Activity 7 – The Shifting Stars

Context

In Activity 2 – The Stars as a Compass you learned that the stars appear to turn around the celestial poles. In this activity you explore the movement of both sun and stars in greater detail, understanding not only the passage of the stars in the night, but the movement of the sun throughout the seasons.

Specific Learning Outcomes

You will make a 3-D model of the sky, called a Sky-in-a-Box, and use it to explore the movement of the sun and stars.

Teacher Planning and Preparation

This activity is in two parts: the first is construction of the Sky-in-a-Box , and the second is exploring the way the sky works using the Sky-in-a-Box.

Construction

Full instructions and details of materials and tools required are provided as part of the downloadable Sky-in-a-Box kit. There is only one version of the Sky-in-a-Box kit – the one model can be used to represent the sky for any location on Earth.

Please read the instructions fully before beginning this project. Construction is straightforward, the instructions are detailed and accompanied by photographs, and the materials are all easy to obtain and work with. However, construction will take several hours and you may need to practice some of the techniques before getting them right.

Classroom teachers will probably not want to get everyone in the class to make a Sky-in-a-Box. Consider getting a group of competent students to complete one as a special project. Make sure you are able to organise sufficient adult supervision with the use of craft knives and hot glue guns. We have had these tools used safely by children as young as five years old, but always with close and competent supervision. It is sometimes necessary to have those tools operated by adults only.

Alternative to construction

You will find that exploring the way the sky works is very difficult without a model to play with. If you find the prospect of constructing your own Sky-in-a-Box too daunting, you may wish to consider purchasing an Earth-Space-Simulator. It works in a similar way to the Sky-in-a-Box, but is factory made.

Exploring

It is difficult for a large group to all be close to the Sky-in-a-Box, yet this is the best way to actually imagine that you are inside the celestial sphere. If you are working with a whole classroom, you will want to plan for small groups or individuals to have time with the device to explore it.

The world tour is quite a big project. You will probably want to organise separate groups to explore each climate zone.

The Sky-in-a-Box is a cardboard
model of the sky.

What You Need

• A globe of the Earth is a very useful resource to use alongside the Sky-in-a-Box.

For constructing the Sky-in-a-Box

Materials

• Print-outs of the Sky-in-a-Box instructions and designs. (Download here)
• Two cardboard boxes, or more (You will need the walls of one or two boxes, and one complete box for the base.)
• Barbecue skewers (between 6 and 8 of them, to make the equator and ecliptic)
• A piece of 8mm or 5/16 inch diameter dowel 270mm or 10½ inches long (to make the axle)
• A plastic bead or marble roughly 10mm or ½ inch diameter (to be the sun)
• Two clothes pegs
• Several hours of time and a reasonable measure of patience

Tools and fasteners

• Hot melt glue gun with sticks of hot melt glue (for construction)
• Bowl of cold water or nearby tap (to avoid burns)
• Glue stick (for sticking paper to cardboard)
• Blu-tack (for sticking sun bead or marble to the ecliptic)
• Packing tape (for taping the base box closed)
• Scissors (for cutting paper)
• Saw (for cutting dowel)
• Large craft knife (for cutting cardboard)
• Replacement blades for the craft knife
• Cutting board or pile of newspapers
• Side cutters (for cutting barbecue skewers)
• Dressmakers pin or push-pin (for marking points through the cardboard)

For taking a world tour using the Sky-in-a-Box

• Seasons worksheets (Download here)

Science Background Knowledge

The Science Background Knowledge section of Activity 2 provides a useful introduction to some of these concepts.

Vocabulary Checklist

Celestial sphere: It looks as if the stars are stuck to the inside of a huge ball. This ball is called the celestial sphere.

Celestial poles: the extension of the Earth’s north pole and south pole onto the celestial sphere

Circum-polar stars: stars near the celestial poles that never set

Celestial equator: the projection of the Earth’s equator onto the celestial sphere

Ecliptic: The ecliptic is the path that the sun appears to follow around the celestial sphere, caused by the Earth orbiting the sun. The ecliptic is tilted away from the celestial equator because the Earth’s axis is tilted.

Equinox: Equinoxes are the times when the sun appears to cross the celestial equator on its journey around the ecliptic. Day and night are equal in length at the equinoxes.

Solstice: Solstices are the times when the sun is furthest from the celestial equator on its journey around the ecliptic. The summer solstice is called the longest day. The winter solstice is called the shortest day.

Helical rise, cosmic rise: the first appearance for the year of a star when it rises ahead of the sun in the dawn

Right ascension, RA: a measure of angular distance around the celestial sphere, used like longitude but to locate places on the celestial sphere (can be measured in hours or degrees, 1 hour of RA = 15 degrees)

Declination: a measure of angular distance from the celestial equator, used like latitude but to locate places on the celestial sphere (measured in degrees)

Meridian: a line from north to south, passing directly overhead, that divides the sky in two

What is the celestial sphere?

