Tag: light/atmosphere

The lingering light on a high plateau after it has left the rest of the earth — the last glow held by altitude when the valleys have already gone dark. Nan Shepherd's word for the phenomenon of summit light persisting after sunset.

The proportion of light that a surface reflects — its reflectivity, expressed as a number from 0 (absorbs all light) to 1 (reflects all light). Fresh snow has an albedo near 0.9; open ocean is around 0.06; a forest canopy around 0.15. Albedo determines how much solar energy a surface absorbs and is one of the key variables in climate: as Arctic ice melts, high-albedo white surfaces are replaced by low-albedo dark ocean, which absorbs more heat, which melts more ice. The feedback loop is as simple as it is relentless.

The sparkle of morning sunlight through hoar-frost on leaves and grasses. A word for a phenomenon that lasts only minutes — the moment when frozen dew catches the first light before it melts.

The figure-eight pattern traced by the sun's position in the sky if you record it at the same time of day from the same place over the course of a year. The asymmetric eight is produced by the combination of Earth's axial tilt and its slightly elliptical orbit. An analemma printed on a globe is the key to reading a sundial. Photographing one requires a year of patience — one exposure per week, same clock time, same tripod position — and the result is one of the most beautiful proofs that the Earth moves.

Curtains, arcs, and ribbons of colored light that appear in the night sky at high latitudes, caused by charged particles from the sun striking the earth's upper atmosphere and exciting gas molecules into luminescence. The aurora borealis (northern lights) and aurora australis (southern lights) are the same phenomenon, mirrored at opposite poles. The colors depend on altitude and gas: green from oxygen at 60 to 150 miles, red from oxygen above 150 miles, blue and purple from nitrogen. An aurora is the planet's atmosphere responding visibly to the sun's breath — the magnetic field channeling the solar wind into the sky and making it glow.

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 cold, intermittent gleam — a glint of faint light, as from sun on ice or water seen through cloud. Not steady illumination but a flicker, a signal from the landscape.

An extremely bright meteor — a fireball — that explodes or fragments as it passes through the atmosphere. Bolides are brighter than any planet, often leave persistent smoke trails, and can produce audible sonic booms. The Chelyabinsk event of 2013, which shattered windows across a Russian city, was a bolide. The word names the rare moment when the sky delivers a rock and the atmosphere fights back.

A nine-level numeric scale for measuring the darkness of a night sky, from Class 1 (the darkest sky attainable on Earth — zodiacal light visible, the Milky Way casting shadows) to Class 9 (inner-city sky — only the Moon, planets, and a few bright stars visible). The Bortle scale is, in effect, a measure of loss: each step up represents another layer of stars erased by artificial light. Most Americans live under Class 5 or worse and have never seen the sky their grandparents knew.

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.

Your own shadow projected onto mist or cloud below you, enormously magnified, ringed by a circular rainbow called a glory. You are the giant.

Shafts of sunlight made visible by haze or dust, fanning out from a gap in clouds or from behind a mountain. The rays appear to diverge but are actually parallel — the divergence is perspective, like railroad tracks. The word means "of twilight," though they occur at any time of day.

Ice crystals suspended in clear, calm air near the ground on extremely cold days, glittering in sunlight like falling dust. Not a cloud, not precipitation — the air itself has frozen and is slowly settling. Diamond dust produces halos, sun dogs, and light pillars.

The rising of the Earth above the lunar horizon, as seen from the Moon or from lunar orbit. The word names a specific, singular shift in human consciousness: on December 24, 1968, Apollo 8 astronaut William Anders photographed the Earth rising above the Moon's surface — a small, blue, achingly beautiful sphere hanging in black nothing — and the image changed how the species understood its home. There had never been an Earthrise before because there had never been eyes in the right place to see one. The word didn't exist until it was needed.

The faint illumination of the dark side of the Moon by sunlight reflected off the Earth — visible as a ghostly glow filling the unlit portion of a crescent moon. Leonardo da Vinci was the first to correctly explain it: the dark part of the Moon is lit by the bright Earth, just as a full moon lights the Earth at night. Earthshine varies with Earth's cloud cover — more clouds, more light reflected, brighter earthshine. It is our planet's light, returning to us from the Moon's surface.

The apparent path of the sun across the sky over the course of a year — the plane of Earth's orbit projected onto the celestial sphere. The ecliptic is the line along which eclipses occur (the Moon must cross it to block the sun), the road the planets travel (all orbiting in roughly the same plane), and the belt through which the zodiac constellations are strung. It is the solar system's equator, drawn on the sky.

 Either of the two moments each year when the sun crosses the celestial equator and day and night are of approximately equal length everywhere on earth. The vernal equinox marks the beginning of spring; the autumnal equinox marks the beginning of fall. The equinoxes are the moments of balance — the year tipping from light to dark or dark to light.

A complex superior mirage that stacks, stretches, and inverts images of distant objects, making coastlines float above the sea and ships hang in the sky. The most elaborate lie the atmosphere tells.

A rainbow formed in fog rather than rain — pale, ghostly, almost colorless, because the water droplets in fog are too small to separate light into a full spectrum. A white arc against grey. The rainbow's quiet cousin.

A faint, oval patch of light in the night sky at the antisolar point — the spot in the sky exactly opposite the sun. Caused by the same interplanetary dust that produces zodiacal light, but seen here by backscatter rather than forward scatter. The gegenschein is one of the faintest naked-eye phenomena in the sky, visible only under near-perfect dark conditions. Most people will never see it. It is the sun's ghost, projected onto the sky behind you.

Enchantment, illusion, a spell cast by landscape or light — the quality of a place that makes it seem other than what it is. A mountain in certain light has glamourie. So does a forest at dusk. The word acknowledges that the landscape can deceive and that the deception is part of the beauty.

