Estimating direction by the moon

The moon circles the Earth about once a month. Depending on where it is, we see more or less of its lit half: new moon, waxing crescent, half moon, full moon and waning again. The lit side always faces the sun. In the evening the sun is in the west, so the moon is lit on its west side; in the early hours of the morning the sun is below the eastern horizon, and the bright side faces east.

The line through the two tips of the crescent runs across the direction to the sun. In the northern hemisphere its downward extension meets the horizon roughly in the south; in the southern hemisphere roughly in the north. It works best when the moon is high and is a clear crescent or half moon.

The full moon is opposite the sun. It therefore rises at about the time the sun sets, stands highest around midnight – in the south where we live – and sets when the sun rises. With the time of night you can estimate a direction this way. During daylight saving time this shifts by about one hour.

Take a straight stick, hold it at arm's length in front of your eye and position it so that it touches both tips of the crescent. Follow the stick down with your eyes to the horizon and pick a tree, a peak or another point there. That is your rough south.

Moonrise and moonset are less reliable than you might think. Over a year the sun rises at different places, and the moon goes through this shift in just one month. On some days it rises far to the north-east, on others far to the south-east. 'It rises in the east' therefore only means: somewhere on the eastern side.

Combine the moon methods with others. If the sky is clear, you can find the North Star; the next morning the sun helps. If the results agree, the direction is usable.

Walking through unknown terrain at night is dangerous. Drop-offs, holes, streams and rock steps are hard to see in the dark, and with a headlamp you only see a small patch. Lay the direction you found on the ground with sticks, make yourself as warm as possible and only set off in daylight.

If you are lost, the STOP rule applies here too. If someone knows your trip plan, it is better to stay at a visible place than to set off on the basis of a moon observation. With phone signal, call the emergency number 112.

The moon methods make sense on nights when the moon is visible but the stars are hidden by thin cloud or haze – and as an extra confirmation. In the evening a look at the crescent can help to fix the direction for the morning.

Around new moon the moon cannot be seen at all for a few days. If it is low on the horizon, the crescent line becomes imprecise. And with thick cloud it is of course no help. Precise navigation needs a map and compass.

You can look up moon phases and moonrise and moonset times before any trip, for example at the U.S. Naval Observatory or in many calendars. That way you know in advance whether there will be a moon in the sky during the night, and when.

A memory aid for the moon phase in the northern hemisphere: if the lit side is on the right (like a 'D'), the moon is waxing; if the lit side is on the left (like a 'C'), it is waning. In the southern hemisphere it is the other way round.