It may seem odd, but when it's winter in the Northern Hemisphere, Earth is closest to the sun. The moon, on the other hand, is not far from the Earth, yet its temperatures drop so low you need a space suit to survive there. Solar radiation alone doesn't determine how hot or cold a planet gets. Several fortunate factors help keep Earth from getting too hot or too cold to sustain life.

Listen to a debate about climate change, and you may hear the phrase "greenhouse effect." While it's true that greenhouse gases cause warming, those gases help keep Earth from getting too cold. When solar energy strikes the planet during the day, the ground, highways and other objects get hot and absorb that energy. As the sun goes down, the Earth cools by giving off infrared radiation. Because greenhouse gases absorb part of this radiation, the atmosphere warms and keeps the Earth from getting too cold.

Gases that produce the greenhouse effect include nitrous oxide, methane and carbon dioxide, though the latter is the one that environmentalists study most intensely. The U.S. Environmental Protection Agency reports that since around 1750, "human activities have contributed substantially to climate change by adding CO2 and other heat-trapping gases to the atmosphere." But natural processes such as volcanic eruptions also contribute to the atmosphere's carbon dioxide concentrations. Venus' smoldering temperatures are one example of how large amounts of CO2 can raise a planet's temperature. The moon has incredibly low temperatures because it has no atmosphere or greenhouse gases to protect it.

Methane contributes to about 30 percent of the greenhouse effect, while nitrous oxide contributes 4.9 percent. Water vapor is also a greenhouse gas, and increased amounts of it help warm the atmosphere. Water vapor occurs when water on the Earth warms and changes to a gas. Eventually, it returns to the ground in the form of liquid water.

When astronomers search for planets that could sustain life, they look for those that lie in the "habitable zone." This is a region near a star where liquid water can exist. Earth lies within the habitable zone that's not too close to the sun and not too far away. Pluto, for instance, is too far away from the sun to have liquid water or sustain life.

Earth's climate adjusts itself so that energy coming in from the sun balances with energy leaving the planet. Reflection and emission help keep the planet from getting too hot. Reflection occurs when parts of the Earth reflect solar energy into space. Clouds, which have white surfaces, reflect significant amounts of energy and help cool the planet. Thick clouds at lower altitudes reflect more solar energy than thinner clouds in the upper atmosphere.