How to navigate using the Moon

Latest News: My latest book, How to Read Water, contains a full, but easy to understand explanation of how the moon influences tides and currents.

The moon is a fascinating and sometimes slippery object for the natural navigator to tame. There are fast and slow methods for finding direction naturally using the moon, but unfortunately no known fast accurate methods. It is no less interesting for this.

The first technique to learn is beautifully simple and extremely quick. Although not perfectly accurate, it can be an excellent guide for getting your general bearings. The ‘crescent method’ works as follows:

Mentally draw a line that connects the horns of a crescent moon and then extend this line down to the horizon. In northern latitudes this will give an approximate indication of south. It works best when the moon is high in the sky and not too near the horizon, when significant errors are possible.

Animation showing how a crescent moon can be used in northern latitudes to find south.

The reason this method works is not very complex. The sun and moon move across the sky in an east-west plane. In other words when they are not aligned (a new moon) then they are roughly either east or west of each other. Since the moon reflects the sun’s light, its bright side will be ‘pointing’ to the direction of the sun, ie. approximately east or west. The line that joins the horns of a crescent moon together is at right-angles to this east/west line and any line that is perpendicular to an east/west line must be a south/north line. This is why it works equally well from southern latitudes, for example in New Zealand it can be used to find north.

If accuracy is needed, then there is only one method that can be used with the moon, but be warned it is not fast and can seriously dent your sleeping time. All celestial objects – sun, stars, planets and moon – arc across the southern sky when viewed from northern latitudes. They all reach their highest point in the sky when they cross your meridian or line of longitude and this will occur when they are exactly due south of you. In other words the moon will be highest in the sky when it is due south from the UK. The best way of working out when a bright object is at its highest point in the sky is by watching and marking shadow tips. It does not need to be a full moon, only a moon that is bright enough to cast a shadow. The curve that joins the tips of the shadow over an evening will make clear when the shadow is shortest and this will be a perfect north-south line. I have used this method in the snow of the Scottish Highlands and it worked well, but it was a very late night!

The moon will of course rise and set very roughly in the east and west, but working out exactly what direction the moon is rising or setting is fiendishly complex. There are some simple rules worth knowing. For example, a full moon will behave in the opposite way to the sun, in the UK it rises close to southeast in midsummer and northeast in midwinter, setting southwest in midsummer and northwest in midwinter.

Lots more information about these methods and lots of others can be found in my books.

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For a complete guide to Natural Navigation read Tristan’s books.

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