What Happens During a Solar or Lunar Eclipse?

Lunar Eclipses

• Lunar eclipses only occur during a Full Moon, when the earth is between the sun and moon. Penumbral lunar eclipse occurs when the moon passes through the earth's penumbral shadow. This is a partial or faint shadow, where the earth only blocks some of the sun's rays. They are very hard to observe. Partial lunar eclipses occur when part of the moon passes through the earth's umbral shadow. This is a dark inner shadow, where the earth blocks all the sun's rays. Total lunar eclipses occur when the entire moon passes through the earth's umbral shadow.

Frequency of Lunar Eclipses

• Lunar eclipses do not occur every month during the Full Moon. This is because the moon's orbit is tilted about five degrees from the earth's orbit around the sun. This causes most Full Moons to pass above or below the plane of the earth's solar orbit. However, the earth's shadow is always cast exactly on this plane. On average, a Full Moon occurs on this plane two to four times a year, resulting in an alignment of the moon, earth and sun that causes at least a portion of the moon to pass through the earth's shadow. Of these occasions, about 35 percent are penumbral, 30 percent are partial and 35 percent are total lunar eclipses.

Solar Eclipses

• Solar eclipses only occur during a New Moon, when the moon is between the earth and sun. Partial solar eclipses occur when the moon's faint outer penumbral shadow passes across the earth. Total solar eclipses occur when the moon's dark inner umbral shadow passes across the earth. The path of this umbral shadow across the earth's surface is called the Path of Totality. Unlike lunar eclipses, which can be observed anywhere on the night side of the earth, total solar eclipses can only be viewed from locations that fall within the Path of Totality. This shadow is only about 100 miles wide.

Frequency of Solar Eclipses

• Total solar eclipses occur everywhere on the earth, including at the poles. However, they only pass over the same location an average of once every 375 years. This is why observing one almost always requires traveling to a location predicted to be within the Path of Totality. Due to the moon's orbital tilt, there is only one total solar eclipse every year or two. On average, a total solar eclipse lasts only three minutes, whereas a lunar eclipse can last for hours.