How often does an eclipse happen?
An eclipse occurs when one celestial object moves into the shadow of another. Here on earth, eclipses involve the sun and the earth’s moon and can occur only when they are nearly in a straight line. More directly, the moon orbits earth on a different plane than the earth orbits the sun, and eclipses can occur only when the moon is close to the intersection of these two planes, which happens twice a year. Eclipses can occur for about two months around these intersection times (called nodes to astronomers). There can be from four to seven eclipses in a calendar year, which repeat according to various eclipse cycles.