A series of concentric colored rings that appear around the shadow of your head on fog or cloud, caused by light backscattering through water droplets. Seen from mountaintops, aircraft, and alongside the Brocken spectre. The glory is always centered on your own shadow — each person sees their own.

Any object whose shadow is used to tell time or direction — a stick in the ground, a standing stone, a column, a human body. The oldest astronomical instrument. Every sundial has one. The word names the principle that the sun writes on the ground if you give it a pen.

A brief flare of emerald light at the upper rim of the sun in the final second before it sets below a clean horizon. Caused by atmospheric refraction separating the sun's light by color, with green the last visible wavelength to disappear. Real, rare, and over before you're sure you saw it.

A shimmering distortion of air caused by heat rising from the ground — the visible convection that makes distant objects waver and dissolve. The landscape melting before your eyes.

A halo of bright light that appears around the shadow of your head on dewy grass, caused by sunlight bouncing back toward you from water droplets. Each droplet acts as a tiny lens, focusing light directly back toward its source. You see the glow only around your own head — each person carries their own halo.

A period of warm, dry, hazy weather occurring in late autumn after the first frost — a false reprieve, the year offering a few last days of warmth before winter closes in. The sky is soft, the light golden, and the air carries a quality of valediction. Indian summers are not guaranteed; they are gifts.

The light reflected off a river at dusk or night, when the water holds the last glow after the land has gone dark. The river remembers the light longer than the ground does.

 Sunlight filtering through the leaves of trees. Not the light itself and not the shadow, but the interplay — the dappled, shifting pattern where the two meet.

A vertical column of light extending above or below the sun, moon, or streetlight, caused by reflection off the flat faces of ice crystals drifting horizontally in calm, cold air. The pillar is not a beam — it's an illusion made by millions of tiny mirrors, each reflecting the light source at a slightly different altitude.

The apparent lifting of distant objects above the horizon by atmospheric refraction, making things visible that should be below the line of sight. Coastlines, ships, and islands that are geometrically beyond the horizon can appear to float above it. The opposite — objects appearing to sink or shrink — is called stooping. The Vikings may have discovered Iceland because it loomed.

 The road-like path of light that the moon makes on water. A shimmering, elongated reflection that appears to lead from the viewer to the horizon.

The optical illusion produced by light refracting through layers of air at different temperatures, bending the image of the sky down onto the ground so that the desert appears to hold water. A mirage is not a hallucination — it is real light, really bending. The physics is clean. Only the conclusion is wrong.

 The collective behavior of starlings flying in dense, shape-shifting flocks at dusk — thousands of birds moving as a single, fluid organism, expanding and contracting in patterns that no individual bird directs. Also the word for a flock of starlings. The sound is part of the name — the low, continuous hum of ten thousand pairs of wings beating in near-unison.

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.

The hiding of one celestial body behind another — a star disappearing behind the Moon, a moon slipping behind a planet. An eclipse is a special case of occultation, but the word applies more broadly: any time one object in the sky passes in front of another, blocking it from view. Occultations are instantaneous — a star winks out in a fraction of a second as the Moon's limb crosses it — and astronomers use the precise timing to measure the positions and sizes of objects with extraordinary accuracy.

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 shift in color sensitivity as eyes adapt to low light: reds fade first, blues persist longest. Named for the Czech physiologist who noticed that his favorite red flowers looked black at dawn while the blue ones still glowed. The word names why dusk doesn't just get darker — it changes color.

The shaman light — a luminous inner fire that enables a shaman to see in the darkness, to perceive what is hidden. Not a physical light but a spiritual illumination, associated with the ability to see beneath the surface of things.

 The shadows of clouds moving across the moorland on a sunny day. The landscape darkening and brightening in slow, silent waves as the sky passes over it.

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.

 Either of the two moments each year when the sun reaches its most extreme position north or south in the sky — the longest day in summer, the shortest in winter. The solstice is the hinge of the year: the point at which the days stop lengthening and begin to shorten, or stop shortening and begin to grow. Every culture that watches the sky has marked this moment.

A bright spot of light on one or both sides of the sun, at the same altitude, caused by refraction through hexagonal ice crystals in high cirrus clouds. Also called a parhelion — a mock sun. On very cold days with diamond dust, sun dogs can be blindingly bright and ringed with color.

The line that divides the illuminated and dark portions of a celestial body — the boundary between day and night on a planet or moon. On Earth, the terminator is the advancing edge of dawn or the retreating edge of dusk, sweeping across the surface at roughly 1,000 miles per hour at the equator. Through a telescope aimed at the Moon, the terminator is where the most dramatic detail appears: craters and mountains cast long shadows at the boundary of light, revealing topography that vanishes under the flattening noon sun.

The darkest, central part of a shadow — where the light source is completely blocked. In an eclipse, the umbra is the cone of total shadow: if you stand within the Moon's umbra during a solar eclipse, the sun is entirely hidden and the sky goes dark. Surrounding the umbra is the penumbra, where the light source is only partially blocked. The umbra is the shadow's heart — the complete absence of direct light.

 The reflection of moonlight on water. Similar to mångata but without the implication of a path — more the shimmer and scatter of light across the surface.

A faint, triangular glow visible on the horizon just after sunset or just before sunrise, caused by sunlight scattering off a vast, thin disk of interplanetary dust orbiting the sun along the plane of the solar system. It is so faint that light pollution renders it invisible to most people alive today. In a truly dark sky, zodiacal light can be brighter than the Milky Way. It is sunlight reflected off dust that has been drifting between the planets since the solar system formed — ancient light bouncing off ancient debris.

A haze or shimmer in the air — the visual distortion caused by heat or moisture, when the landscape wavers and the distance dissolves. The South African version of heat haze, with its own name and its own quality of light.