Polaris Star Trails

Star trails surround the north star in this 8-hour exposure. In the center stands a saguaro cactus, sentinel of the Sonoran Desert. The distinctive square peak on the right is Castle Dome Peak, a regional landmark.

This photograph appeared on NASA/USRA's Earth Science Picture of the Day site on June 7, 2004.

In the past I've called star trail photography "the lazy person's astrophotography," since it seems that all one has to do is plop the camera down on a tripod and open the shutter! That may be true of relatively short exposures, perhaps an hour or two. But I've come to realize that to get a classic shot of really long trails (10+ hours), in which countless arcs seem to completely encircle the pole, is a very challenging proposition ... indeed I've never done it myself! At 8 hours, this photo is the longest exposure I've achieved to date. To get a longer exposure, all of these circumstances would have to coincide:

• It has to be the right time of year. Summer nights are too short! In the mid-northern latitudes in mid-summer, there are only about 6 dark hours each night. But in winter, one could theoretically get 12-hour star trails! This photo was taken within a week of the winter solstice, when the nights are longest.
  
  
• You have to be very far away from any cities. Any artificial light in the sky will overexpose the film during the long exposure. Also, there must be no distant city's light dome in the direction you are shooting (north, for circumpolar star trails in the northern hemisphere). This photo was taken in a remote desert location in southwest Arizona; the nearest town was Yuma, which was small, 40 miles away, and behind the camera. To the north for hundreds of miles was nothing but open desert.
  
  
• You have to stay at this remote location the whole night. Typically this means camping at the site. I usually do my star trail shots when I am staying overnight anyway, at a star party or, as here, on a camping trip. I just walk a short distance into the darkness and set up my camera on a tripod. But even in the desert, mid-winter nights can get cold. The temperature this night got down around freezing!
  
  
• The night must be moonless. Just like city lights, moonlight will overexpose the sky, washing out the star trails. This is the only circumstance that kept this photo from being close to 12 hours; a thick crescent moon rose about 4 hours before morning twilight, forcing me to stop the exposure at 8 hours.
  
  
• The night must be cloudless. Even thin clouds blowing through will cause "gaps" in the star trails. This night was completely clear the entire night.
  
  
• You must remember to wake up in time to close the shutter! You certainly wouldn't want to do an exposure lasting all night, only to have it ruined by the light of dawn! An alarm clock is a vital piece of equipment!
  

How often are we presented with all of these opportunities at once? With planning and effort, one can improve the odds and get that classic shot ... and that is definitely not being "lazy"!

Date: December 27-28, 2002
Time: 7:14 p.m. to 3:14 a.m. MST
Location: Kofa National Wildlife Refuge, Arizona
Camera: Olympus OM-1 35mm SLR on fixed tripod
Film: Kodak Elite Chrome 100 Extra Color slide
Focal length: 24 mm
Aperture: f/8-11
Exposure time: 8 hours
Scanner: Nikon Coolscan LS-2000