Pictured here is a full moon nearing a total lunar eclipse on Dec. 21, 2010, just before dawn in the Shetland Islands of Scotland. A lunar eclipse occurs when a full moon is opposite the sun in the sky. But why do we not see a lunar eclipse each month with the full moon? The answer lies in orbits. The moon's orbit around Earth is tilted 5 degrees to the Earth's orbit around the sun, so a lunar eclipse can only happen when the sun is exactly aligned with the Earth and the moon. This happens two to four times a year. Still, lunar eclipses are more common than solar eclipses (when the moon is between the sun and the Earth) because the moon is much closer to the Earth than the sun.