Why do Moon Phases Occur

Have you ever wondered why the Moon is not always fully illuminated as you look at it in the night sky? The answer lies in its perpetual and intriguing lunar phases.

The revolution of the Moon around the Earth, in partnership with the Earth’s revolution around the Sun, causes different parts of the Moon’s surface to be illuminated as seen from Earth. These differing shapes of illumination are referred to as the ‘phases’ of the Moon, which occur in a perpetual cycle over a 29.5-day (synodic) period.

There are eight main identifiable phases which follow each other sequentially as the Moon orbits the Earth according to the changing relative positions of the Earth, Moon and Sun. These phases cause a certain percentage of the Moon’s sunlit surface – note that half of the satellite’s surface is always fully illuminated by the Sun however – to be illuminated as seen from Earth, with a total sweep between 0 per cent and 100 per cent visible during the eight main phases due to different viewing geometries.

Indeed, to understand this process fully, it is the idea of viewing geometry that is key. The Moon does not appear only partially illuminated due to the Earth blocking light from the Sun, or casting a shadow (umbra) over it but merely due to the Earth’s respective viewing angle on the Moon’s sunlit surface. For example, during the first quarter phase, the Moon is positioned at a 90-degree angle with respect to the Earth, hence only 50 per cent of its half-illuminated side is visible on the planet. Equally, in the phases of new moon and full moon respectively, due to the three bodies’ approximate alignment (syzygies), that viewing angle is either 0 or 180 degrees, allowing either no or full illumination of the satellite’s sunlit side as seen from the Earth.

If the Moon both sits between the Earth and Sun while in a new moon phase, and on the opposite side of the Earth during a full moon phase, this logically raises the question: why don’t the Earth and Moon block the Sun’s light from falling on each other if they are in alignment? The answer rests on the fact that the plane of the Moon’s orbit around the Earth is inclined (tilted) by about five degrees with respect to the plane of the Earth’s orbit around the Sun (plane of the ecliptic). As such, while the bodies are in approximate alignment during these phases, the Moon typically is positioned just off a direct line, allowing illumination to still occur.

Interestingly, the aforementioned perfect alignment can occur on rare occasions due to the Moon’s elliptic orbit around the Earth, and the Earth’s elliptic orbit around the Sun. This leads both planet and satellite to experience changes in orbital velocity – faster at both their perihelion and aphelion – and cause the five-degree orbital plane offset to collapse at certain infrequent occasions. It is due to these rare events that solar and lunar eclipses occur.

Lunar eclipse

At specific times of the year, the Earth, Sun and Moon line up acutely, with the typical five-degree Earth-Sun orbital plane inclination removed. When the Moon blocks the Sun – or a proportion of it – we experience a solar eclipse on Earth, an event that always occurs during a new moon phase. This is a well-known occurrence. However, what many people don’t realise is that there can also be lunar eclipses. A lunar eclipse occurs when the Sun, Earth and Moon align acutely but do so when the Moon is in its full moon phase, with the Earth blocking the Sun’s illumination of its satellite due to the collapse of the Earth-Sun orbital plane’s inclination.

The moon phases

1. New moon – The start of the synodic period begins with a new moon, which occurs when the Moon is positioned between the Earth and the Sun in approximate alignment (five degrees off from the Earth-Sun orbital plane). The sunlit side is not visible from Earth.

2. Waxing crescent – As the Moon’s lunation begins from the new moon phase, gradually more of the sunlit surface of the Moon becomes visible from Earth. This is referred to as a waxing crescent as only a thin crescent-shape portion is visible.

3. First quarter – The first quarter phase occurs when the Moon is at a 90-degree angle with respect to both the Earth and the Sun. This leads to a perception of 50 per cent illumination of the Moon’s sunlit side from Earth.

4. Waxing gibbous – In the waxing gibbous phase gradually more and more of the Moon’s sunlit surface becomes visible, rising from 50 per cent through to a full moon’s 100 per cent. The illuminated portion is therefore a convex semi-circle.

5. Full moon – As with the new moon, here the Earth, Moon and Sun are in approximate alignment, however at this phase the Moon is on the opposite side of the Earth from the Sun and is therefore illuminated fully on the sunlit side as viewed from Earth.

6. Waning gibbous – Mirroring but reversing the waxing gibbous phase, the waning gibbous sees the sunlit portion of the Moon’s surface decrease from 100 per cent down to the third quarter phase’s 50 per cent.

7. Third quarter – As with the first quarter phase, here the Moon is once more at a 90-degree angle with respect to the Earth and the Sun. As before, approximately 50 per cent of its sunlit surface is illuminated as viewed from Earth.

8. Waning crescent – Just as an increasing crescent-shaped portion of the Moon’s sunlit side becomes visible after a new moon, after the third quarter’s 50 per cent illumination, less and less becomes visible – the crescent wanes (shrinks).