In reality the stars are scattered through space, all at different distances from Earth. But how far away do they look?

When you look at something your brain compares the images from each of your two eyes. Your brain uses the differences between the two images to work out how far away things are. All stars are so far away that there are no differences. Because of this, your brain cannot determine the distance to the stars. It simply assumes that, whatever distance they are, they are all the same distance.

If you were surrounded by thousands and thousands of stars, all the same distance away, what shape would they form? They would form a ball or sphere.

For thousands of years people assumed that the stars were attached to a sphere simply because that’s what it looked like. This sphere was called the “celestial sphere.” We now know that the stars are not attached to a sphere, but we still use the idea of a celestial sphere because it accurately describes what the starry sky looks like.

The Sky-in-a-Box is a model of what the sky looks like. That is why it is made in the shape of a celestial sphere. When you use the Sky-in-a-Box you pretend that you are standing on the Earth in the very middle of the cardboard celestial sphere.

The axis and the celestial poles

The Earth rotates around itself, causing day and night. The “axle” it rotates around is called the axis.

We cannot feel this rotation. The result is that it looks as if the celestial sphere is rotating around the Earth, instead of the Earth rotating around its own axis. We use the idea of a rotating celestial sphere because it accurately describes what the sky’s movement looks like.

The celestial sphere appears to rotate around the Earth’s axis. If we extend the Earth’s axis out into space, it becomes the celestial sphere’s axis. Just like the Earth, the celestial sphere has north and south poles: the north celestial pole and the south celestial pole.

In the Sky-in-a-Box, the axis is made from a rod (called the axle) which is mounted in two cardboard bearings, one at the north celestial pole and one at the south celestial pole.

The sun and the seasons

Make sure you understand the Earth's rotation and orbit. See “Rotate or Orbit?” in Activity 2.

Apart from the sun itself (which is a star) all of the stars are much, much further away than the very outer reaches of the Solar System. So we can think of the sun and Earth occupying the centre of the celestial sphere. Something like this: (looking down on the North Pole from above the Solar System)

 

The Earth spins on its axis causing day, when you cannot see the stars, and then night when you see only those stars on the other side of the celestial sphere from the sun. In the above diagram you will see the constellation Virgo high in the sky at midnight. We say the sun is “in Pisces” because it appears to be in the same part of the celestial sphere as Pisces.

After the Earth has travelled along its orbit of the sun for a further three months it will look like this:

 

The sun appears to be in a different part of the celestial sphere, and we see a different set of constellations at night. Now we will see the constellation Sagittarius high in the sky at midnight and the sun is in Gemini.

As the Earth completes its orbit the sun appears to move through all 12 of the constellations shown here. The path the sun appears to take is called the ecliptic, and the constellations it appears to pass through are called the constellations of the zodiac. As the sun passes through each of the constellations of the zodiac it also passes through each of the four seasons spring, summer, autumn and winter.

Classroom Lead-In

It is a good idea to discuss and demonstrate the Science Background Knowledge topics in this activity before exploring the Sky-in-a-Box.

A good way to explain the principle in The sun and the seasons above is to get students to role-play it. Have a large circle of 12 students. Give each student a label with one of the 12 zodiac constellation names on it. Have another two students in the middle to be the sun and Earth. Get Earth to say which constellation the sun appears to be in. Get Earth to rotate and say which constellations will appear at night (when facing away from the sun). Then get Earth to orbit to a new position later in the year and again report which constellation the sun is in and which constellations are visible at night.

Instructions

Construction

Full instructions for making and setting the Sky-in-a-Box are in the downloadable kit.

Exploration

Today and tonight

First set the Sky-in-a-Box for your latitude. (Instructions for setting it are in the downloadable kit.) Then blu-tack the sun onto the ecliptic at roughly the correct place for today. (The ecliptic is the ring of barbecue skewers labelled “Ecliptic.” The sun is a yellow bead or marble.) Four of the arms are labelled with months: March, June, September and December. For the months in between, just estimate the location of the sun. Any intermediate location will do, so long as it is on the ecliptic.

Now get close to the Sky-in-a-Box. The opening in the base represents the horizon. The part of the celestial sphere you can see above the opening represents the sky you can see from your latitude on Earth. Get your eyes level with the horizon, and as close as possible to the celestial sphere. Look through the near side of the sphere and focus your attention on the far side of the sphere, especially on the stars. Imagine that you are standing inside the sphere, and that everything below the horizon is not visible to you.

Turn the celestial sphere so that the sun rises in the east, and sets in the west.

• Does the sun rise due east, or further to the south-east or north-east?
• Does it go high into the sky, or cross the sky quite low?
• Does it set due west, or further to the southwest or northwest?

