The Moon's phases, and why it rises later each day

Earth's natural satellite

• A satellite is an object that travels in a path (an orbit) around a larger object.
• The Moon is Earth's only natural satellite. It is a roughly spherical ball of rock, about a quarter as wide as Earth.
• It takes about a month to go once around Earth, held in its orbit by gravity.
• Human-made satellites, such as weather and GPS satellites, also orbit Earth, but they are artificial.

Half the Moon is always lit

• The Moon makes no light of its own. We see it because sunlight bounces off it.
• At every moment, the half of the Moon facing the Sun is lit and the other half is dark, just like day and night on Earth.
• As the Moon orbits, we on Earth see different amounts of that lit half. This changing view is what we call the phases.
• When the Moon is between Earth and the Sun, its lit half faces away from us and we see almost nothing: a new moon. When it is on the far side of Earth from the Sun, we see the whole lit half: a full moon.
• The diagram looks down from far above the South Pole, with sunlight coming from the left. From there the Moon travels clockwise around Earth.

The cycle of phases from New Zealand

• After new moon, a thin crescent appears. The lit part grows each night: this is called waxing.
• About a week after new moon, half the disc is lit: the first quarter. Then comes a waxing gibbous moon (more than half lit), and then full moon.
• After full moon the lit part shrinks, called waning: waning gibbous, last quarter, waning crescent, and back to new.
• From New Zealand, a waxing Moon is lit on the left and a waning Moon on the right. In the Northern Hemisphere it looks the other way round.
• One full cycle, from new moon to the next new moon, takes about 29.5 days.

Try it: the Moon's phases from New Zealand

Why moonrise moves later

• Earth spins once a day, which makes the Sun, Moon and stars appear to rise in the east and set in the west.
• While Earth spins, the Moon is also moving along its orbit in the same direction.
• So after one full spin, Earth has to turn a little further to bring the Moon back into view. On average this takes about 50 minutes extra, so the Moon rises about 50 minutes later each day.
• That is why the phase and the time you can see the Moon go together.

Try it: waxing or waning?

Collecting your own Moon data

• Scientists find patterns by recording the same things again and again over a long time.
• A Moon diary records the date, the time, the shape of the Moon (draw it) and where it is in the sky, such as low in the east or high in the north.
• Over a few weeks you will see the phases change in order, and the Moon appear later each evening.
• Over several months you can check that the cycle really does repeat, and predict when the next full moon will be.

Try it: tonight's Moon

The maramataka

• The maramataka is a traditional Māori calendar based on the movements and phases of the Moon (te marama) and the stars (ngā whetū).
• Each night of the lunar month has its own name, linked to a particular phase of the Moon. Most iwi count 12 months in a year, each of about 30 nights.
• Maramataka differ between iwi and regions. Some iwi begin the month at the new moon, with the night called Whiro; others begin at the full moon, with Rākaunui.
• A local maramataka guides planting, fishing and gathering food, together with signs in the environment (tohu). To use one, find the maramataka for your own area, from your local iwi or marae.