Cool lesser-known astro-expressions

Graphic Fairy

Every hobby and profession has its own language, and we astronomers certainly have ours. Some terms (zenith: the point straight up) are known by almost everyone. Others (Schwarzschild radius) are familiar only to professionals and serious amateurs.

Some are pure fun. They’re fun either because they sound intriguing, or else denote a little-known phenomenon that deserves wider recognition.

Some merely sound cool, like exeligmos. It’s a period of time: 54 years and one month. An exeligmos is how long you must wait before the same kind of eclipse returns to your neighborhood. So the fabulous six-and-a-half-minute North American total solar eclipse of July 11, 1991, seen from Baja California, gets a reprise in the US on August 12, 2045: an exeligmos later. That’ll be the longest totality in US history.


How about syzygy? Good Scrabble word (except you need a blank). That’s simply the three-way lineup of Earth, Moon and Sun, like at Full Moon.

Parhelion is the mellifluous name for a bright, colorful spot of light 22 degrees to the left or right of a lowish Sun. Its popular name is just as attractive: sundog. How can anyone not like that?

I like quadrature, too: the position of any superior planet – one outside of our orbit – when 90 degrees from the Sun in our sky. This month Jupiter is at quadrature, so it will hover in the south at its highest at nightfall. Look up this Saturday evening, June 3, and you’ll see Jupiter right next to the Moon. It’s a very cool conjunction – worth a look, trust me.

How about terminator? It sounds cold and icy, thanks to the movies, but that day/night line – which, on the Moon, creeps along the surface at 10 miles per hour to deliver exquisite high-relief shadowing – makes the terminator the place to look when hauling out any backyard scope. These nights, the terminator is optimally placed for peering at our nearest neighbor.

Event horizon is an appealing phrase; we need more like it. It’s a location at the Schwarzschild radius: the distance from a black hole’s singularity inside of which no natural object or even light can escape, because space/time is infinitely curved. Astro-terms don’t get much better than these.

Twilight wedge deserves to be here, too; and it’s something out of everyday life, rather than esoteric astrophysical theory. That’s the gray/blue horizontal band hugging the eastern horizon each evening at sunset. It’s nothing less than Earth’s shadow thrown into space. It’s striking and obvious, so why isn’t everyone aware of it?

While we’re out at dusk, check out crepuscular rays. They’re not visible every clear evening, like the twilight wedge, but they’re not rare. You’ve seen them. Those dramatic fairyland rays, emanating from just below the horizon, where the Sun lurks soon after sunset, come from shafts of light poking over the tops of clouds beyond the horizon. Crepuscular means “pertaining to twilight.” It sounds so lovely, I use the word as much as possible – even when it’s not appropriate.

Lagrangian point: Does that have zing or what? ’Twas named for Joseph-Louis Lagrange, who, two centuries ago, identified five places where small bits of rock like asteroids (or, these days, spacecraft) can float in happy stable positions in equilibrium with the gravities of two larger objects like a planet and the Sun, or the Moon and Earth. At such positions we’ve placed spacecraft like SOHO, monitoring the Sun, and WMAP, examining the cosmic microwave background. Next year the James Webb Space Telescope will be parked at a Lagrange point, too.

Two-word science terms that contain a color are automatically so catchy, they’re often repeated by the mass media: green flash, red giant, black hole, brown dwarf, red shift. Maybe too well-known to qualify for our list of esoterica, but snappy all the same.

Cerenkov radiation (sometimes spelled Kerenkov): That’s the blue-violet glow emitted when something breaks the light-speed barrier. How cool is that? Of course, nothing outruns light in a vacuum. But in denser media, like in a nebula or a nuclear reactor’s water-filled tank, that violet glow means subatomic particles are zooming around like faster-than-light roaches. Eerie.

I love the word Maria. It has two totally different meanings. Pronounced Ma-RYE-uh, as in Mariah Carey, it’s the first name of the great astronomer Maria Mitchell. A century ago she trailblazed the way for women astronomers from her observatory on Nantucket (worth a visit), then as Vassar’s first female professor. But pronounced differently, as MAR-ee-uh, they’re the black lava seas on the moon: the most prominent lunar features visible to the naked eye. Singular is Mare (MAR-ee or MAR-ay).

If you’d like to make this a competitive exercise, award yourself a point for each definition you already knew. Ten points earn you a one-year subscription to Modern Vacuum.