The Moon

My recent article ‘Finding Direction By The Moon’, sparked off a flurry of correspondence on the topic of the usefulness of this celestial body in navigation and in response, the next few articles will be on this subject.


The diagram below shows, that as the Moon completes its 27.3 day orbit around the Earth, we see it pass through various phases of illumination.   It goes from New Moon, to Full Moon and back to new Moon again.moon phases

The Phases.

New Moon.  When the illuminated side of the Moon is facing away from the Earth.  The Moon and the Sun are lined up on the same side of the Earth, so we can only see the shadowed side.  During a new moon, we can see the reflected light from the Earth, since no sunlight is falling on the Moon – this is known as ‘earthshine’.

Waxing Crescent – The waxing crescent moon is the first sliver of the Moon that we can see after the new moon. From the northern hemisphere, the crescent moon has the illuminated edge of the Moon on the right. This situation is reversed for the southern hemisphere.  “Waxing” means that the Moon becomes more illuminated night-by-night,

First Quarter – This occurs when the Sun and the Moon make a 90-degree angle compared to the Earth.   Although it’s called a quarter moon, we actually see it as half illuminated.

 Waxing Gibbous – This phase of the Moon occurs when more than half of the Moon is illuminated but it is not yet a full Moon.

Full Moon – This is the phase when the Moon is brightest in the sky. The Moon and the Sun are lined up on opposite sides of the Earth, so from our perspective here on Earth, the Moon is fully illuminated by the light of the Sun.

Waning Gibbous – In this lunar phase, more than half of the Moon is illuminated but it is not yet a full moon. The term “waning” means that it’s getting less illuminated each night.

Last Quarter – At this point of the lunar cycle, the Moon has reached half illumination again. Now it is the left-hand side of the Moon that’s illuminated, and the right-hand side is in darkness (from a northern hemisphere perspective).

Waning Crescent – This is the final sliver of illuminated moon we can see before the Moon goes into darkness again.

Links:    Lunar Distance       Tidal Effects of the Moon.     Altitude corrections for the Moon

Survival – Finding Direction From The Moon

Where to buy books of the Astro Navigation Demystified series:

Astro Navigation Demystified at

Astro Navigation Demystified at

Applying Mathematics to Astro Navigation at Amazon .com

Applying Mathematics to Astro Navigation at Amazon .uk

Astronomy for Astro Navigation at

Astronomy for Astro Navigation at



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