|On the afternoon of Christmas Day, sun dogs made this spectacular halo, seen from Herring Cove Beach.|
A couple of sun dogs appeared in the western sky off Provincetown shores on the afternoon of Christmas Day, giving a remarkable visual treat to those of us lucky enough to be at Herring Cove Beach during these few moments. A sun dog is a parhelion, the bright spot, or false sun, that can appear about 22 degrees to either side, or both sides, of the sun, and at the same elevation as the sun, with ice crystals in the atmosphere refracting the light in a particular way.
As the sun rises or descends, these bright spots follow, or "dog" the sun, which may explain the name. No one can say for certain where the term came from, but it seems to have been in use since the early 1600s, around the time when the Mayflower Pilgrims landed on Provincetown shores. The easiest time to see these bright spots following the sun is early morning or late afternoon, when the sun is closest to the horizon.
In the photo above a couple of sun dogs (a pair of parhelia) give the appearance of two bright spots on either side of the sun, creating a luminous circle, or a sort of halo around the sun, with the area inside the halo appearing slightly darker than the surrounding sky. These halos can appear when flat, hexagonal ice crystals, often in wispy, cirrus clouds, are drifting in random orientations, refracting the light and bending the rays that pass through them. As these crystals sink through the atmosphere they tend to align vertically, refracting the light horizontally, which can produce sun dogs.
A sun dog will always appear at the same height above the horizon as the sun, whether or not the complete halo is visible. This phenomenon can be seen anywhere around the world, in any season, but it seems to me that we have a few more than our fair share of these remarkable displays here in PTown, and I'm going to chalk that up to the unique quality of the light that surrounds us. The light in Provincetown truly is different from the light in other spots on the globe.
Sun dogs will sometimes exhibit a sort of rainbow effect. with a reddish color nearest to the sun, morphing to blue on the outer edge. See my post Sun Dog Appears Over Race Point, from October of last year. You'll find a rather vibrant photo I managed to get one afternoon, illustrating this rainbow effect.
Of course, this cosmic sight was visible from many vantage points around town, but the view from Herring Cove, looking out over Cape Cod Bay, was particularly splendid. One of the greatest things about living in this amazing spot is just how often each of us ends up "in the right place at the right time" to see the spectacular show that Mother Nature puts on for us, several times a day, from any viewpoint one might choose.