Finding North By The Moon In The Southern Hemisphere

In the bush at night you wouldn't normally be using the Moon to navigate, but a good understanding of the relationship between the Sun and Moon will help you to make good decisions about travel in the bush. Especially if you are planning to travel by night or you are in a situation where you run out of daylight and a decision needs to be made about whether to stop or continue. In open country you can see quite well without a torch when there is half a Moon or better. But it is also good to know that the Moon can be used to give you a sense of direction.

The method described, can be adapted to be used in the Northern Hemisphere but it is a little more complex. It is also an approximation only. Many factors affect it’s accuracy, for example the width of the time zone you are in or how accurately you can estimate the phase of the Moon. However, for use in the bush or a survival situation it’s a whole lot better than nothing.

 

For starters, you need to know how to find North by the Sun using an analogue wristwatch. This is done as follows:

Step 1 - With the face of the watch flat and horizontal, point the 12 on the watch face towards a point directly below the Sun.

Step 2 - Read the hour hand. Say it's 10:00am.

Step 3 - North is the direction midway between the 12 and the hour hand. I.e. in this case the direction pointed to by 11 on the watch face.

This works for any time of the day. At 5pm, the direction North will be midway between the 12 and the 5. I.e. the direction that the hour hand points to when it's 2:30pm.

Now let's bring the Moon into the picture.

 

Sun and Moon Orbits

The Moon orbits the Earth in a path that is very close to the path taken by the Sun - for this purpose, we'll assume that they are the same. We also know that the Sun appears to orbit the earth. It’s helpful for this exercise to image that the Sun is orbiting the Earth and having a race with the Moon. Imagine that you are an observer situated in the middle of the racetrack.

The race starts at New Moon. At new Moon, the Moon is very close to the Sun. You'll see the Moon  set on the Western horizon, very soon after Sunset. It'll be a dark night.

The Sun and Moon are very close to each other, but the Sun moves faster.

Each day, the Sun will gain on the Moon by about 1 hour. 14 days later, the Sun and Moon will be on opposite sides of the earth. It will be Full Moon. The Moon will rise at a time close to Sunset.

The next day after Full Moon, the Moon will rise "an hour" after Sunset. The following day "two hours" after Sunset. By now the Sun will have a lead of more than a half a lap on the Moon and it is clear that the Sun will eventually lap the Moon. The Moon will appear in the sky closer to dawn. It will appear that the Moon is now leading the race.

Eventually ( 28 days after New Moon ) the Sun will lap the Moon and the cycle starts all over again.

 

Position of the shadow line on the Moon throughout the Moon’s cycle

If you look at the Moon throughout the 28 day period you'll notice that the shadow line on the face of the Moon moves from left to right across the it’s face. It starts on the left side which is the side illuminated by the Sun. Eventually, the shadow line reaches the right side and it disappears completely during Full Moon. Later, the shadow line reappears on the left side, but this time the illuminated side of the Moon is on the right. The shadow line moves from left to right until the Moon is completely in shadow. This is at new Moon.

 

The significance of the shadow line

The position of this shadow line tells you where the Sun is.

Imagine a line across the face of the Moon that is perpendicular to the midpoint of the shadow line.

The shadow line moves by approximately 1/14th  of a Moon diameter to the right every 24 hrs. After 14 days ( Full Moon ) it has crossed completely from left to right. The Moon is now 12 hours behind the Sun. Remember, the Moon now rises at sunset, this is 12 hours behind the Sun.

 

Time difference between the Sun and Moon

As an approximation, the proportion of the Moon that is illuminated gives an indication of the distance between the Moon and the Sun ( in terms of the race of course ).

When the illuminated side of the Moon is on the left, the Sun is leading, and conversely, when the illuminated side of the Moon is on the right, the Moon is leading.

The amount of lead is approximately equal to the number of 12ths of the Moon that are illuminated. ( using our imaginary line ). Each 12th corresponds to one hour of time.

 

Finding the Sun’s position from the Moon

Even though you cannot see the Sun at night, it’s position can be determined by looking at the Moon and judging the lead time between the two.

Example with a waxing Moon. – If it’s almost first half Moon, the sun could be say 5 hours ahead of the Moon. ( The illuminated part of the Moon is 5/12ths wide ). If the time is 8pm, the Moon’s current position is where the Sun would have been 5 hours ago. I.e. at 3pm.

Point the 12 of your watch to the Moon and take a line half way between the 12 and the 3. That is approximately where North is.  

Example with a waning Moon. – The time is 3am, the moon is 5/12ths illuminated. i.e. 5 hrs ahead of the sun. The sun will be in the Moon’s current position in 5 hours time. i.e. 8am. Point the 12 of your watch at the Moon. Take a line halfway between the 12 and the 8. i.e. 10. That is approximately where North is.   

 

Other Lunar Relationships

There three pieces of information, which relate to one another in such a way that if you know any two of them, you can deduce the third ( with one exception ).

They are:

  •      The direction North
  •      The time of day
  •      The phase of the Moon

In the method above, we used the time of day and the phase of the Moon to get the direction North, but if you know where North is already, you could tell the time, by the Moon.

Similarly if you know what phase of the Moon it is, you can fairly easily figure out where the Moon will be in the sky at any time of day or night. 

Obviously you can’t figure out the phase of the Moon just from the time of day. But if you understand how it all works, you will be able to make a pretty close guess just by knowing what phase of the Moon it was a few days ago or by knowing the phase of the Moon on a recently passed date.

 

 

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Tohunga
Comment by Simon Amos on June 19, 2012 at 14:43

I loved the science.....I enjoyed the maths........I appreciated your explanations which made the process of divining a whole heap of information seem simple. Whilst the method is undoubtedly quite clever and sound..........if I want to find/confirm (approx) North - but neither the time nor the Moonphase - I use the stick-and-two-stones method (which works just as well using the Sun). Done properly, it's delivers a decent approximation; and it has the bonus of being ultra-simple and easy to remember. 


Shrafter
Comment by Supple Existence on July 26, 2011 at 20:43

well explained however i think i will have to read 6 times before it sinks in thanks

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