Lessons · The sky · 3

The Moon: Phases, Nodes and Eclipses

Goal: After this lesson you will be able to tell the Moon’s phase from its appearance and explain when and why solar and lunar eclipses happen.

Why the Moon changes its face

The Moon does not shine on its own; it reflects sunlight. At any moment exactly half of it is lit. As the Moon orbits Earth, we see a changing portion of that lit half, and these changes are the phases. A full cycle from new Moon to new Moon takes about 29.5 days and is called the synodic month.

The phases in brief

At new Moon the Moon lies between Earth and the Sun, and the side facing us is dark. A few days later a thin crescent appears and grows. About a week after new Moon we see first quarter, a half disc. Then comes full Moon, when Earth is between the Sun and the Moon and the whole disc is lit. After that the Moon wanes through last quarter and becomes a crescent again.

A practical hint for the Northern Hemisphere: a waxing Moon is lit on the right, and a waning Moon on the left. A thin crescent is often visible low in the west in the evening or low in the east before dawn. Next to it you may sometimes notice a faint glow on the dark part. This is earthshine, sunlight reflected first by Earth and then by the Moon. Read more in Moon phases.

The tilted orbit and the nodes

The Moon’s orbit is tilted by about 5° to the ecliptic, the Sun’s apparent path across the sky. For that reason, at most new Moons the Moon’s shadow passes above or below Earth. At most full Moons the Moon passes above or below Earth’s shadow. No eclipse occurs.

The two points where the Moon’s orbit crosses the plane of the ecliptic are called lunar nodes. One is ascending and the other descending. The nodes slowly shift backwards along the ecliptic and complete a full circle in about 18.6 years. See Lunar nodes in astronomy for details.

When eclipses happen

An eclipse is possible when a new or full Moon coincides with being near a node.

• Solar eclipse: the Moon passes in front of the Sun at new Moon. A total eclipse is visible only along a narrow track on Earth, because the Moon and the Sun appear almost the same size in our sky. Outside that track the eclipse is partial. See Solar eclipse.
• Lunar eclipse: the Moon enters Earth’s shadow at full Moon. It often turns reddish, because light bent through Earth’s atmosphere still reaches it. See Lunar eclipse.

Eclipses come in seasons, roughly twice a year. They also repeat in a cycle called the saros, about 18 years and 11 days.

A note on safety: never look at the Sun with the naked eye, through ordinary sunglasses, binoculars or a telescope without a proper solar filter. You need eclipse glasses that meet the ISO 12312-2 standard. A lunar eclipse is completely safe to watch.

What the tradition says

In the astrological tradition the lunar nodes are called the North Node and the South Node. They are read as symbols of direction and of familiar habits, an invitation to reflect on where we wish to go. Eclipses are traditionally linked with themes of endings and new beginnings. This is a cultural interpretation, not science. Astronomy does not show that eclipses or nodes determine events in our lives. See Lunar nodes in astrology.

What to remember

Phases come from our viewpoint on the lit Moon. Eclipses need a precise alignment of the Sun, Earth and Moon near a node. That is why they happen only a few times a year rather than every month.

Exercise in the sky

1. 1. Open the Astronativa 3D sky and select the Moon. Note its phase today. 2. Move the date forward day by day for one week. Watch how the lit part changes and from which side it is lit. 3. Find the nearest new Moon and full Moon. Compare where the Moon is relative to the ecliptic and why there is usually no eclipse. 4. Go outside in the evening if the sky is clear. Decide whether the Moon is waxing or waning from the side that is lit. 5. Optionally, keep a notebook of your observations for a week.

Check yourself

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