This simulation brings together everything we’ve covered on previous pages. We’ve looked at the Moon’s orbit around Earth, which is fundamental to understanding how eclipses work. The second page in the series explains what nodes are; these nodes are key to understanding how eclipses work.
Now we can put it all together and see all the moving parts. With the simulation below, click on “Focus on Earth” and make sure the line of nodes checkbox is selected. Also, click the “show lunar perigee/apogee” checkbox and zoom in so you can see them clearly. If you speed up the time, you’ll see both of those lines move independently of each other. The line of nodes spins backwards (retrograde), and the line linking perigee and apogee goes the opposite way (prograde). This spinning of those two lines is what defines the Saros cycle and how eclipses are predicted.
Key Terms
Full Moon — The phase when Earth lies approximately between the Sun and Moon.
Solar eclipse — An eclipse that occurs when the Moon passes between the Sun and Earth and its shadow reaches Earth.
Lunar eclipse — An eclipse that occurs when the Moon passes through Earth’s shadow.
Eclipse season — A period when the Sun is close enough to the line of nodes for eclipses to occur.
Umbra — The darkest central part of a shadow, where the light source is completely blocked.
Penumbra — The outer part of a shadow, where the light source is only partly blocked.
Antumbra — The region beyond the end of the umbra where the blocking object appears smaller than the light source.
True anomaly — The angle that describes an object’s current position in its elliptical orbit, measured from periapsis.
Eccentricity — A measure of how much an orbit differs from a perfect circle.
Semimajor axis — Half the longest diameter of an elliptical orbit and a measure of its overall size.
Lunar latitude — The Moon’s angular distance north or south of the ecliptic.
Lunar libration — The apparent rocking of the Moon that allows slightly more than half of its surface to be seen from Earth over time.
Synodic month — The time from one New Moon to the next, about 29.53 days.
Sidereal month — The time the Moon takes to orbit Earth relative to the background stars, about 27.32 days.
Anomalistic month — The time from one lunar perigee to the next, about 27.55 days.
Draconic month — The time from one passage through a lunar node to the next, about 27.21 days.
Next Steps
As a bit of an extension we can have a look at plotting all of these cycles to see when and where eclipses occur.
