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To coincide with Earth Day on April 22, a day we take to consider just how incredible our planet is, a beautiful atmospheric phenomenon known as the "Belt of Venus" will be visible just after sunset. This pink band across the sky can be seen on clear days at sunrise and sunset – though it's more pronounced and easily seen at sunset, hence its other name, an "anti-twilight arch".
Named for the girdle worn by the Roman goddess of love, not the planet, you should be able to see it as the sun dips below the horizon in the west (in the Northern Hemisphere) if you do an about-turn and face east. You'll see a band of pink-orange with a band of blue underneath. This blue stripe, which is darker than the surrounding twilight, is Earth's "shadow" being cast on the atmosphere. These bands of color will appear to be moving upwards in the minutes following sunset, eventually forming an arch across the sky before fading as night sets in.
So, what is happening here?
Well, Earth's shadow rising can be seen on any clear day in the opposite direction to sunset, ascending at the same rate as the Sun going down. As Earth rotates, sunlight ceases to reach parts of the atmosphere near Earth's horizon on the opposite side of the sky than the Sun. Earth's shadow is being cast on the atmosphere, as it is no longer reflecting sunlight.
The anti-twilight arch also appears in the opposite direction to the Sun; anti-twilight meaning opposite twilight. We see this arch when the Sun is just below the horizon but some light is still making its way through the atmosphere. It appears a rosy-hued color because red light has the longest wavelength, so it's able to travel farther through the atmosphere. At the point opposite the sunset, the light is scattered through the denser atmosphere and back towards our eyes. This "backscattering" creates the pink band.
As Earth's shadow is emerging it appears to make the pink band separate from the horizon, emphasizing the arch, and that is the Belt of Venus.
