Try this: Finding your way

Many birds, including pigeons, have tiny particles of iron oxide in their beaks. This helps them to navigate by sensing magnetic fields. Since humans can't do this, this week we will look at some other ways we can navigate.

You will need

• Ppin or needle
• Bowl of water
• Lleaf, slightly larger than the pin
• A magnet. Don't use a flexible fridge magnet.
• Analogue watch (one with hands)
• Sunny day

What to do

Making a compass

1. Stroke the pin along the magnet at least 60 times.
   • Always stroke it in the same direction.
   • Always use the same end of the pin and the magnet.
2. Carefully place the leaf on the surface of the water, so it floats.
3. Place the pin on top of the leaf.
4. The leaf will slowly spin on the water until one end of the pin points north. You may need to occasionally give it a gentle push away from the edge of the water bowl.
5. To work out which end of the pin is pointing north, it may help to remember that in the southern hemisphere, the sun rises in the east, goes across the sky to the north, then sets in the west. If it is morning, the sun will be somewhere in the north-eastern quarter of the sky, if it is afternoon it will be in the north-western quarter.

Finding north using a watch

1. Start by pointing your finger directly at the sun, then drop it straight down until you are pointing at a spot on the horizon.
• In the Southern Hemisphere
2. Point the 12 on the watch at the spot on the horizon that you found in step 1.
3. North is halfway between the 12 and the hour hand.
• For northern hemisphere
2. Point hour hand at the spot on the horizon that you found in step 1.
3. South is halfway between the 12 and the hour hand. North is the opposite direction.

Please note, this will not work well in the tropics.
During daylight savings time, use 1 instead of 12 on your watch.

What's happening?

We have used two very different methods to find north and south.

The Earth has a magnetic field, with poles near the geographic North and South poles (the points that the Earth spins around).

When you stroke the pin against a magnet, you magnetise the pin. The Earths magnetic field pulled on the pin to line it up with the Earth's magnetic field. Since it was floating on a leaf in the water, this force was enough to twist it around.

Finding north using a watch is based on how the Sun moves across the sky. It assumes the sun rises at around 6 am, reaches its highest point in the sky at 12 pm, then sets at 6 pm. Depending on where you are on the Earth, it will either move across the sky to your north (southern hemisphere) or to the south (northern hemisphere).

Because we use watches where the hour hand goes around twice in one day, during the day the angle between the hour hand and the 12 is normally around twice the angle between the sun and north. So to find north, all we need to do is know where to point our watches and to halve the angle between the hour hand and the 12.

Using a watch is not as accurate as using a compass, because on most days the rising and setting of the Sun won't really occur at 6 am and 6 pm, for a few reasons:

• During summer the sun rises earlier and sets later, during winter it's the other way around (to see why, check out www.csiro.au/helix/sciencemail/activities/Seasons1.html ).
• The time for sunset and sunrise also depends on where you live in your time zone. The further east you are, the earlier the Sun will rise and set. For example, Perth and Kalgoorlie are in the same time zone. This morning (12 February 2004) the Sun rose at 5:31 in Kalgoorlie, but 600 km to the west, in Perth, the Sun rose at 5:51.

Although this method is not perfectly accurate, it is still pretty good. It has the advantage that a watch is easy to find. Even if you have a digital watch instead an analogue watch, you could draw a picture of one showing the current time and use it.

This method would not be reliable if you were in the tropics, because during some months the Sun would move across the sky to your south even though you were in the southern hemisphere, or to the north while you were in the northern hemisphere.