Trace the Moon's phases, eclipses and the seasons from space and from a pin on Earth, to see how one tilted, turning system makes all three.
Model details
Both orbits are drawn as circles. Earth’s very nearly is one, and a visibly stretched orbit would teach that the seasons come from distance. The cost is that the equinox and solstice dates land within about two days of the real ones.
The Moon’s orbit is tilted 5.145 degrees to Earth’s, and its two crossing points slide backwards around the year. Nothing else is needed to make eclipses as rare as they are.
An eclipse of the Sun is declared when the Moon comes within 1.41 degrees of Earth’s orbit plane at a new moon. An eclipse of the Moon needs 0.92 degrees at a full moon. Over ten simulated years the model produces about 22 of the first and 13 of the second.
The space view’s sizes and distances are wrong on purpose, and the button beside Not drawn to scale says by how much. In Eclipses, which is drawn at true scale, the Sun is given its real angular size so the Moon can cover it.
Sunrise and sunset are counted geometrically, when the centre of the Sun crosses the horizon. Air bending light near the horizon adds a few minutes to every real day and is not modelled. The clock reads local mean solar time at the pin.
How do I explore?
- Pick a pattern. Each one sets up both views for you.
- The big picture and the card beside it are one moment, seen from two places.
- When you pick a pattern, a ring pulses round the one body you can take hold of. Drag that body to wind the clock. It is the Moon in Phases and Eclipses, and Earth in Seasons.
- Press one of the latitude buttons to stand somewhere else on Earth. Phases has none, because the phase looks the same from everywhere. In Eclipses you always stand where the Moon is up.
- The two arrow buttons under the date run the clock backward and forward.
What should I do/notice?
- In Phases, run one month. Half the Moon is always lit, so what is changing?
- The pale blue dot and its stalk on the Moon face Earth all month. So the Moon turns once as it goes round once.
- In Seasons, watch Earth from the side. The lit half never moves, but the axis leans through it.
- Then press a different latitude and watch the daylight curve change shape.
- The Moon passes the Sun every month, but eclipses are rare. The tilted ring shows why.
- Earth’s axis always points at the same star. Check it in June and in December.
- Press 66.6 north and stop the clock on 21 June. The Sun circles the sky without setting.
Key moments
The first eclipse of the Sun and the first of the Moon stop the clock and wait. In Eclipses the clock stops for every eclipse, not just the first. Press Continue when you are ready, or Watch the shadow to see it in Eclipses.
What this model leaves out
Both orbits are drawn as circles, because they very nearly are. So there are no supermoons here, and no ring eclipses.
The equinox and solstice dates can be two days out. Use an almanac for tonight’s real sky.
In the sky view the Sun and the Moon are drawn four times wider than they look.
The space view’s sizes and distances are wrong on purpose. The button beside Not drawn to scale says by how much.