As the sun goes down, the angle that the light projects next to the surface tends to 180º, to flatten out. If we see our own shadow, we can see how it gets longer and longer, and if the surface is flat enough and we do not have obstacles that prevent our silhouette, the shadow can lengthen to long distances. The Brocken spectrum is based on this principle and that there is fog, named after Mount Brocken 1142 meters above sea level in the Harz Mountains, Germany.
The mountaineers who came there, could see in the evening, leaving the Sun behind them, its long silhouette projected in the mist, which is commonly formed. Sometimes, looking into the distance, the sun's rays form an aura of the colors of the rainbow. That halo is the Brocken specter.
Why is it so curious?
Because the Brocken spectrum can only be the one who casts the shadow. It doesn't matter if other people go with you, the halo can only be seen by the person whose shadow is projected. So, if everyone is ready to see their shadows, you will see only their own colored aura and the other shadows of the companions projected in the mist with nothing else. Another of the accompanying phenomena is that it seems that it is really covering the body. Since the shadow is reflected in the mist, the human silhouette does not appear lying down, but in a blurred way standing.
The spectrum, even and having its origin in Brocken, can be seen elsewhere. In ancient times, this phenomenon was more than just an optical effect. The existence of halos or areolas around the body or head was like a kind of divine sign that God had chosen that person for a special purpose.