Sunset delay.

Get ready: October 30 marks the end of daylight saving time. But the fall ritual of turning the clocks back will soon be postponed by one week. The hope: That people will save energy by making better use of autumn's daylight hours.

Because Earth rotates on a tilted axis, the number of daylight hours on Earth varies during the year. When the planet's Northern Hemisphere tilts away from the sun--from September to March--the region experiences later sunrises, earlier sunsets, and fewer daylight hours. When the northern half of Earth tips toward the sun, it has more daylight hours.

In the early 1900s, officials and scientists said that people could save energy in the daylight-filled months if they set their clocks forward one hour. Why? When it's light out, you use less electricity to brighten your home. But early morning daylight is wasted because most people are asleep. So by "springing" clocks ahead, one energy-saving hour of sunlight shifts from the morning to the evening--when people are awake.

Hoping to save even more energy, the U.S. Congress recently passed a law extending daylight saving time starting in 2007. There's an added bonus, says David Prerau, a scientist who consulted with Congress on the new law: "Most people prefer to have the extra hour of evening light to do things after school or work."