An eclipse is the shadowing of one celestial body by another. In a lunar eclipse, the moon passes through the earth’s shadow. In a solar eclipse (referred to by some purists as an eclipse of the earth), the moon’s shadow touches the earth.
An eclipse occurs every time the earth, the moon, and the sun form a straight line. If the moon orbited the earth in exactly the same plane as the earth orbits the sun (the ecliptic) there would be an eclipse at every new and full moon. Since the orbit of the moon is tilted 5° from the ecliptic, the moon is too far north or south of the earth-sun line for eclipses to occur most of the time. Eclipses only occur when the moon is near the ecliptic when it is new or full. Since the nodes are the points where the moon’s orbit crosses the ecliptic, this means eclipses are possible only when the moon is near its nodes.
When the earth gets between the moon and the sun it casts a shadow considerably wider than the moon at the moon’s distance from earth. There is a darker central shadow called the umbra surrounded by a less complete shadow called the penumbra. The umbra’s diameter at the moon’s distance is 1.4° (compared to the moon’s 0.5°).
In a penumbral eclipse (also called an appulse) the moon goes through the penumbra without ever entering the umbra. Only a slight dimming can be noticed.
In a partial eclipse part of the moon’s surface passes through the umbra, so you can see a full moon with a bite taken out of it.
In a total eclipse the whole moon enters the umbra. The moon is not darkened to the point of complete invisibility, however, because the earth’s atmosphere refracts some light into the umbra. The refracted light turns the moon a deep coppery red during a total lunar eclipse.
Lunar eclipses are easy to observe because they can be seen anywhere in the whole hemisphere for which the moon is above the horizon at the time. There can be anywhere from 0 to 3 lunar eclipses in a given year.
When the moon gets between the earth and the sun, its shadow is not large enough to cover the earth. Only observers in the shadow path will see the sun eclipsed. The path of the umbra, the dark shadow producing a total eclipse, is even narrower than that of the penumbra (a few hundred kilometers is the widest it can get). Sometimes the umbra doesn’t touch earth at all, and there is only a partial eclipse.
Both the sun and the moon appear to have a diameter of about half a degree of arc. Since both the earth’s orbit around the sun and the moon’s orbit around the earth are elliptical, the apparent diameter of both sun and moon varies. When the sun is closer than average or the moon farther than average, the sun will appear larger than the moon. If an eclipse occurs at such a time, the moon’s umbra falls short of the earth and a total eclipse is impossible. If the moon moves across the center of the sun (rather than being displaced north or south) there is an annular eclipse with a ring of the sun still visible around the moon.
Of the three types of solar eclipse (total, annular, and partial), scientists are especially interested in total eclipses (for tests of relativity and coronal study especially). Since the path of the umbra on the earth is never very wide and it travels along the earth’s surface at about 3500 miles per hour, total eclipses don’t last long. Under the best of circumstances, with the moon as close as it can get (perigee) and the sun as far as it can get, a total eclipse can last 7 1/2 minutes for a stationary observer. Most eclipses don’t even last that long. Even so, people travel thousands of miles to observe total solar eclipses. The eclipse that just occurred on February 26, 1979 was the last total eclipse visible from the United States or Canada till 2017.
A note on timing. Astrologers generally set up eclipse charts for the moment of conjunction or opposition in longitude. When astronomers want a moment as the center or peak of an eclipse, they take the moment of conjunction in Right Ascension. The point of central eclipse occurs at local apparent noon for the point on the path of totality at the moment of the conjunction in Right Ascension.
Since eclipses can occur only when the moon is near its nodes, and eclipses are either conjunctions or oppositions of the sun and the moon, it follows that eclipses can only occur when the sun is near the moon’s nodes. This means that eclipses can only occur at certain times of year. Because of the backward motion of the moon’s nodes, eclipse seasons are a little under six months apart (the eclipse year—sun on node to sun on same node—is 346.6 days). The eclipse seasons move backward through the calendar year with the 18.6 year cycle of the lunar nodes. The eclipse season goes from 19 days before the sun crosses a node to 19 days after. During that time, the sun and moon will be close enough in latitude for an eclipse to occur at New Moon. Since the season is more than a lunar synodic cycle in length, at least one New Moon (and therefore eclipse) must occur during the season (and two can occur). If the New Moon is within 11 days of the sun’s node crossing, the eclipse will be central (either total or annular). If a Full Moon occurs within 13 days of the sun’s node crossing, the sun and moon will be close enough together in latitude for a lunar eclipse to occur. Since this period is shorter than a lunar synodic cycle, a lunar eclipse doesn’t have to occur every season. Since the eclipse year is shorter than the calendar year, it is possible to have two and a fraction eclipse seasons in one year. So there can be 0 to 3 lunar and 2 to 5 solar eclipses in a year.
Since eclipses occur at New and Full Moons near node crossings, two cycles are involved. The synodic month of 29.53 days is the time from New Moon to New Moon. The draconic month of 27.21 days is the time from the moon being on a node to the moon back at the same node (the name comes from the old designation of the nodes as dragon’s head and tail). The interaction of these two periods produces the saros cycle. One saros equals 223 lunations equals 241.99 draconic months equals 6585.32 days equals 18 years and 11 1/3 days (or 10 1/3 days—depending on how many leap years there have been). Eclipses one saros apart are usually very similar. Since one saros is also 238.99 anomalistic months (perigee to perigee), even the apparent sizes of sun and moon will be very similar. Because of the 1/3 day in the length of a saros, eclipses move about 120° west on the earth from one cycle to the next. Longitudes repeat after three saroses, or 54 years and about a month. Ancient recognition of these cycles helped make eclipse prediction possible.
I leave it to others how eclipses are to be interpreted. Perhaps they are just one more astrological inkblot.