Does it ever get really dark?

After sundown, there is still enough light to pour your gin and tonic and sit on a desert rock and gaze at a glowing landscape, for the Sun’s disc is just below the horizon and its light is scattered by the Earth’s atmosphere. Cat thieves and burglars do not yet merit the stiffest sentences reserved for night offences. You might get off for not having your car lights switched on. The brightest planets will be visible, along with zero magnitude stars such as Sirius, Vega, Capella and Arcturus, soon followed by 2nd and 3rd magnitude stars. You will have about half an hour of this civil twilight which ends officially when the Sun’s centre is 6 degrees below the horizon.

The night now moves into nautical twilight. On the open seas, the horizon is still visible and a mariner can navigate by measuring the angle between the horizon and familiar stars. Fourth and fifth magnitude stars appear through the gathering darkness and it’s no longer possible to read outside. By the end of nautical twilight, the Sun’s centre is 12 degrees below the horizon, the horizon has become indistinct and there is no trace of light distinguishing the western from the eastern horizon.

Yet the atmosphere continues to scatter a faint sunlight, finally extinguished when the sun falls 18 degrees below the horizon. Not until the dimmest stars of sixth magnitude appear at the zenith can we say it’s completely night and astronomical twilight has come to an end.

The time taken for the light to fade depends on the time of the year and your latitude. In the north in summer, the Sun skims below the horizon and civil twilight can last till the morning. Near the equator, as the Sun plunges straight down, it can be night within 20 minutes.

But it’s still not completely dark! A sky glow lightens the sky compared to the shadows that cling to forests and mountains. About half of this luminosity comes from the starlight; the other half from air glow, a chemiluminescence produced by molecules disrupted by sunlight during the day recombining during the night and shaking off their excess energy as light. Have a look at about 15 degrees above ground level and you’ll see air glow at its most intense.

This entry was posted in Stars 2012. Bookmark the permalink.