Eclipses are astronomical events during which light from the sun or reflected light from the moon is blocked from reaching the earth, causing tempo­rary darkness. A solar eclipse occurs when the earth and the sun are in alignment with the moon positioned between them, and the moon casts its shadow on the earth. A lunar eclipse occurs when the moon and the sun are aligned with the earth positioned between them. In this case, the earth casts a shadow onto the moon. The timing of solar and lunar eclipses can be determined based on astronomers’ knowledge of the orbits of the earth and the moon.

Periodicity of Eclipses

The earth completes an orbit of the sun in approx­imately 365 days, and the moon completes one orbit around the earth in 29.5 days. Both orbits are elliptical, but the moon’s orbit is more elongated and the plane of its orbit is tipped by an angle of 5° from the plane of the earth’s orbit. This con­figuration means that the moon is closer to Earth at some times during the year, and for most of the year, the moon is slightly above or below the sun as viewed from Earth. The nodes are the two points where the two orbits’ planes intersect. Solar eclipses occur when the moon nears the node closest to the sun and comes into alignment between the sun and the earth. Lunar eclipses occur when the moon nears the node farthest from the sun, and Earth is aligned in between the sun and the moon.

Astronomers can predict eclipses forward and backward in time based on knowledge of the earth’s and moon’s orbits. In the 1880s the Viennese astronomer Theodor von Oppolzer cal­culated all of the eclipses that did and will occur between 1208 BCE and 2161 CE. Based on his calculations, solar eclipses occur 238 times per century and lunar eclipses are less frequent at 154 times per century. With current knowledge, astron­omers have determined that the minimum number of eclipses per year is two solar eclipses, and the maximum number is seven eclipses with the ratio of solar to lunar eclipses being 5:2 or 4:3.

Another pattern in eclipse frequency is the saros series, which was first identified by the Babylonians. Any one solar eclipse has a specific geometric arrangement between the sun, the moon, and the earth, which determines its length and the location on Earth where it is visible. That precise arrangement repeats itself every 18 years and 11 1/3 days, meaning a solar eclipse of the same length results. However, the location is dif­ferent because Earth has rotated for an additional 1/3 of a day. The eclipse will be at approximately the same latitude but will occur 120° west of where the solar eclipse did 18 years and 11 1/3 days earlier.

Solar Eclipses

There are three types of solar eclipses: total, partial, and annular. The first two depend on where an observer is in relation to the moon’s shadow, and the last depends on the moon’s distance from the sun. During an eclipse, the moon casts a total shadow, or umbra, over the earth in an area that is 60 to 100 miles in diameter. The penumbra, the
area of partial shadow, encircles the umbra and has a much larger diameter of 4,000 to 4,500 miles. A person observes a total eclipse only when located within the umbra and when the moon is at one of its closest points to Earth. The moon appears to completely blot out the sun in this case, turning day to night. While the sun is about 400 times bigger in diameter than the moon, it is also 400 times farther from the earth than the moon; thus the perception is created that the sun and the moon are the same size. The frequency of total solar eclipses is one per

1. 5 years, and the longest recorded period of total darkness is slightly more than 7 minutes. Although solar eclipses happen every year, a total eclipse can be observed in any one particular location on the northern hemisphere approximately only once every 330 years, and in the southern hemisphere only once every 540 years.

An observer sees a partial solar eclipse when located in the penumbra of the moon’s shadow. The moon appears as a black disk moving over only part of the sun, so a partial solar eclipse does not have the dramatic darkness characteristic of a total eclipse. However, many more people can observe a partial eclipse because of the greater diameter of the penumbra. The frequency of par­tial solar eclipses is approximately 84 per century, or one partial eclipse per 1.2 years.

An annular eclipse occurs when the moon is aligned between the earth and sun and is approaching or leaving its apogee, the point in its orbit that is farthest from the earth. Then the moon appears to cover the sun only partially because it is closer to the much larger sun than it is during a total eclipse. When the moon and sun line up during an annular eclipse, the moon appears as a dark disk centered over the sun and ringed by its bright light.

Lunar Eclipses

Lunar eclipses also can be total or partial depend­ing on whether the moon passes through the earth’s umbra or penumbra, respectively. The frequency of lunar eclipses is 1.5 eclipses per year. They are less frequent than solar eclipses at 2.4 per year because the moon is a smaller target for the earth’s shadow to hit than vice versa. However, lunar eclipses have a longer duration than solar eclipses, lasting up to several hours, including total coverage of the moon for as long as 1.5 hours. This phenomenon occurs because the earth’s shadow is much larger than the moon’s, therefore it takes the moon longer to traverse the shadow than the earth does to cross the moon’s shadow.

Erin M. O’Toole