A look ahead to the rest of 2007: This year there are a total of four eclipses. They started with a total lunar eclipse of the Moon on March 3. The end of this eclipse was visible from North Carolina; the Moon was in eclipse as it rose. About two weeks later, on March 19, there was a partial eclipse of the Sun visible from eastern Asia and parts of northern Alaska. Following a gap of almost six months we have another total lunar eclipse on August 28 which begins in the wee hours of the morning such that the Moon will be in totality as it sets. Finally, there is another partial solar eclipse on September 11 visible from parts of South America, Antarctica and the South Atlantic.

Now, let’s analyze the above data and see if we can find a pattern. We note that there are two pairs of eclipses separated by a little bit less than six months. Is this just coincidence or is there a physical explanation for it? There is, of course, a reason. Let’s first review the mechanism of eclipses. The Moon orbits the Earth approximately every 29½ days with respect to the Sun. Thus, full moons are 29½ days apart as are new moons. We call these alignments of the Earth, Sun and Moon syzygy. The Moon’s orbit is tilted just over 5° to the ecliptic, the plane of the Earth’s orbit. Thus, there are two places, or nodes, where the Moon’s orbit crosses the ecliptic, the ascending node where the Moon is moving up from below the ecliptic and the descending node where it is moving downward. At new moon the Moon passes between the Earth and the Sun…sort of. An eclipse occurs only if the Moon is close to the ecliptic, i.e., close to a node in its orbit. Otherwise, it will be above or below the Sun as viewed from the Earth and there will be no eclipse. Likewise, a lunar eclipse occurs when the Moon moves into the Earth’s shadow which also lies in the ecliptic. This occurs only at full moon and only if the Moon is close to a node; otherwise we have a full moon without an eclipse since the Moon passes above or below the Earth’s shadow in space.

If we draw a line between the ascending and descending nodes of the Moon’s orbit, we call this the line of nodes. As the Earth makes its annual journey around the Sun, the line of nodes lines up with the Earth-Sun line twice a year. Thus, we have two times during the year when we can have eclipses, i.e., two eclipse seasons. In other words, the Moon passes through each node of its orbit once every 29½ days but, unless it is near syzygy, there will be no eclipse. Solar eclipses occur if new moon occurs during a 36-day period centered on the Moon’s passage through either node. Since the month is only 29½ days long, there must be a solar eclipse in each eclipse season! There could be two! Lunar eclipses occur if full moon occurs during a 24-day period around a node passage. Thus, there could be one and only one lunar eclipse per eclipse season and there could be an eclipse season with no lunar eclipses. Since there are two eclipse seasons per calendar year, we can have as many as four solar eclipses and as many as two lunar eclipses. We have to have at least two solar eclipses per year. We do not have to have even one lunar eclipse.

But, go back a few years. In 1982 there were four solar eclipses and three lunar eclipses! In view of the above, that is not possible. But there’s a fly in the ointment. Each year the line of nodes of the Moon’s orbit slips backwards about 19°. This means the intervals between eclipse seasons are about nine days short of six months. Thus, it is possible to have parts of three eclipse seasons in a calendar year (early January, mid-year and late December). The upshot of that is that we could possibly have a total of seven eclipses in a given year, either five solar and two lunar or, like 1982, four solar and three lunar. The geometry of the situation does not permit five solar and three lunar eclipses in a calendar year.

So, in 2007, we observe two eclipse seasons with a lunar and a solar eclipse in each.

References:
Observer’s Handbook 2007, The Royal Astronomical Society of Canada
HM Nautical Almanac Office, http://www.eclipse.org.uk/eclbin/query.cgi