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The phases of the Moon

The Moon's appearance changes as it moves in its orbit around the Earth. The cycle is referred to as the phases of the Moon.

The phases of the Moon
The phases of the Moon. The outer circle shows how the appearance of the Moon changes with its position relative to the Earth and the Sun.

New Moon occurs when the Moon is between the Earth and Sun (position 1 in the above diagram). At this time the Moon is in silhouette for observers on the Earth. The Earth-facing hemisphere is in darkness and the Moon is close to the Sun in the sky. The Earth-facing hemisphere is fully illuminated and the Moon is at its highest point at local midnight at Full Moon. At this time the Earth lies between the Moon and the Sun (position 5).

Between New Moon and Full Moon, the Moon's phase is waxing as the illuminated area we see from Earth increases. Between Full Moon and New Moon, the Moon's phase is waning as the illuminated area we see from Earth decreases.

The difference between a synodic and sidereal lunar month
The difference between a synodic and sidereal lunar month
The Moon completes one orbit around the Earth with respect to the background stars - a sidereal month - in 27.3 days.

However the Earth has moved around the Sun in that time, so the length of time for the Moon to return to the same position with respect to the Earth and Sun in 29.5 days. This is a synodic month - it is the interval between two successive same lunar phases.


The tides

Tides result from the gravitational pull of the Moon and Sun on the Earth's oceans. As the Earth rotates, each point on its surface passes through two high tides and low tides each day.

The Moon's influence on the oceans of the Earth
The Moon's influence on the oceans of the Earth

The effect of the Sun's gravity on the oceans is only half that of the Moon. However, when they are aligned (every 14.5 days) their pulls reinforce to create spring tides, when the tidal range is greatest.

The formation of spring tides
The formation of spring tides

Similarly, when the Sun and Moon are at right angles to the Earth, their pulls offset each other and neap tides result. At this time the tidal range is at a minimum.

The formation of neap tides
The formation of neap tides

It should be noted that tides are a very complex subject so making accurate predictions is extremely difficult.


Questions to think about

1. If the Moon is a waxing crescent in the UK, what phase will it show in Australia on the same date?


2.

Why are spring tides the time when the risk of flooding is especially great?



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