Tag: celestial

An informal, recognizable pattern of stars that is not one of the 88 official constellations. The Big Dipper is an asterism within Ursa Major. The Summer Triangle connects stars from three different constellations. Asterisms are the patterns people actually see and use — the folk astronomy that predates and outlasts the official maps.

The technique of looking slightly away from a faint star or object to see it more clearly, placing its image on the more sensitive rod cells at the edge of the retina rather than the less sensitive cone cells at the center. You see the faintest things in the sky by not looking directly at them. The peripheral is more honest than the focal.

A pink-purple band of light visible just above Earth's shadow on the opposite horizon at dusk or dawn. The shadow rises blue-gray; the Belt glows above it where sunlight still reaches the upper atmosphere. Best seen from coastal slopes with a low, clear horizon.

A halo around the sun or moon — a luminous ring caused by ice crystals in high clouds refracting light. In Scotland, a broch around the moon means rain is coming.

Parallel rows of cumulus clouds aligned with the wind, marking alternating bands of rising warm air and sinking cool air. The spacing between streets is usually two to three times the cloud height. The atmosphere organizing itself into lanes.

A condensation trail left by aircraft exhaust freezing in cold upper air. A persistent contrail means humid air aloft and possibly approaching weather; a vanishing one means dry air. The sky's simplest hygrometer.

The inverse of a contrail: a thin line of blue sky punched through an existing cloud layer by an aircraft's heat. Sometimes called a hole punch when the plane climbs through at an angle. The cloud erased rather than created.

A circular or elliptical gap in a cloud layer where supercooled water droplets have frozen into ice crystals and fallen out, leaving a hole fringed with wispy cirrus. Eerie, geometric, and often mistaken for something stranger than it is.

Electric flickers in the sky — atmospheric electrical discharges, sheet lightning, or the shimmer of aurora seen from below. Not a bolt but a wash of light, as if the sky briefly caught fire.

The point directly below you, opposite the zenith — through the earth and out the other side. In common use it means the lowest point of anything. The Arabic astronomers who named it were mapping the geometry of the sky, and the lowest point was the one you couldn't see.

Luminous clouds visible in the upper atmosphere around twilight — so high (about 50 miles up) that they catch sunlight long after the sun has set below the horizon. Electric blue, rippled, and rare. They are made of ice crystals coating meteoritic dust.

An arc of light passing through the sun, parallel to the horizon — caused by sunlight refracting through hexagonal ice crystals in high cirrus clouds. One of the rarer atmospheric optical phenomena. When it appears, the sky becomes geometry.

A mock moon — a bright spot flanking the real moon, caused by moonlight refracting through ice crystals. The lunar equivalent of a sun dog. Fainter, rarer, and more ghostly.

Bright spots flanking the sun, caused by ice crystal refraction — commonly called sun dogs. They appear at the same altitude as the sun, about 22 degrees to either side, and are often ringed with color. In extreme cold, parhelia blaze.

The twinkling of stars, caused by light refracting through turbulent, moisture-laden atmosphere. Stars that twinkle more than usual are forecasting incoming weather — more moisture aloft means more refraction. Pacific Island navigators read the sky this way, noting which part of the horizon twinkled most to predict where weather was arriving.

Large-scale electrical discharges above thunderstorm clouds — faint, reddish flashes extending upward into the stratosphere, lasting milliseconds. Sprites are lightning's mirror image: where lightning strikes downward, sprites reach up. They were not photographed until 1989.

A luminous plasma discharge from pointed objects during a thunderstorm — a blue or violet glow appearing on ship masts, aircraft wings, church steeples, or even the tips of horns on cattle. The air itself becomes electric. Not fire but ionized gas.

A dark patch on the underside of cloud cover that indicates open water below, visible from a distance across ice. Arctic navigators read the sky as a map of the surface — water sky is dark; ice blink, its opposite, is bright. The clouds become a mirror, and the navigator reads the reflection.

The point in the sky directly above you. Your zenith is yours alone — no two people standing in different places share the same one. The opposite is nadir.