Activity 2 – The Stars as a Compass

Context

Activity One involved landmarks that indicate north, south, east and west. Landmarks make it very easy to get oriented, but they only work at a particular stargazing place. It is also possible to find directions from the stars themselves, and this works wherever you are. In this activity you will learn how to find north and south from the stars.

There are several different systems for finding north and south from the stars. They all work. We favour the system presented here because it uses the “three hands” principle. “Three hands” is easy to remember, and the same measurement works in both the Northern and Southern Hemisphere.

Practice is vital to acquiring a new skill. In this activity you will practise on paper (Star Hunt sheets) and on the computer screen (Computer Challenge) prior to finding north and south from the night sky.

Specific Learning Outcomes

You will be able to find north or south using specific stars.

Teacher Planning and Preparation

Students can do most of this activity at school, but the final test of their skills in the night sky needs to be done as a homework exercise.

There are three star groups that we use for finding the celestial poles: the Big Dipper, Cassiopeia, and the Southern Cross. For each star group there is a different method. The complete system works anywhere in the world and involves learning all three star groups and methods.

While it is very useful for the globe trotter to know all three methods, many people live in parts of the world where you can only see one star group. It makes sense to teach only the appropriate methods for your latitude.

To teach only the methods for your latitude, download the appropriate kit.

Preparation for Teaching the Method

The A4/Letter size posters used in this section are all designed to enlarge to A3/Tabloid on a photocopier. You can also use them on an overhead projector, or copy them onto a whiteboard.

Preparation for Star Hunt

Each set of Star Hunt sheets contains 8 different sheets showing the stars at different times. It is suggested that you give them out so that neighbours get different sheets. Once students have completed a sheet, you can give them extra practise with another sheet from the set.

Preparation for Computer Challenge

It is best to do this activity after at least one of the Star Hunt sheets has been completed.

Unless you have access to a suite of computers we suggest that you get students to take turns to work in pairs. Successful “finds” can be recorded on a chart.

You will need to download and install the program Mirapla Sky for Windows. (At this time we do not have a version for the Macintosh.) This is a very small download and installation is simple. You may need to adjust the brightness and contrast of your monitor to enable the stars to be clearly seen. On a badly adjusted monitor they will be invisible! The program has an “Adjust your monitor…” button, which works best when your display settings are either High Colour (16 bit) or True Colour (32 bit).

Classroom Lead-In

Things you might discuss with the students as a warm up to this activity:

• How do the stars move in the sky?

• What are some ways for finding north and south?
• Who knows a method for finding north or south from the stars?

Science Background Knowledge

Vocabulary checkpoint

North Celestial Pole: The North Celestial Pole is the “North Pole in the sky.” It can be found in the sky, above the horizon, due north. The Pole Star marks the approximate location of the North Celestial Pole. The further north you go the higher it appears in the sky. It cannot be seen from the Southern Hemisphere at all.

South Celestial Pole: The South Celestial Pole is the “South Pole in the sky.” It can be found in the sky, above the horizon, due south. There is no bright star to mark the location of the South Celestial Pole. The further south you go, the higher it appears in the sky. It cannot be seen from the Northern Hemisphere at all.

Circumpolar Stars: The stars in the vicinity of the celestial pole that do not set (do not go below the horizon), even during the day. Instead of setting, they go round and round the celestial pole. There are more circumpolar stars when the celestial pole is higher in the sky. Therefore from different locations on Earth there are different numbers of circumpolar stars.

Axis: The imaginary line that the Earth spins around. It runs through the centre of the Earth, connecting the North Pole and the South Pole.

How do the Stars Move?

The stars near the celestial poles do not rise and set; instead they go round and round the celestial pole. Here is a question and answer sequence that leads to a greater understanding of this:

Questions

• What causes day and night?
• Which moves, the Sun or the Earth?
• Which seems to move, the Sun or the Earth?
• Where does the Sun seem to move?

Answers

• The Earth rotates, creating day and night.
• The Sun does not move. (This is not strictly true, but its movement is negligible when considering night and day.)
• The Sun seems to move, appearing to rise in the east and set in the west.

Note to teachers: If your students cannot produce answers like this in response to these questions, it may be necessary to teach more about day and night before going on.

Questions

• Do the stars move?
• Where do the stars seem to move?

Answers

• The stars do not move, but they seem to rise in the east and set in the west.

