As the moon wheels around Earth every 28 days and shows us a progressively greater and then stingier slice of its sun-lightened face, the distance between moon and Earth changes, too. At the nearest point along its egg-shaped orbit, its perigee, the moon may be 26,000 miles closer to us than it is at its far point.
And should the moon happen to hit its ever-shifting orbital perigee at the same time that it lies athwart from the sun, we are treated to a so-called supermoon, a full moon that can seem embraceably close — as much as 12 percent bigger and 30 percent brighter than the average full moon.
If the weather is good where you are, please, go out Monday or Tuesday night and gawk for yourself: A supermoon will be dominating the sky. It's the last of this summer's impressive run of three supermoons, and the final one of the year.
Some astronomers dislike the whole supermoon hoopla. They point out that the term originated with astrology, not astronomy; that perigee full moons are not all that rare, coming an average of every 13 1/2 months; and that their apparently swollen dimensions are often as much a matter of optical illusion and wishful blinking as of relative lunar nearness. The superstar astronomer Neil deGrasse Tyson grumbled archly on Twitter that the "perennially hyped" term debases the legacy of Superman, supernovas and the video game character Super Mario.
Nevertheless, astronomers concur that whatever the reason, yes, you should look at the moon early and often, whether it's waxing or waning, gibbous or crescent, and appreciate the many features that set our moon apart from the other 100-plus moons of the solar system, and even celebrate our loyal satellite as a planet in its own right.
"I know it goes contrary to the nomenclature currently used," said David A. Paige, a professor of planetary science at the University of California, Los Angeles, referring to the definition of a planet as (among other things) the dominant gravitational object in its orbit. "But where I come from, anything that's big enough to be round is a planet." Unlike most moons of the solar system, ours has the heft, the gravitational gravitas, to pull itself into a sphere.
Scientists say that while the public may think of the moon as a problem solved and a bit retro — the place astronauts visited a half-dozen times way back before Watergate and then abandoned with a giant "meh" from mankind — in fact, lunar studies is a vibrant enterprise that is yielding a wealth of insights and surprises.
One research group reported new evidence that the moon was born violently, in an act of planetary suicide that left faint but readable fingerprints at the scene. Another team proposed that the moon's cataclysmic origins could explain the mysterious lunar features we know as the man in the moon.
Partly on the basis of data from NASA's Lunar Reconnaissance Orbiter, a multi-instrument spacecraft that has been orbiting, mapping and analyzing the moon since 2009, researchers have determined that the moon is a place of thermal lunacy, of searing heat crossed with sub-Plutonic cold, and of pockets that may be the most frigid spots in the entire solar system. Recent measurements taken inside impact craters at the lunar poles, where no solar light is thought to have penetrated for a billion years or more, showed temperatures of about 30 degrees Celsius above absolute zero, the point at which even atoms cease to move, Paige said.
Andrew Jordan of the University of New Hampshire and his colleagues have calculated that these temperature extremes could give rise to a novel form of sparkiness, tiny bolts of lightning that dance silently through the moon's airless landscape and fluff up the soil as they flash.
Reporting in The Journal of Geophysical Research: Planets, the researchers proposed that charged particles from the sun could be getting trapped at slightly different depths of the frigid lunar surface, forming electric fields. Those fields would gradually build up strength until, zap, serious sparks start to fly, which in turn would vaporize particles of soil.
Sparking events, the researchers said, could explain the bafflingly foamy appearance of soil recently detected by NASA's orbiter. The lunar surface "may be far more active than we thought," Jordan said. "It's amazing to have this kind of natural laboratory almost in our spatial backyard."
At an average distance of 238,855 miles, the moon is indeed on Earth's patio: string together just 11 round trips from New York to Tasmania, and you're there. The moon is not the largest satellite in the solar system — three moons of Jupiter and one of Saturn are bigger — but with a diameter almost 30 percent of Earth's, it is by far the largest relative to its planet. Jupiter's Ganymede, for example, which tops the lunar size chart and even outgirths Mercury, measures just 4 percent the diameter of its gas-giant sponsor.
