Arctic terns fly about 70,000 kilometers a year, the longest known migration of any animal.

Arctic terns nest around the Arctic in the northern summer, then fly south to spend the southern summer around Antarctica. Following the sun this way means they experience more daylight than any other animal. Rather than flying in a straight line, they take long, curving routes that use prevailing winds, which makes the distance much longer.

In 2010, researchers fitted tiny geolocators to terns breeding in Greenland and Iceland. The data showed an average annual round trip of about 70,900 kilometers, with some birds flying even farther. Later studies on terns from the Netherlands recorded similar or longer distances.

Arctic terns can live for 30 years or more, so a long lived bird may fly millions of kilometers in its life.

For years, the Arctic tern's migration was estimated from sightings and ringed birds recovered far from where they were banded. These showed the birds travelled between the poles, but not their exact routes. The breakthrough came with geolocators weighing only a fraction of a gram, light enough for a bird of about 100 grams to carry.

The devices recorded light levels, from which researchers could calculate a bird's position each day. Results published in the Proceedings of the National Academy of Sciences in 2010 revealed an S shaped route down the Atlantic, with a long stopover in the North Atlantic to feed. Birds then split, some following Africa's coast and others crossing to South America before reaching Antarctic waters.

If a tern flies around 70,000 kilometers a year and lives for 30 years, it could cover more than two million kilometers in its life.

That is roughly equal to three round trips between Earth and the Moon, all on wings powered by fish and small sea creatures.

Migration is one of nature's great spectacles. Bar tailed godwits fly nonstop from Alaska to New Zealand, more than 11,000 kilometers, without landing to feed. Monarch butterflies travel thousands of kilometers across North America. But measured by total yearly distance, the Arctic tern remains the champion.

Because Arctic terns spend summer near both poles, they see more daylight than any other creature, chasing long polar days in both hemispheres.

Climate change may affect these journeys. Shifts in sea ice, ocean temperatures and fish populations could change where terns feed along their routes, and scientists continue to track them to understand those effects.