Lunar Standstill Model

This model, designed by Dr. Erica Ellingson and modeled by Andrew Widner (me) at the University of Colorado is designed to illustrate a lunar standstill. It is designed for astronomy education, with teaching notes below, written by Dr. Erica. Printing and assembly instructions are attached.

Lunar Standstill Model Teaching Notes:

Hold the model so that the North/South axis is up/down (the handle will be slightly tilted). identify North and South directions and how objects can be seen in the sky farther or less far north.

This can be seen most easily by watching the rising or setting positions. Imagine a horizon (or use the real one) and point your hand in different directions to indicate “more north” and “more south” rise/set positions. This is an important set-up for what comes next.

Optional: The flat plane around the Earth is the plane where the Earth orbits the Sun each year. You can think of this as the apparent path of the Sun around our sky across the year, aka the ecliptic.

The Moon’s orbit is tilted from the plane of the Earth’s orbit by about 5 degrees- this is exaggerated in the model! Note also that the moon is MUCH closer to the Earth in the model than in reality. Hold the Moon’s orbit steady and move the moon around (counterclockwise as seen from the top) and note how it goes north and south each month (moonth).

Optional: you can demonstrate the phases of the Moon as seen from the Earth by identifying where the sunlight is coming from (use the real sun if you can!).

The Moon’s orbit “regresses” backwards around the Earth. Turn the moon’s orbit (clockwise as seen from the top). This cycle takes 18.6 years.

Ask your students: find the configuration when the Moon will be as far north as it can possibly go- this is the “northern lunar extreme” position that only happens during the “major lunar standstill season”. They should be able to figure out how to rotate the Moon’s orbit so that the orbital tilt aligns with the Earth’s N/S axis.

Run your finger around the Moon’s orbit to show how the northernmost extreme is followed by the southernmost extreme ½ month later. Sweep your hand horizontally “north and south” to indicate a wide and fast swing of the position of moonrise or moonset along the horizon from day to day.

Rotate the Moon’s orbit to the “minor lunar standstill” position, 9 years later. Re-identify North and South and show how the Moon now never goes very far north or south, in comparison. Sweep your hand along the horizon in a smaller range to indicate the swing of the moon on the minor standstill. Finish with demonstrating the pattern of the Moon’s monthly swing getting wider and then narrower and then wider over 18.6 years.

Optional: the “nodes” mark where the Moon and Sun orbits intersect and the technical term for this cycle is the “Regression of the nodes.” Eclipses happen when the Sun and Moon are both on the nodes. You can use the model to show that there are two eclipse seasons each year, each one having one lunar and one solar eclipse (and sometimes a third). The regression of the nodes means that the seasons are separated by slightly less than ½ year.