The Phases of the Moon: Concept Map

Concept Map

Organise the vocabulary, quantities and relationships into a connected model.

This extension applies that lens specifically to The Phases of the Moon.

At every moment, exactly half of the Moon is lit by the Sun — that never changes. What changes is how much of that lit half we can see from Earth, and that depends on where the Moon is in its orbit around us.

The cycle takes about 29.5 days and runs in this order:

1. New Moon — the Moon is between Earth and Sun, its lit half faces away from us, and we see nothing.
2. Waxing crescent — a thin sliver appears and grows ( waxing means growing ).
3. First quarter — we see half of the disc lit; the Moon has completed a quarter of its orbit.
4. Waxing gibbous — more than half lit, still growing.
5. Full Moon — Earth is between Sun and Moon, the whole lit half faces us.
6. Waning gibbous, 7. Last quarter, 8. Waning crescent — the lit part shrinks ( waning means shrinking ) back to New Moon.

The phase also tells you when to look: a Full Moon rises at about sunset and sets at about sunrise, while a New Moon is in the sky during the day, right beside the Sun, which is why it is impossible to see.

A southern-hemisphere detail worth knowing. Most textbook diagrams are drawn for the northern hemisphere, where a waxing Moon is lit on the right. Seen from South Africa we are, in effect, standing the other way up: here a waxing (growing) Moon is lit on the left, and a waning Moon is lit on the right. If you use a northern diagram to predict tonight's sky in Polokwane, you will get it backwards.

Worked example. There is a Full Moon on 3 March. When is the next Full Moon?

One full cycle is about 29.5 days. March has 31 days, so 3 March $+\ 29.5$ days lands about $32.5 - 31 = 1.5$ days into April — that is, about 1 April. This is also why the months of a calendar do not line up neatly with the Moon: 12 lunar cycles come to about 354 days, eleven days short of a year.