Newton’s first law holds that an object in motion tends to stay in motion and an object at rest tends to stay at rest. What is true of physics, in this case, is also true of history, and a parallel law of historical inertia might be formulated. Empires that exist tend to keep existing, long past the point when logic dictates that they should expire.But once the end finally comes, their non-existence also tends to become absolute: A fallen empire tends to be forgotten so completely that, in the minds of most people, it might as well never have existed at all.

The Byzantine Empire is a perfect illustration of both parts of this grim historical law. Its very name is a historian’s invention, applied retrospectively to the state governed from Constantinople, the great city founded in 330 A.D. on the site of the Greek town called Byzantium. The people we call Byzantines would not have recognized the name: They considered themselves Romans, the legitimate heirs to the empire founded by the Caesars.

Yet Constantinople spent most of its history presiding over the slow decline of that empire. By the time Constantine moved the imperial capital from Rome to the shores of Bosphorus, symbolically straddling Europe and Asia, the empire was already past its zenith. When its Western European provinces fell to the barbarians, in the fifth century, the eastern capital preserved the name, laws, and prestige of Rome. But it gradually lost touch with Roman institutions and the Latin language, which was replaced by the local tongue, Greek. A sixth-century attempt to reconquer the West, by the emperor Justinian and his general Belisarius, succeeded only briefly, and at tremendous cost. Italy was soon lost to the Lombards; then, with the rise of Mohammed in the seventh century, the empire’s Near Eastern domains were lost to the Arabs.

In 1204, Byzantium received its deathblow — this time, ironically, from its nominal allies in the Christian West, when the knights of the Fourth Crusade decided it would be more profitable to sack Constantinople than to fight for Jerusalem. For the next 250 years, the empire straggled along as little more than a name, until the Ottoman Turks finally came to deal the coup de grace in 1453.The Turks rebaptized Constantinople as Istanbul, and the New Rome began its slow fade from popular memory.

Today, the very name of Byzantium is known to many readers only thanks to W.B. Yeats, who used it as a blank canvas on which to project his poetic fantasies.The title of “Sailing From Byzantium” (Delacorte, 336 pages, $22), Colin Wells’s smart and accessible new history, pays homage to Yeats’s famous poem, “Sailing to Byzantium,” which makes “the holy city” a metaphor for the eternal realm of art:

Once out of nature I shall never take
My bodily form from any natural thing,
But such a form as Grecian goldsmiths make
Of hammered gold and gold enamelling
To keep a drowsy emperor awake;
Or set upon a golden bough to sing
To lords and ladies of Byzantium
Of what is past, or passing, or to come.

Yet as Mr. Wells shows, there is more to Byzantium than a metaphor. Its history, he insists, should not be considered as simply an excruciatingly long decline, a pathetic sequel to the glory that was Rome. For the historical inertia that preserved Constantinople for 1,000 years also kept alive, in a kind of suspended animation, the culture of the ancient world. Like one of those spores that lay buried in the Pharaohs’ sarcophagi for 4,000 years, only to flower again when the tombs were opened, so the literature, philosophy, and science of Greece and Rome burrowed into the splendid corpse of Byzantium. When younger, ruder, and more vital hosts came along, they absorbed that lore and set it on a series of improbable new careers.

As his title suggests, Mr. Wells is concerned less with Byzantium than its heirs — the cultures that sailed away from it, not in headlong flight, but as a merchant ship leaves a port bearing precious cargo. Rather than tell his story chronologically, Mr. Wells divides his book into three sections, one for each of Byzantium’s pupils: the Western Europeans, the Arabs, and the Slavs. As different, and at times mutually hostile, as these cultures have been, all three are heirs of Byzantium.Indeed, each of them can look back on a golden age sparked by contact with Constantinople.

To tell this whole story would be beyond the powers of any one historian. And Mr. Wells, an amateur whose interest in the subject dates from his college years, forthrightly acknowledges that “Sailing From Byzantium” is “a work of popular synthesis with no pretensions to original scholarship.” Rather than delving deeply into cultural and religious history, Mr. Wells offers capsule summaries of important intellectual developments, while keeping vivid personalities — kings, monks, philosophers, travelers — to the fore. The reader comes away, accordingly, with a broad outline of a complex subject, and a whole bushel of interesting anecdotes.

The first part of Mr. Wells’s story, dealing with Byzantium and the West, is the most familiar. It is no coincidence, he reminds us, that the first Italian humanists embarked on their revival on classical learning just at the moment, in the early 15th century, when Byzantium was entering its death throes. The decline of the empire meant that a steady stream of educated, Greek-speaking refugees descended on Italy, many of them diplomats seeking a last-ditch reconciliation between the Catholic and Orthodox churches

Remarkably, the whole revival of Greek learning in the West was made possible by a handful of these emigrés, who gathered disciples in Florence, Venice, and Rome. One of the most important, Manuel Chrysoloras, taught Greek in Florence for just three years, starting in 1397. Yet “the roster of [his] pupils,” Mr. Wells writes, “reads like a who’s who of early Renaissance humanism,” including future masters like Leonardo Bruni and Niccolo Niccoli. This cultural handover occurred in the nick of time: If Chrysoloras and a few other teachers hadn’t arrived when they did, the fall of Constantinople might have prevented the West from ever rediscovering Plato and Thucydides.

By the time the Christian West found its way back to the Greek classics, the Muslim East had cherished them for many centuries.The rise of Islam in the seventh century took place on Byzantium’s doorstep, and the stunning Arab conquest brought much of the empire’s territory under Muslim control. Yet Constantinople’s prestige long survived its power, and the successors to Muhammad yearned to take over its imperial mantle. The first Umayyad Caliph, Muawiyah, consciously adopted Byzantine court rituals, explaining that his capital, Damascus, “was full of Greeks, and that none would believe in his power if he did not behave and look like an emperor.” The Dome of the Rock in Jerusalem was designed in Byzantine style. More important still, the classics of Greek philosophy and medicine were translated into Arabic, thus making possible the great intellectual flowering that produced Avicenna and Averroes.

In between the Arabs and the Latins, the Slavs made their own approach to Byzantium. Here it was the religious legacy that became most important, as the pagan tribes to the empire’s north gradually fell under the sway of Orthodox belief.The first appearance of Russia in the history books came in 860 A.D., when a band of Russian marauders descended on Constantinople and plundered it. Yet as Mr. Wells writes, “one day these ‘arrogant savages’ would become the empire’s best and most faithful allies.” Thanks to missionaries like Cyril — who invented an alphabet for the previously illiterate Slavs, the prototype of modern Cyrillic — the rising Slavic states in Eastern Europe became part of what Mr. Wells calls a “Byzantine Commonwealth.” After 1453, the Russian Tsar — his very title a version of “Caesar” — would claim Moscow as the “Third Rome,” the only legitimate successor to Rome and Constantinople as capital of the world.

The history of these three civilizations, Mr. Wells shows, is like a palimpsest. The writing on top may be Italian or Arabic or Russian, but just underneath lies a Greek script, now all but forgotten. By bringing it to the surface, “Sailing From Byzantium” offers the reader a fascinating lesson in the strange transience, and even stranger endurance, of empires.

akirsch@nysun.com