What it does depends on your latitude and the time of year, but it should mimic what the actual sun is doing in the sky this month.

Keep turning the celestial sphere while watching the stars.

• Can you identify any circum-polar stars?
• Can you see some stars that never rise at all?
• Can you see that Scorpius rises whenever Orion sets?
• Can you see that Orion rises due east and sets due west?

The changing seasons

As the Earth rotates it causes day and night. The sun and stars appear to rise in the east, cross the sky, and set in the west.

As the Earth orbits it causes the seasons. Earth’s axis always points in the same direction, towards the same stars. The northern end of the axis points to the pole star while the southern end points to a point between Achernar and the Southern Cross. This means that the stars always follow the same pattern; the same stars rise at the same places year-round. When you turn the celestial sphere on your Sky-in-a-Box you can see that the star paths are fixed.

(There is a very slow variation in the direction of the earth’s axis called precession. Precession takes place over period of thousands of years, with a full cycle of precession taking 25,000 years. It causes a slow variation in the star paths. Because this effect is so slow, we ignore it in this activity. It is a more advanced topic, and cannot be modelled with a Sky-in-a-Box.)

As the Earth orbits, it causes the sun to appear against a different background of stars every month. If you connect up these “background” stars they form a single line called the ecliptic. The sun makes one complete journey around the ecliptic every year, caused by one complete orbit.

This means that the sun’s path through the sky is different every day. It also means that some stars cannot be seen at all during parts of the year when the sun is in the sky at the same time as those stars. You can model this on your Sky-in-a-Box by placing the sun on the ecliptic, and then turning the celestial sphere to see what happens during one day and night cycle. We will now take a tour through the four seasons of the year.

For thousands of years people have watched the slow passage of the sun around the ecliptic, and used its progress to tell the time of year. The ancient Greek and Roman people did this. Their new year began when the sun passed a point that indicated spring time: the March equinox. We will follow this convention and start our exploration of the year at the March equinox.

Blu-tack the sun to the ecliptic at the point labelled “March Equinox.” Now turn the celestial sphere and see what happens during one day and one night.

• Do you notice that the sun rises due east and sets due west?
• How high in the sky does the sun go during the day?
• Do you notice that the sun spends half of its time above the horizon, and half below?
• Do you notice that when the sun sets Orion is high in the sky, and that by sunrise Scorpius is up (as high as it gets in your part of the world).

Equinox comes from the Latin words for ‘equal’ and ‘night.’ It means that day and night are equal in length.

Move the sun to the point on the ecliptic labelled “June Solstice.” Now turn the celestial sphere and see what happens during one day and one night.

• Do you notice that the sun rises to the north of due east and sets to the north of due west?
• How high in the sky does the sun go during the day?
• Is it summer or winter?
• Does the sun spend more than half of its time above the horizon, or less than half of its time?
• Do you notice that Scorpius is at its highest at midnight?

Solstice comes from the Latin words for ‘stopped sun.’ It means the sun has stopped getting further north, and is beginning to move south again. (At the December solstice it stops moving south, and begins to move north again.)

Move the sun to the ecliptic at the point labelled “September Equinox.” Now turn the celestial sphere and see what happens during one day and one night.

• Do you notice that the sun rises due east and sets due west?
• How high in the sky does the sun go during the day?
• Do you notice that the sun spends half of its time above the horizon, and half below?
• Do you notice that the sun’s movement is exactly the same as it was at the March equinox?
• Do you notice that when the sun sets Scorpius is up (as high as it gets in your part of the world), and that by sunrise Orion is high in the sky.

Move the sun to the point on the ecliptic labelled “December Solstice.” Now turn the celestial sphere and see what happens during one day and one night.

• Do you notice that the sun rises to the south of due east and sets to the south of due west?
• How high in the sky does the sun go during the day?
• Is it summer or winter?
• Does the sun spend more than half of its time above the horizon, or less than half of its time?
• Do you notice that Orion is at its highest at midnight?

Now that you have completed this tour of the year, you should be able to work out which of the solstices is the summer solstice, and which is the winter solstice. The summer solstice is often called the longest day and the winter solstice is often called the shortest day. Can you see why?

In the Northern Hemisphere the December solstice is the winter solstice and the June one is the summer solstice. In the Southern Hemisphere it is the other way around with the June solstice being winter and December solstice being summer.

In the Northern Hemisphere the March equinox is the spring equinox, and the September equinox is the autumn equinox. In the Southern Hemisphere it is the other way round.

A calendar of stars

Let’s look at what happens to the Pleiades during the course of a year. At the March equinox the Pleiades rise during the day, and they can only be seen for a short time in the evening after the sun sets and before they set. As the year wears on the sun moves, little by little, towards the June solstice. Each day the sun gets a little closer to the Pleiades. Each day they can be seen for less and less time after sunset. Try it with your Sky-in-a-Box. Move the sun a little bit closer to the Pleiades along the ecliptic, and then turn the celestial sphere through one day.

