Three animations explaining the anomalistic, synodic, and draconic months. These were motivated by work on my Eclipse Metacycles project where I learned about repeating patterns of eclipses and the Saros.
Videos of the explanimations below. But first, some context.
Different Types of Eclipses #
Solar eclipses occur two to five times a year but not all eclipses are equal.
The moon orbits the earth in an ellipse so sometimes it’s closer and other times it’s farther. When the moon is closer to the earth, it looks bigger and it’s able to cover the sun completely, giving us a total eclipse. When the moon is farther it appears smaller and results in a ring-like annular eclipse.
The alignment of the sun, the earth, and the moon also affect the appearance of each solar eclipse. If the three bodies are not close enough to a straight line then the moon may only cover part of the sun, giving us a partial eclipse. The moon may appear to travel past the sun a little too high or a little too low rather than going straight across the middle.
Your view of the eclipse will depend on your location on earth. People in one region may see a total eclipse while neighbouring areas will only see a partial eclipse. Outside of the path of the eclipse the sun won’t be covered by the moon at all.
Repeating Eclipses #
Eclipses repeat on an 18-year cycle. If there was an eclipse today then we could predict that in approximately 18 years there will be another very similar eclipse. That 18-year period is known as a Saros.
The sun, the earth, and the moon move in repeating cycles. Some of these cycles are very familiar to us, such as the earth’s year-long journey around the sun or the moon’s month-long journey around the earth.
There are other less well-known cycles which affect the relative positions of the sun, the earth, and the moon. These cycles are all different ways of measuring a month: the anomalistic month, the synodic month, and the draconic month.
Together, these three cycles conspire to return the sun, the earth, and the moon to the same relative positions every 18 years.
The anomalistic month #
The moon’s path as it orbits the earth is an ellipse, and that ellipse is also slowly rotating. An anomalistic month is the time it takes the moon to go from, for example, the point where it’s closest to the earth—the perigee—and back. Since the ellipse rotates in the same direction as the moon’s orbit, the moon has to travel a little farther each month to return to the perigee.
This affects the size of the moon as seen from the earth because over the anomalistic month, the moon gets farther and closer. This can affect whether an eclipse is total or annular.
An anomalistic month is approximately 27.6 days long.
The synodic month #
Draw a line from the earth to the sun. A synodic month is the time it takes the moon to travel from that line, around the earth, and back to that line. Since the earth is moving around the sun at the same time, the moon has to travel a little farther to reach that line.
Eclipses only occur when the sun, earth, and moon are in a straight line. Once every synodic month when these three bodies are aligned there is an opportunity for an eclipse.
The synodic month is approximately 29.5 days long.
The draconic month #
The earth orbits the sun on a plane called the ecliptic plane. The moon also orbits the earth on a plane, however, these two planes are different. The moon’s orbit is tilted by about 5°. During its orbit, the moon will intersect with the ecliptic plane twice at the the orbital nodes. A draconic month is the time it takes the moon to travel from one node to the other and back.
The sun’s gravity causes these nodes to slowly rotate around the earth. They rotate in the opposite direction of the moon’s orbit so the moon doesn’t have to travel as far to return to the node.
Eclipses only occur when the moon is on the ecliptic plane, that is, when the moon is at one of the two orbital nodes.
The draconic month is approximately 27.2 days long.
The Saros #
The Saros is a period defined as exactly 223 synodic months. During that time, 242 draconic months and 239 anomalistic months will have elapsed, give or take a couple hours. All three types of month will have completed an integer number of cycles so the sun, earth, and moon will have returned to similar relative positions. That is why eclipses repeat every Saros.
Technical Details #
The animations were created in Processing. Nothing is to scale since the vastness of space doesn’t lend itself well to any kind of meaningful visual representation.
Learn More #
NASA has resources where you can learn more about Saros and Periodicity of Solar Eclipses.