long . lines and ripples

About the Pole Star

The last thing that I have been skimming each night is Ian Newton's book Bird Migration.1 I know of few other popular science books that are so relentlessly detailed and through, committed to taking the amateur reader seriously. For what amounts to a literature review of the state of bird migration research from a decade ago, the book is more than readable; it is a little addictive.

In the section on navigation, Newton describes what early, pre-scientific bird observers had long suspected: that birds look at the stars too, and use an internal "star compass," among other tools, to guide them during migration. What birds look for is the sky's stable center point:

"With the star compass, the key factor was the rotation of the night sky about the Pole Star, and in experimental conditions, birds learned to respond to an extremely simplified and reduced star pattern, provided that it rotated about a single conspicuous star. Thus, nestling Indigo Buntings raised under an artificial sky with the star Betelgeuse (in the constellation Orion) as the point of rotation treated Betelgeuse as the Pole Star when subsequently tested."2

This suggests that the avian celestial compass long predates the current configuration of our sky. The Earth "wobbles" (precesses) along its axis, changing the pole star along a circle during a regular period of around 26,000 years. If birds evolved to look for a pole star, they gained that ability through hundreds of thousands, even millions, of generations. Today's birds look at tonight's sky, anchored on the current pole star Polaris, and know it through their ancestors. Their compass has seen Polaris come and go as the pole star hundreds, even thousands, of times. The lesson of the experiment that Newton cites above, with the star Betelgeuse as the pole star, is that it does not matter to birds what star they follow. They count on a more stable feature of the night sky, the mechanic of rotation itself.

Polaris is a relatively conspicuous star. I can see it well under porch lights from my back steps in light-polluted Chicago. Thousands of years in the future, there will be other bright stars, like Capella in the constellation Auriga, that become the pole.

In the city, my sky also contains large blank patches around Polaris, where no stars shine through. This would not have been the case in eras past, when most areas of sky would have contained at least a faint star. Under modern conditions, there will be gaps of hundreds, even thousands of years in which there is effectively no pole star due to the effects of light pollution.

If birds continue to navigate by starlight in future centuries, their star compass will have to be up to the challenge of discerning an invisible point at the center of an obscured sky.

  1. Originally published 2010, a paperback reissue in July 2020 speaks to its relevance. The book is part of the spectacular New Naturalist series put out by HarperCollins, a multi-decade achievement in serious publishing that is still going.
  2. Ian Newton, Bird Migration, 167