The "Brocken Specter" is a sort of optical illusion that most often happens in the mountains. It is named after the Brocken, a mountain in Germany where the illusion happens frequently, due to its geographic situation.
Imagine that you are standing on the sidewalk on a sunny day. You turn so that the sun is at your back. You look down and see your shadow cast upon the sidewalk.
Now imagine that you are standing on a mountain side. Your climb started out in the foggy lowlands, but you have climbed above the fog (or low-lying clouds) and now stand in the sun. You turn so that the sun is at your back. You look down and see your shadow cast upon the fog below you.
A few weird things happen. For one thing, because your shadow is being cast on fog instead of a solid surface, it's hard for your brain to compute where the shadow actually is. It can seem both far away and very close, because the shadow on the fog particles throws off your depth perception.
For another thing, because of the angle of the light and the depth of the fog, your shadow is distorted into a triangular shape. It seems very wide at the feet, and very narrow at the head. This can give the illusion that the shadow is very tall, like a giant striding along beside you.
Just to make things a little more unsettling, diffraction often causes a halo or circular rainbow to appear to encircle the head of the figure, like the holy glow on a Renaissance painting of baby Jesus.
This phenomenon is well known to people who spend a lot of time in the mountains. (Its name comes from the German word Brockengespenst, coined by the people who live near the mountain Brocken.) But it can be startling to visitors, to whom it seems almost an apparition appearing below or beside them in the mist.
It is even more disconcerting when, due to some very specific circumstances, the specter appears to be walking alongside you just like another person! For this to happen you need very flat land, a low angle of sun, and foggy weather. These conditions occur in the Arctic and in Antarctica, and the Brocken Specter is no doubt what is referred to as the "extra person" who is said to have haunted many expeditions. I recall having read about this shadowy extra man in an account of Shackleton's Antarctic expedition.
This "ghost who walks alongside you" can also occur in the mountains if you are very high, as when crossing certain mountain passes. A Brocken Specter haunts Byron and Shelley as they cross the Alps in Tim Powers' novel The Stress of Her Regard.
Brocken Specters may also be responsible for many strange sightings that occur in foggy or mountainous areas. It doesn't take too much imagination to see a Brocken Specter as the potential source of many Sasquatch and Yeti sightings, as well as more abstract monsters like the Jersey Devil or the Mothman.
Photo credit: Flickr/Mouser NerdBot