Moon Phases and Eclipses

The Moon does not make its own light. We see it only because it reflects sunlight, just as a tennis ball lights up when you shine a torch on it. Here is the key idea: the Sun always lights up exactly half of the Moon, the half facing the Sun. That never changes. What changes is how much of that lit half we can see from Earth as the Moon travels around us. This is the idea in the NSW syllabus dot point you are learning today (SC4-OTU-01).

The Moon takes about 29.5 days to go once around Earth, which is roughly one month. As it moves, the angle between the Sun, the Moon and us keeps changing. Sometimes we see the whole lit half (a full moon), sometimes only a thin sliver (a crescent), and sometimes none of the lit side at all (a new moon). The shape we see is called a phase.

Bust the myth: a lot of people think the phases are caused by Earth's shadow falling on the Moon. That is not true. The phases are caused by the changing relative positions of the Sun, the Earth and the Moon, which change how much of the lit half points towards us. Earth's shadow only matters during a lunar eclipse, which is a different and much rarer event you will meet later in this lesson.

The phases follow the same pattern every month, like clockwork. We give the eight main phases names, in this order:

• New moon: the Moon is between us and the Sun, so its lit half faces away from us and we see almost nothing.
• Waxing crescent: a thin sliver of light appears and starts to grow.
• First quarter: we see exactly half of the lit side.
• Waxing gibbous: more than half is lit and still growing.
• Full moon: the whole lit half faces us, so we see a full bright circle.
• Waning gibbous: the lit part starts to shrink.
• Last quarter: we again see half, but the other half from before.
• Waning crescent: only a thin sliver is left before the cycle starts again.

Waxing means the lit part is growing, on the way to full moon. Waning means it is shrinking, on the way back to new moon. The whole cycle takes about 29.5 days, then it repeats. This cyclic pattern is exactly what the diagram below shows: the Moon at eight points around Earth, with sunlight coming from one side.

Scientists often use a physical model, a real object you can move and look at, to explain something we cannot easily see in space. The syllabus asks you to use a physical model or a virtual simulation to explain the cyclic pattern of phases and eclipses. Here is a model you can build with simple gear.

What you need: a bright lamp (this is the Sun), a large ball such as a basketball (this is Earth), and a small ball such as a tennis ball on a stick (this is the Moon). Work in a darkened room.

• To show phases: stand the lamp at one side of the room. Hold the small Moon ball at arm's length and slowly turn yourself in a full circle, keeping the ball up. Watch the lit part of the ball you can see. It grows from nothing, to a crescent, to half, to full, then shrinks again. You have reproduced the whole cyclic pattern of phases.
• To show a solar eclipse: line up the small Moon ball directly between your eye and the lamp. The little ball blocks the lamp and casts a tiny shadow on you. In space, the Moon blocks the Sun and casts a shadow on part of Earth.
• To show a lunar eclipse: put the large Earth ball between the lamp and the small Moon ball. Earth's shadow falls across the Moon ball, hiding its light. In space, Earth's shadow falls on the Moon.

A model like this is powerful because it lets you change the positions of the Sun, Earth and Moon with your own hands and see exactly what happens. That is how a model helps explain a pattern we could never stand back and watch from space.

An eclipse happens when the Sun, Earth and Moon line up almost perfectly so that one casts a shadow on another. There are two kinds, and it is important not to mix them up.

• Solar eclipse: the Moon passes between the Sun and Earth, so the Moon's shadow falls on part of Earth. For a short time the Moon blocks the Sun from view. A solar eclipse can only happen at new moon, because that is the only time the Moon is on the Sun's side of Earth.
• Lunar eclipse: the Earth passes between the Sun and the Moon, so Earth's shadow falls on the Moon. The Moon often turns a dim red colour. A lunar eclipse can only happen at full moon, because that is the only time the Moon is on the far side of Earth from the Sun.

So why is there not an eclipse every month? You might expect a solar eclipse at every new moon and a lunar eclipse at every full moon. The reason we do not is that the Moon's orbit is tilted about 5 degrees compared with Earth's path around the Sun. Most months the Moon passes a little above or a little below the exact line, so the three bodies do not line up perfectly and no shadow lands where we are. Only a few times a year does everything line up well enough for an eclipse.

Safety, very important: never look directly at the Sun, even during a solar eclipse. The Sun's light can permanently damage your eyes. To watch a solar eclipse safely you must use special certified eclipse glasses or project the image with a pinhole. A lunar eclipse is safe to watch with the naked eye, because you are only looking at the dim Moon.

Australia gets some wonderful views of eclipses. On 20 April 2023, a rare total solar eclipse crossed the Exmouth area in Western Australia, and crowds travelled there to see day briefly turn to twilight. Total lunar eclipses, sometimes called a "blood moon" because the Moon glows dim red, are seen across large parts of Australia and are completely safe to watch with the naked eye.

Because the pattern of phases and eclipses is so regular, astronomers can predict eclipses years in advance. This is the same predictable Sun, Earth and Moon system you have been studying. The Bureau of Meteorology and observatories such as Sydney Observatory publish the dates so people can plan to watch.

Remember the safety rule one more time: a lunar eclipse is safe to view directly, but for a solar eclipse you must use certified eclipse glasses or a pinhole projector, and never look straight at the Sun.