Another outstanding feature of the moon is its origin. Most of the other moons in the solar system are thought to be celestial passers-by that were pulled into a planet's orbit, or to have formed contemporaneously with their planet from an initial starter disc of dust, gas and rock. The moon, by contrast, is thought to have a bloodier past.
According to the reigning hypothesis, about 4.5 billion years ago, shortly after Earth had accreted down into a sphere from its little slub of circumsolar material, another newborn planet, still shaky on its feet, slammed obliquely into Earth with terrifying force.
That "giant impactor," named Theia, who in Greek mythology was mother to the goddess of the moon, is thought to have been roughly the size of Mars and to have been pulverized in the encounter, along with a good chunk of proto-Earth. From that fiery cloud of all-Theia and part-Earth, the scenario goes, our moon soon condensed.
The impactor hypothesis made sense and comported with computer models, but hard evidence for it proved elusive. If the moon was partly the offspring of a non-Earth body — Theia — there should be chemical fingerprints attesting to the foreign parentage.
Astronomers who have analyzed a wide array of extraterrestrial material have determined that the many residents of the solar system differ measurably in their isotope ratios, the forms of the chemical elements they carry. (Heavy oxygen or light? Titanium with more neutrons or fewer?) But when researchers checked the isotope content in rocks from the moon, the ratios looked identical to rocks on Earth. Where were the traces of Theia?
Now it looks as if the evidence has arrived. This summer, Daniel Herwartz, a geochemist working at the University of Göttingen in Germany and his colleagues reported in the journal Science that they had detected isotopic ratios of oxygen in lunar rocks that were unlike the forms of oxygen found on Earth. It is, Herwartz said, "the difference between Earth and moon predicted by the impact theory."
The researchers stumbled to victory accidentally, he said. As geochemists, they had developed new techniques for more precisely measuring oxygen isotopes to address Earth-based geological problems. "When that succeeded," Herwartz said, "we thought we'd have a look at the moon question again."
Initial efforts foundered. "NASA doesn't hand out Apollo samples to everybody," he said, referring to rocks brought back decades ago by astronauts. So the scientists first tried to work with meteorite fragments, which proved too disturbed to be useful.
The researchers then persuaded the space agency to hand over a baby aspirin's worth of pure Apollo rock, and, sure enough, there was Theia's isotopic thumbprint.
Other signs of the fiery collision may linger in the moon's familiar patchwork of dark and light splotches that has long been likened, dubiously, to a man's face. It's the only side of the moon we ever see from home base, a result of Earth's having yanked its satellite into a so-called tidal lock: The time it takes the moon to rotate once on its axis is the same as the four weeks it takes to orbit Earth, which means the same side is always turned toward us.
"It's the minimum energy configuration, the most stable configuration the two can take," said Arpita Roy, a doctoral student in astronomy and astrophysics at Penn State. Roy is an author of a new report in The Astrophysical Journal Letters about how tidal locking and the great impact together shaped the lunar look.
Ever since the dawn of the space age, when astronomers glimpsed the first photographs of the far side of the moon, they've wondered why it differed visibly from the near side, particularly in its absence of the dark flat plains called maria, from the Latin for seas. In the new paper, the researchers applied insights from the study of exoplanets that circle close to their stars and, like the moon, are tidally locked, with one half facing ever sunward.
In the immediate aftermath of the giant impact, Roy said, the Earth would have been as hot as a small sun, which means the half of the moon that faced us would have remained hot as well, while the opposite side, which faced out into space, had a chance to cool down. Some metals and silicates from the dust cloud surrounding the young orb would preferentially settle onto the cool side, thickening that portion of the crust.
As a result, future meteor impacts on the far side would fail to puncture below the crust, while those hitting the thin-crusted near side would expose the moon's soft inner layers. "The craters would fill with the gooey stuff underneath," Roy said.
That goo then hardened into maria, the seas we see when we have the good sense to look up and lock eyes with the moon.