Eventually there comes a time when the Pleiades cannot be seen at all. They rise when the sky is light, and set before it gets dark.

But keep moving the sun little by little on its year-long journey around the ecliptic. Soon the Pleiades are rising ahead of the sun. Soon they will rise early enough to be seen in the morning twilight. Every day they rise 4 minutes earlier. If you watch enough sunrises around this time of the year, eventually you will see the Pleiades rise, twinkle in the sky close to the eastern horizon for a minute or two, and then fade from view as the sun lights the sky. This first visible rising of the Pleiades before the sun is called the helical rise (also known as the cosmic rise) of the Pleiades.

While some cultures used equinoxes and solstices as their main calendar markers, some used helical rises of stars. It seems that all cultures knew about both, its just that one particular marker was used more than others were. For example, the Greek and Roman cultures used the spring equinox as the start of their new year, while the New Zealand Māori used the helical rise of the Pleiades, known to them as Matariki.

Sirius is known as the dog star. The saying “the dog days” means the time of the helical rise of the dog star, always the hottest time of summer in the Northern Hemisphere. In the Southern Hemisphere the same helical rise is a sign that the coldest part of winter is upon us. Sirius is called Takurua (winter star) by the New Zealand Māori.

The star compass

In Activity 2 you learned how to find north or south from the stars. This technique is very effective north of latitude 35° north where the pole star is reasonably high in the sky and the Big Dipper is always above the horizon to help find it.

Close to the equator the pole star is always low on the horizon where it is often difficult to see due to haze or low cloud. The Southern Cross is of limited use at these latitudes. Polynesian seafarers could not afford to depend on the pole star and the Southern Cross. They developed a method known as the star compass for finding their way when crossing the ocean by canoe.

Now that you have completed a tour of the whole year, you will have realised that each star in the sky (apart from the sun) always rises at the same point on the horizon. At certain times of the year you cannot see certain stars, but when you do see a star rise, you always see it rise at the same place.

If you learned to name and recognise all of the stars that rise in a particular direction, then you could use them for direction finding. Orion is easy – it always rises due east. But if you wanted to sail due east, you would need to learn something like 20 or 30 other stars that also rise due east. Then you could sail towards whichever one of the 20 or 30 happened to be rising at the time.

Of course for every direction there is a different set of stars. Polynesian navigators probably knew at least 300 constellations and stars off by heart. Before setting out on a journey they would sit up all night looking in the direction they planned to sail, refreshing their memory of exactly which stars to use.

This system may sound cumbersome, but it was accurate and reliable. Polynesian mariners could cross a vast area of empty ocean, arrive at a tiny island, and then return home to an equally tiny island.

There is a lot more information about star compasses at the Polynesian Voyaging Society web site.

Does a star compass work anywhere in the world? You can use your Sky-in-a-Box to research this question. Try starting with your Sky-in-a-Box set for the equator. Where does Orion rise? Where does the Southern Cross rise? Now change the latitude to 45° south. Where does Orion rise? Where does the Southern Cross rise? Stars closer to the celestial poles do change their rise positions when you travel a long way north or south. They can even stop rising altogether when they become circum-polar. The star compass was only part of a complex system of navigation, and it was not used for long north/south migrations.

A world tour

Now that you have explored the way the sky works for your latitude, let’s take a world tour and see how the rest of the world sees the sun.

The world can be divided into three main climate zones: tropical, temperate, and polar. The boundaries of these zones are the two tropic lines and the two circum-polar circles.

Use the Sky-in-a-Box to simulate visiting typical latitudes from each zone. Latitudes to visit are 80° North, 45° North, 0°, 45° South, and 80° South.

For each climate zone, answer these questions four times over (once for each of the two equinoxes and once for each of the two solstices):

• Does the sun rise due east, or does it rise further to the north/south?
• Does the sun set due west, or does it set further to the north/south?
• Are the days long, short, or equal to the nights.
• How high does the sun get at its highest: low, medium, high, or right to the top of the sky?

For each climate zone, answer this question: Do the four seasons bring huge changes, or are all four seasons almost identical?

The seasons worksheets (download here) have places for you to answer all of these questions for each of the five climate zones.

A summary of answers to these questions has not been included here. We rely on you to discover the answers using the Sky-in-a-Box.

Follow Up and Extension

Completing this activity gives you a good grounding in the movement of the sun and stars. Research topics that further extend this could include precession, timekeeping, and celestial navigation.

Downloadable Resources

	Sky-in-a-Box kit – Instructions  (1.1 MB)
	Sky-in-a-Box kit – Designs  (0.8 MB) Preview as a web page here.
	Seasons worksheets  (0.2 MB)
	Help with printing and downloading