If the stars don’t move, and the Sun doesn’t move, it follows that the stars will appear to move in more-or-less the same way as the Sun. That is, they rise in the east and set in the west. With a little prompting younger students can probably work this out for themselves.

• Close to the celestial poles the stars do something quite different to the Sun and Moon. The circumpolar stars circle around the celestial pole and they never set.

Question

• What does the Earth rotate around?

Answer

• The Earth rotates around itself. The “axle” it rotates around is called the axis.

At this point some students may confuse rotation with orbiting. See the Science Background Knowledge topic Rotate or Orbit? for an exercise to help reinforce the correct use of these terms.

Question

• What are the ends of the axis called?

Answer

• The ends of the axis are the North Pole and the South Pole.

Question

• What are the celestial poles?

Answer

• Extend the Earth’s axis out into space, into the stars, and you get the North Celestial Pole and the South Celestial Pole. In the Northern Hemisphere there is a bright star very close to the North Celestial Pole, known as Polaris or the Pole Star. In the Southern Hemisphere there are no bright stars near the South Celestial Pole. There is nothing there; it is just the point in the sky that the stars appear to turn around.
• The North Pole is directly beneath the North Celestial Pole. The South Pole is directly beneath the South Celestial Pole.

Rotate or Orbit?

Rotating is spinning around yourself. Orbiting is spinning around someone else. This can be reinforced with a simple classroom exercise: get students to stand in pairs with plenty of space around them. One student will be the Sun, one the Earth. Get the Earth to rotate (student will twirl on the spot). Get the Earth to orbit (student will walk around their partner). This simple exercise reinforces the separate meanings of the two words rotate and orbit. Make sure that students understand the Earth does both.

Rotation causes day and night.

Orbiting causes the year.

The Earth does 365¼ rotations to every one orbit.

Instructions

Part 1 – A built in ruler

Your hand, held at arms length, can be used to measure distances in the sky. We do not measure distances in the sky in metres or feet. We measure them in degrees.

Hold your arm at arm’s length, fingers together, with the hand up as if you were trying to signal “stop.” The width of your hand will cover 10° of sky. It doesn’t matter how big you are; if your arm is longer your hand will be wider.

You can measure greater distances by putting your hands one beside the other. A simple way to test this is to start at the horizon and measure 9 hand widths up. This will be 90° and will leave you looking straight up towards the centre of the sky.

The star groups we use for finding north and south are each 30° away from the celestial pole. So we will use three hand widths when finding north and south.

This works for the real sky, but not when we use a map of the sky. For a map we use a cut-out hand. The Star Hunt sheets all have a cut-out hand that works for them. Mirapla Sky has cut-out hands that work for the computer screen.

An interesting class project is to measure the hand widths and arm lengths of everyone in the class. The angle covered by the hand can be found by taking the inverse tangent of hand width divided by arm length. In Microsoft Excel and most spreadsheets this function is called atan. It gives an answer in radians. Convert to degrees with *180/PI(). In this way you can investigate how true the 10° rule actually is for your class.

Part 2 – The star groups and how to find the poles

Use the poster set provided as part of the kit for your latitude. These posters explain the method for finding north or south from the stars. Once the method is understood, the Star Hunt and Computer Challenge can be used for practise.

Part 3 – Star Hunt

Use the Star Hunt set from the kit for your latitude. The Star Hunt set contains 8 sheets, each of which has instructions printed on it.

Part 4 – Computer Challenge

Use the Computer Challenge Instructions sheet from the kit for your latitude. You will also need copies of the Mirapla Sky Hands worksheet from the kit. Cut out the correct size hand for your computer screens.

Follow Up and Extension

Encourage students to use the first available clear night to practise finding the North or South Celestial Pole in the night sky. (It is recommended that you do this even if you have a stargazing night planned. Students need to practise their skill as soon as possible.)

Get students to report back when they have done this. Success can be recorded on the same chart as the computer challenge.

Using the Whole Earth kit, learn the methods for the opposite hemisphere and practise finding north/south there. You will not be able to see those stars in your hometown night sky, but perhaps one day you will travel that far. When you do, you will be able to find your way by the stars.

Downloadable Resources

	Stars as a Compass kits Download the appropriate kit containing all of the posters and worksheets for this activity in three ready-to-print files. Not a subscriber? Preview as a web page here.
	Mirapla Sky for Windows
	Help with printing and downloading