Eclipse Paths caused by the Moon's shadow sweeping across the Earth are very narrow but very long. The animation in Fig. 4 shows an example of the path of the Moon's umbral and penumbral shadows during a total solar eclipse. The eclipse is that of 2017 August 21, the next total eclipse that will occur in the United States. (See Prolonged Eclipse Drought for North America below.) Upper right corner shows the Universal Time (Greenwich Civil Time) as the animation runs. Lower right corner shows instantaneous duration of the total eclipse.

Fig. 4. 2017 August 21 Total Solar Eclipse Path Animation. Path of totality sweeps the entire width of the USA. Maximum duration of totality is 2m40s in Southern Illinois. (Animation from F. Espenak from a program by British astronomer Andrew Sinclair)

The penumbra appears as a large grayish region thousands of kilometers or miles in diameter sweeping across the Earth from west to east. Everyone within the penumbra's path sees a partial eclipse of the Sun. Outside the penumbral path, no eclipse is visible.

The Moon's umbral shadow appears as a very small black dot (typically only a few hundred kilometers or miles wide) at the center of the penumbra. The dot moves quickly so look carefully since the Moon's shadow moves rapidly across the Earth's surface at velocities of several thousand kilometers or miles per hour. Only those within the narrow umbral path see a total eclipse, which usually lasts for only a few minutes at a given location.

The moving dark blue area shows the nighttime areas of the Earth. From start to finish, the penumbra can take more than five hours to sweep across the Earth.

Short Movie of Lunar Shadow View a short movie of the Moon's shadow sweeping across lower Baja, Centeral American and Northern Venezuela during the 1991 July 11 total solar eclipse! We produced the movie from GOES-7 Satellite images. (See National Optical Astronomy Observatories.)

Frequency of Eclipses As the Moon orbits the Earth, the Sun, Earth and Moon approximately line up about every 29.5 days. At this time the Moon is either at New Phase (when the Moon is between the Earth and Sun) or Full Phase (when the Earth is between the Moon and Sun). However, the Moon usually does not pass in front of the Sun (causing a solar eclipse), nor does the Moon pass into the Earth's shadow (causing a lunar eclipse) since the Moon's orbit is inclined about 5 degree to the plane of the Earth's orbit around the Sun.

Solar and lunar eclipses can only occur when the Moon is near (±18°) one of the two points where the Moon's and Earth's orbits cross (called nodes). This allows the Moon to pass at least partially in front of the Sun (a solar eclipse), or at least partially move into the Earth's shadow (a lunar eclipse). That is, for eclipses to occur the Moon must be at New or Full Phase and the line of nodes must point toward Sun (points 1 or 3 in Fig. 5). Otherwise the Moon will not lie in the plane of the Earth's orbit and eclipses will not be possible (such as point 2 in Fig. 5).

Fig. 5. Lunar Nodes and Eclipses. The Moon's orbit is inclined about 5° to the plane of the Earth's orbit around the Sun.

Every six months these nodes line up with the Sun's direction and solar eclipses become possible at the time of New Moon. Likewise lunar eclipses become possible at the time of Full Moon. These two intervals of time when solar and lunar eclipses are possible are called eclipse seasons. Because the Moon's orbit slowly rotates in space, the nodes regress slowly with a period of approximately 18.6 years causing the two eclipse seasons to occur about 19 days earlier each year.

Currently (2005) the eclipse seasons are occuring shortly after the times of the vernal and autumnal equinoxes (April and October).

The Number of Eclipses Per Calendar Year Thus, each calendar year usually has four eclipses (the minimum number), two of the Sun and two of the Moon (as in both 2004 and 2005). These eclipses occur as solar-lunar eclipse pairs, each eclipse pair separated by about six months. However, sometimes there may be as many as seven solar or lunar eclipses of some type (as in 1982). This can happen if the Moon can move from New back to New (or Full back to Full) before the Earth-Moon direction gets too out-of-line with the Sun to prohibit an eclipse. For examples of some yearly eclipse frequencies, see Table 2.

Note: The number of lunar eclipses per calendar year can vary from two to five if penumbral lunar eclipses are counted. (In a penumbral lunar eclipse, the Moon only passes through the lighter, penumbra or outer part of the Earth's shadow.) Since penumbral eclipses are often inconspicuous, the number of conspicuous (i.e., umbral) lunar eclipses can range from zero to three each calendar year. The number of solar eclipses possible each calendar year of all types is also two to five. Counting only annular and total solar eclipses, the number is again none to three as with umbral lunar eclipses.

The Saros Eclipses occur in families. The Saros cycle is a period of about 6,585.3 days (18 years 11 days 8 hours). Two eclipses separated by one Saros cycle have similar geometry (similar duration, same time of year, etc.).

The periodicity and recurrences of solar eclipses as governed by the Saros is useful for organizing eclipses into families. A typical Saros series lasts about 12 to 13 centuries and contains 70 or more eclipses.

Prospects of Seeing Eclipses Solar eclipses are thus more numerous than observable lunar eclipse. But more people see lunar eclipses than annual or total solar eclipses! Why? Since penumbral lunar eclipses are inconspicuous, the number of observable solar eclipses (partial, annual or total) over a period of time typically outnumbers observable umbral lunar eclipses (partial or total) by about three to two. However, lunar eclipses are more common for any one location on Earth as long as the Moon is above your horizon at the time of the eclipse. That is, about one half of the Earth sees the lunar eclipse.