Science in Focus: Shedding Light: Highlights
Since its formation 4.5 billion years ago the Earth has undergone dramatic climatic changes. For most of its history, the Earth was considerably warmer than it is now. But beginning about 3 million years ago, the Earth became cooler. In the last 2 million years, there have been seven ice ages. During these more than 30 percent of the Earth was covered by ice. The most recent major ice age occurred about 18,000 years ago. At that time ice sheets up to 3 km thick covered most of North America, all of Scandinavia, the northern part of Britain and the Ural Mountains. In the Southern Hemisphere, much of New Zealand, Australia, Argentina and South Africa were under ice.
About 12,000 years ago, warming began and by 5000 BC most of the glaciers had melted. This resulted in a rapid rise in sea levels. At this time Britain was cut off by water from the rest of Europe.
- Ice Ages
http://www.historyoftheuniverse.com/iceage.htmlFrom the History of the Universe
- Ice Ages
http://www.museum.state.il.us/exhibits/ice_ages/From the Illinois State Museum Exhibits
More light energy is absorbed by Earth when the Sun is high in the sky (about June 22) and less is absorbed when it is low ( about December 22). Because the Earth is a sphere, its surface is heated unevenly. The Sun is high in the sky, all year, over the area of the Earth between the tropics. It is low in the sky all year over the area of the Earth near the Arctic and Antarctic Circles.
Geometry of a Sphere
The Arctic Circle describes the points on a rotating Earth where rays of sunlight just miss and continue into space. On the winter solstice at the North Pole, the Sun never rises above the horizon. At the summer solstice, the Sun never sets. The noon Sun is directly overhead on the tropic of Cancer at the summer solstice and directly overhead on the tropic of Capricorn at the winter solstice. The noon Sun is directly overhead on the equator at each equinox.