The image above shows the Belt of Venus, a rosy-hued band of light between Earth’s shadow and the upper atmosphere that occurs just after sunset, or just before sunrise. However, this atmospheric phenomenon has nothing whatsoever to do with the planet Venus; the only connection between the planet and the phenomenon is that it is thought by some historians to be named after the girdle supposedly worn by the goddess Venus.
What causes the Belt of Venus?
In some ways, the Belt of Venus is similar to the Alpenglow phenomenon, which is also caused by the scattering of reddened sunlight. However, unlike the alpenglow, the Belt of Venus is caused by fine dust particles that are suspended in the atmosphere, which means that the Belt of Venus persists for much longer than other, similar phenomena.
Typically, the Belt of Venus occurs between about 10 degrees and 20 degrees above the horizon and can be seen from almost any latitude for periods that range from as short as a few minutes, to as long as 15 minutes just after either sunset or just before dawn. In practice, though, the best views of the Belt of Venus occur over a clear horizon, such as over the ocean, and at a point opposite the setting (or rising) Sun.
How the Belt of Venus forms after sunset
When you look in a direction opposite to where the Sun is setting, all the light from the Sun is progressively being tilted more and more with respect to your observed horizon. However, some of the Sun’s light still passes through the haze in the atmosphere, and it is this band of light that appears red to our vision (due to the scattering of red light) and which forms a band of rosy-hued light between Earth’s partial shadow, and the upper atmosphere. However, the band of red light is only visible because an observer’s line of sight is approximately parallel to the upper edge of Earth’s shadow.
As the Sun sinks lower behind the horizon (behind you), its light is tilted evermore upwards through the atmospheric haze, which has the practical effect that the Belt of Venus seems to lift up off the horizon, and take on a more curved appearance the higher it climbs. By the time the Sun reaches a position of about 18 degrees below the horizon, though, its light is tilted so far that the band of red light is moved beyond the point where it is visible (i.e., it is moved out of the observer’s line of sight). Eventually, after about 15 minutes or so, the red light is extinguished when Earth’s shadow overtakes it as the Sun sinks even lower below the western horizon.
Common feature of Belt of Venus photographs
Although the Belt of Venus usually appears as a vivid pink in photographs, it almost never appears quite that bright to people with normal vision. In fact, the Belt of Venus is frequently barely discernible to our eyes, and it seems bright in pictures only because cameras capture light in ways that human vision cannot. One other common feature of photographs of the Belt of Venus is that when the picture includes the Moon, the Moon is always full. There is nothing weird about this, though, as the Moon is always full when it rises exactly opposite the setting Sun.
Where is Venus?
It is often said that the planet Venus is always visible within the Belt of Venus when Venus is visible. However, while the Belt can extend over a huge segment of the horizon opposite the setting Sun, the fact is that to human vision, the brightest part of the Belt is always 180 degrees away from the setting Sun. Therefore, since the furthest Venus can ever be away from the Sun is about 45 degrees (because its orbit is smaller than Earth’s), the planet Venus can never be inside the part of the Belt of Venus that we can see.