Tag: light/atmosphere

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.

 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.

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 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.

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.

 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.

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.

 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 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 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.