The shape of Earth’s orbit becomes more or less oval (eccentricity), Earth wobbles as it spins (precession), and Earth's axis changes too (tilt). All these changes, over thousands of years, causes Earth's climate to change. Click on image for full size (14K GIF) Windows to the Universe

The 11-Year Solar Cycle
When the Sun has fewer sunspots, it gives off less energy, less energy makes its way to Earth, and our planet cools down. More than three hundred years ago, when the climate was cooler for a time called the "Little Ice Age", people noticed there were no sunspots for several decades. Over time, scientists have noticed a pattern in the number of sunspots. About every 11 years the number of sunspots reaches a high and then decreases again.

Milankovitch Cycles
Over thousands of years, changes in Earth's orbit cause changes in the amount of the Sun's energy that gets to the planet. Over the past several million years these changes have caused cycles of global warming and cooling.

There are three ways that Earth's orbit changes over time.

• Eccentricity: The shape of Earth's orbit around the Sun becomes slightly more and then less oval every 100,000 years.
• Precession: Earth wobbles on it axis as it spins, completing a full wobble every 23,000 years.
• Tilt: The angle of the Earth's axis relative to the plane of its orbit changes about three degrees every 41,000 years.

Once the Sun's energy reaches the Earth, several things can happen. The energy can be absorbed by the planet, reflected back into space, or become trapped in the atmosphere.