Eclipses

Why eclipses cluster near the nodes

At every new moon the Moon passes between Earth and the Sun, and at every full moon Earth passes between the Sun and the Moon. So you might reasonably expect a solar eclipse every new moon and a lunar eclipse every full moon. We do not get them, and the reason is a small tilt. The Moon's orbit is tipped about 5 degrees out of the plane in which Earth circles the Sun, the plane astronomers call the ecliptic. Five degrees sounds tiny, but it is roughly ten times the apparent width of the Moon itself. So at most new moons the Moon rides a little above or below the Sun and its shadow misses Earth entirely, shooting off into space; at most full moons the Moon passes above or below Earth's shadow and sails on untouched. The alignment is just slightly off.

A tilted circle and a flat circle can only meet at two points, opposite each other. For the Moon's orbit those two crossings are the lunar nodes, the only places where the Moon sits exactly in the ecliptic, level with the Sun and Earth. An eclipse therefore needs two conditions at the same instant: the Moon must be new or full, and it must be passing through a node. You can watch the rule at work in the view above. Set the time of year so the line of nodes points toward the Sun, then move the Moon through its month, and you get a solar eclipse at new moon and a lunar eclipse at full moon. Now turn the nodes away from the Sun, and a whole month slides by with no eclipse, because the new and full moons fall where the orbit is tilted out of line.

Solar versus lunar

• A solar eclipse happens at new moon, when the Moon slips between Earth and the Sun and casts its shadow onto us. Because that shadow is small, the eclipse is visible only from the narrow track it sweeps across Earth's surface.
• A lunar eclipse happens at full moon, when Earth lies between the Sun and the Moon and the Moon drifts through Earth's shadow. It often glows a deep coppery red, lit only by sunlight bent through the edges of Earth's atmosphere, and it can be seen from the entire night side of Earth at once.

Solar eclipses come in three kinds: total, annular, and partial. Whether a central eclipse turns out total or annular comes down to distance. The Moon's orbit is slightly oval, so its distance from Earth, and with it the Moon's apparent size, changes through the month. When the Moon is near its closest point it looks just large enough to cover the Sun completely, and we get a total eclipse. When it is near its farthest point its disk is a touch too small, and a brilliant ring of sunlight is left showing all around it, an annular eclipse (from anulus, Latin for ring). The month-to-month change in distance comes simply from the orbit being an ellipse: the Moon runs from its closest point to its farthest and back over about 27.5 days. The long axis of that ellipse also turns slowly, all the way around about every 8.85 years, a drift called the orbit's apsidal precession, which shifts the longer-term pattern of where total and annular eclipses fall.

Eclipse seasons and the Saros

Because an eclipse needs the line of nodes aimed roughly at the Sun, eclipses do not scatter evenly through the year. They cluster into eclipse seasons, windows about a month long that come around every 173 days, a little under six months apart. Meanwhile the nodes themselves slowly swing backward around the sky (the subject of the lunar nodes), which nudges each eclipse season a little earlier from one year to the next. A longer rhythm is hidden here too. Wait about 18 years and 11 days, a span the ancient astronomers called the Saros, and the Sun, the Moon, and a node return to almost exactly the same positions, producing an eclipse that is nearly a twin of the one before. You can find the Saros, the eclipse year, and the nodal cycle on the cycles by length page.

Look up real eclipses

To see exactly when and where eclipses have happened and will happen, NASA keeps a complete catalog going back thousands of years and forward for centuries: NASA Eclipse Web Site.