Uniformitarianism is the principle or assumption that the same natural laws and processes that operate in the universe now have always operated in the universe in the past and apply everywhere in the universe.

In science, a half-life is the amount of time it takes for half of a substance or entity to undergo some specified process.

Pangaea, Laurasia, and Gondwana were ancient supercontinents that existed during different geological periods, with Pangaea being the most recent and comprising Laurasia in the northern hemisphere and Gondwana in the southern hemisphere.

The Hertzsprung–Russell diagram, or H–R diagram, is a plot of stellar luminosity against surface temperature.

Animals in a subarctic climate have to be well-adapted to the cold, with extra layers of fat and thick fur. Some of the world's largest hunters live within the Arctic Circle: bears and wolves, and smaller ones such as wolverines, red and arctic foxes, and lynxes. They hunt herd animals such as caribou, moose, elk, reindeer, musk oxen, mountain goats, Dall sheep and deer. There are no reptiles - whose ectothermic metabolism means they cannot adapt to the cold - but there are a host of small mammals: snowshoe hares, beavers, river otters, mice, rats, lemmings, voles and squirrels.