Displacement versus Distance

The Arctic Tern Migration Project showed that the Arctic Tern, a bird which weighs a little more than 100 grams (4 ounces), can travel upwards of 71000 kilometers (44000 miles) per year.

Displaced distance versus traveled distance:  

1. The distance between the Arctic and Antarctic destinations of the Arctic Tern  migration along the surface of the Earth, is about 15000 kilometers (9300 miles). Therefore the round trip should be about 3000 kilometers (18600 miles).
2. The average distance traveled by each Arctic Tern travel was measured to be 71000 kilometers (44000 miles).

1. The traveled distance is much greater than the distance displaced during the migration. Why is this so?

Image from the Arctic Tern Migration Project (Follow link for higher resolution image)

Note: With this example the students can be encouraged to go to the website and find the information about the distance traveled by the Arctic Tern.  This can be used along with a discussion on how to measure distances along the surface of the earth.  For example, from the paper “Tracking of Arctic terns Sterna paradisaea reveals longest animal migration” we get that the Arctic Tern begins its migration at a latitude of 75N and travels to 60S, which represents an arc of 135 degrees along the surface of the earth. Since the circumference of the earth is 40000 km the displaced distance of the migration is the fraction of the total circumference covered by the 135 degree arc, or 40000 km x 135/360 = 15000 km. The fact that the displaced distance is clearly much less than the traveled distance, can be used to spark a discussion about why this is so.