of those figures in the sky reveal a great deal: The stars move each day as one, the carousel experience revealing our own Earthly spin. The planets move in their own dance about the sun. The sun rules night and day—disclosing Earth spin again—while moon is but an Earth partner.
It does not take much effort to notice that the time for a planet's circuit depends on the size of its orbit. (It seemed less likely to Copernicus that the paths go through empty space than that a crystal sphere bore the planets around.) Copernicus's key was that link: The longer the period, the more distant the sun. The decision to link Earth and moon as companions was essential; monthly moonshine correctly orders the planetary distances only if the fellow travelers Earth and moon are held as if one planet sharing their waltz around the glowing solar center.
Clearly, the closest orbit of all is moon's about Earth. The deduction is natural; the moon is a visibly bright, relatively very large disk, disclosing its surface details and shadow phases. Both moon and Earth ring the sun, our distance from the sun being 400 times greater than the relative arm's length that separates the two partners.
The Earth-moon pair models in itself the entire planetary cortege. The orbits are not circles but slightly irregular ellipses; their orbital sizes are not proportional to their periods but rise more rapidly with distance from the center. (It was left to Jonhannes Kepler to find the true relation and to Newton to explain it, two Copernicans who spanned the 17th century.) Our moonlight account leads by rich analogy to a modest understanding of the system. It was no secret that certain classical scholars a millennium earlier and lslamic scholars centuries before had considered the same possibilities.
What of the unchanging stars that rotate each day, to return annually with the Earth's year? The "fixed star" background circles daily as a whole with little internal change; the stars are simply very far away, so far off that even their journeys appear tiny to us. Moon and Earth round the sun linked as a tight couple. Meanwhile, we here also spin daily with Earth's solid surface, carrying our viewpoint around the sky as we too rotate about the polar axis of planet Earth so that star motions bear the mark of our own simple movement, not of their vast and individual intricacies.
Copernicus postulated around 1510 or 1512 (others had touched the issue in ancient times) that the moon is not a sun-encircling planet like the others, but a partner with our Earth as together they dance around each other and around the far-off sun. One can go on to name a succession of orbit sizes of the planets as they circle the sun: moon is closest to Earth, and looking out from the sun we reach in turn Mercury, Venus, Earth with its moon close at hand, Mars and, beyond, the big planets with their adagio dances. Extrapolating the moonshine arguments gives correctly the order of the planets.
I concede that measurements—or Newton's gravitational physics—must be called on for more detail; the two concur, and Galileo's magnifying lenses, simply by contracting the visual distance, showed that the tiny disks of planets do mimic the moon, showing the phases of a sun-lit disk. Indeed, the lens did more; it showed that other planets too could be alternative centers of encirclement for many moons, their own companions. Earth-moon is a planet like the others. The lengthy volume of tables needed to convince the professionals was a later demand, whose publication was delayed by about three decades for fear of opposition within the Church.
But lengthy tables are not altogether necessary. Moonlight itself discloses the broad appearance of the solar system to the unaided eye—and to reason, based on everyday, observant experience.
© Philip Morrison