As clear May evenings grow late, a great starry umbrella spans the darkening western sky: the Arch of Spring. In the next few days, the waxing crescent moon will march up under its shelter.

Look high in the west as dusk deepens. There you’ll find the top of the arch: two moderately bright stars lined up horizontally. They’re Pollux and Castor, the heads of the Gemini twins. Hold your hand at arm’s length, and they’re about three fingers apart.

They have been up there much longer than people have been down here to look at them.

The legend of the Gemini twins, as we received it from the Greeks and Romans, was apparently piled together, in its dreamy confusion, from the imaginings of many peoples earlier. Castor and Pollux were half-twins with different fathers: a king of Sparta and Zeus himself, both of whom impregnated a swan.

The swan laid an egg from which Castor and Pollux hatched. They were often depicted as a matched pair of athletes with caps of eggshell still on their heads. It must have made sense at the time.

Temples were built to them. They became the patron demigods of sailors, a role that survived long after Christianity displaced most of the elder gods. When your granddad swore “by Jiminy!’’ he was, wittingly or not, swearing a sailors’ oath by Gemini.

Look lower left of Pollux and Castor, and there’s Procyon, Greek for “before the dog.’’ It rises before the brighter Dog Star, Sirius, but sets after it; Sirius is already disappearing down to the horizon as twilight fades. See if you can still catch it twinkling very low to Procyon’s bottom left.

Farther away to the lower right from Pollux and Castor shines Capella, the Goat Star, part of Auriga the Charioteer. The chariot driver was supposed to be carrying one or more baby goats on his shoulder, which must have been awkward while holding a horse’s reins in his hands.

The Greeks picked up the starry goats from Arab desert dwellers and conflated the whole tableau together. Look for three fainter stars forming a long triangle below Capella, as plotted on the sky-scene here. These are still called “The Kids.’’

Just before you reach Capella from Castor and Pollux, you hit a lesser star to its upper left: Menkalinan, from the Arabic for “shoulder of the rein-holder.’’ After the collapse of Greek and Roman civilization, Arab scholars took over stewardship of its astronomy and translated many of the old star names into Arabic.

Medieval Europeans later picked these up and rendered them phonetically, more or less. Those are the names by which we know many stars today, from Algenib to Zubenelgenubi.

Way down below the Arch of Spring, due west as twilight fades, glimmers orange-red Betelgeuse. It’s the last of wintry Orion’s bright stars to sink from view. The name is from the Arabic for “armpit of the giant,’’ which is where Betelgeuse resides in Orion’s ancient figure.

On the other hand, the Arch of Spring is a very recent invention. Will it catch on? People who gaze up at night are constantly connecting the dots to make new patterns. Most never gain traction, but a few do, for fluky little reasons no one knows.

The Big Dipper and the Sagittarius Teapot were named only in the 19th and 20th centuries, respectively, even though they look very obviously like those two kitchen implements – to modern eyes. The Gemini twins, on the other hand, have roots at least as far back as ancient Mesopotamia and perhaps in prehistoric times.

In the last few centuries, sky lore of a new and different kind has filled the heavens in a way unlike anything before — in terms of quantity, quality, and reality. Thank the invention of telescopes in the 17th century, precision sky-measuring tools in the 18th, and spectroscopy in the 19th. These gave enormous new life to a way of astronomical thought that began among a few of the ancient Greeks but then died out for some 1,500 years.

This way of thought was to examine the sky for evidence of what was actually going on in it, rather than just lying back, dreaming up stories, and declaring them to be true.

To put aside stories and hunches, and pursue difficult, evidence-based investigation wherever it leads, is an unnatural thing for the human mind to do. But the power of this idea finally began to take hold about four centuries ago, and it made us masters of the world. Astronomy led the way.

Alan M. MacRobert is a senior editor of Sky & Telescope magazine in Cambridge (SkyandTelescope.com). His Star Watch column appears the first Saturday of the month.