Looking into Space, Grade 8
This is the one Grade 8 topic where South Africa is genuinely world class, and where a class in Sutherland or Soweto has a better view than a class in London.
The Southern Cross, Alpha Centauri and the centre of our own galaxy are all southern sky objects. So is SALT, the largest single optical telescope in the southern hemisphere, and MeerKAT, and the Square Kilometre Array.
And the constellations have African names that are older than the Latin ones. They are in the curriculum, they are the best material in the topic, and they are the part most often skipped.
This page covers constellations, African star lore, how to find the Southern Cross, how telescopes work, and South Africa's telescopes. For the Milky Way and light years, see beyond the solar system. For the Sun and planets, see the solar system practical.
Where this fits in the curriculum
| Subject | Natural Sciences |
|---|---|
| Grade | 8 |
| Term | 4 |
| Strand | Planet Earth and beyond |
| Topic | Looking into space |
| Status | Informal activities. This grade puts all three assessed pieces in Terms 1, 2 and 3 |
| Marks | 45 on our worksheet. The topic revision section is 30 |
What a constellation is
A constellation is a pattern of bright stars that people have named. The stars in one are usually nowhere near each other in space. They only look like a group from where we happen to be standing.
| How many | 88, agreed internationally |
|---|---|
| The smallest | The Southern Cross, or Crux |
| Visible from South Africa all year | Carina, Centaurus and the Southern Cross |
| Seasonal from here | Orion and Taurus in summer, Scorpius and Sagittarius in winter |
One attribution to correct. You will read that "the ancient Egyptians made remarkably accurate calculations about the distance from the Earth to the Sun and even the size of Earth itself".
Those measurements were Greek. Eratosthenes measured the circumference of the Earth around 240 BCE, and Aristarchus attempted the Earth to Sun distance. Both worked at Alexandria, which is in Egypt, which is almost certainly how the credit drifted.
Worth fixing rather than skipping, because Eratosthenes' method is a free practical in its own right: two sticks, two towns a known distance apart, and the difference in shadow length at noon.
The African names for the stars
Teach this properly. It is in the curriculum, it is uniquely ours, and it is what learners remember.
| Constellation | African name | The story |
|---|---|---|
| The Pleiades | Selemela (Tswana), Isilimela (Xhosa) | "The digging stars." When they reappear in spring it is time to plough and plant. The Xhosa new year is marked by their return |
| The Southern Cross and the Pointers | Dithutlwa (Sotho, Tswana, Venda) | Giraffes. The Pointers are the males and the stars of the Cross are the females, grazing above the trees |
| Orion's belt and sword | Dintsa le Dikolobe (Tswana) | Three hunting dogs chasing three piglets |
| Orion's belt | Khoisan | A girl with magical powers looked at three fierce lions and turned them into stars |
| Antares, in Scorpius | The Fire-finishing Star (!Xu) | It is red, and it sets late at night when the camp fires have burnt down |
Ask the class for their own family's names for the stars. In most South African classrooms somebody will have one.
Practical 1: find the Southern Cross, and use it to find south
Free. A clear night and a view towards the southern horizon.
- Face south, from somewhere you can see the horizon
- In winter, scan low around the horizon. In summer, look higher up
- Look for four bright stars in the shape of a kite, plus a faint fifth
- Check the two Pointers. Two very bright stars, Alpha Centauri and Beta Centauri, should point at the top of the Cross
- Follow the long axis of the Cross downwards, from the top star through the bottom star, and keep going. That direction is roughly south
The False Cross, and the check that catches it
There is another cross in the southern sky and it fools people every year.
The False Cross sits in Carina and Vela. It is bigger, its stars are more evenly spaced, and it has no Pointers.
That is the test. If you cannot find two brilliant stars pointing straight at it, you are looking at the wrong cross.
Alpha Centauri is not in the Southern Cross
You will read in a Key concepts box that "Alpha Centauri forms part of the Southern Cross constellation". It does not.
Alpha and Beta Centauri are the two Pointers, and they sit in the constellation Centaurus. They point at the Cross. They are not part of it.
The same material states this correctly further on, where it tells you to use the Pointers to check you have found the right cross. Use that version.
You may also read that Alpha Centauri B is "also called Hadar". Hadar is Beta Centauri, which is the other Pointer, and that is very likely where the confusion began.
Practical 2: the observation log
Free, and it runs across two months. It is the only long investigation in the Grade 8 year.
Go outside every second week, at roughly the same time, and record:
| Record | Why |
|---|---|
| Date and time | The Cross moves through the night and through the year |
| The position of the Cross on a drawn semi-circle of sky | This is the actual measurement |
| How far above the horizon, and the angle of the Cross | |
| Cloud cover and how dark it is | The controlled conditions |
| Whether the Moon is up | It washes out faint stars |
After four sightings, describe how the position and appearance changed. The Cross moves because Earth turns on its axis during the night, and because Earth goes round the Sun across the months.
Start it at the beginning of Term 4 and it finishes with the term. The recording sheet is on our worksheet.
Who invented the telescope
| 1608 | A Dutch spectacle maker puts two lenses in line and invents the telescope |
|---|---|
| 1609 | Galileo builds one that magnifies about 30 times |
| What Galileo saw | Craters and mountains on the Moon, the four largest moons of Jupiter, and spots on the Sun |
SAFETY, AND YOU WILL NOT FIND IT IN THE TOPIC.
The material tells you Galileo looked at the Sun through a telescope and says nothing at all about the danger.
Looking at the Sun through any telescope or binoculars causes immediate and permanent blindness. The optics concentrate the light, so it is far more dangerous than looking with the naked eye, which is already dangerous.
To show sunspots, project the image onto card. Never look through.
How telescopes work: refracting and reflecting
| Refracting | Reflecting | |
|---|---|---|
| Uses | Lenses | Mirrors |
| The light is | Bent, or refracted | Bounced, or reflected |
| The big one is called | The objective | The primary |
| You look through | The eyepiece, or ocular | The eyepiece |
| Advantage | Simple, sealed tube | Mirrors can be made far larger and cost less, so they gather more light |
Every large research telescope is a reflector, including SALT, because a mirror can be supported from behind while a lens can only be held at its edge.
Calculating the magnification of a telescope
Magnification = focal length of the telescope divided by focal length of the eyepiece.
Worked example: a 1 000 mm telescope with a 25 mm eyepiece gives 1 000 divided by 25 = 40 times.
| Telescope | Eyepiece | Telescope focal length | Magnification |
|---|---|---|---|
| A | 20 mm | 600 mm | 30x |
| B | 24 mm | 1 200 mm | 50x |
| C | 4 mm | 800 mm | 200x |
| D | 12 cm | 120 cm | 10x |
Watch telescope D. It is given in centimetres while the others are in millimetres. It still works, because magnification is a ratio and the units cancel, as long as both numbers are in the same unit. That is the point of the question.
Why the biggest magnification is not the best telescope
You will read: "So the bigger the magnification, the bigger the object looks."
True, and badly misleading. It is the exact sentence printed on the box of every "600 times!" toy telescope that shows a grey blur.
What actually limits a telescope is its aperture, the width of its main lens or mirror. Aperture decides how much light you gather and how much fine detail is there to magnify at all.
Push the magnification past what the aperture supports and you get a bigger, dimmer, blurrier picture of nothing new. Telescope C above gives 200x, and whether that is usable depends entirely on an aperture the table never mentions.
If you take one buying lesson from this topic: look at the aperture and ignore the magnification on the box.
Radio telescopes and arrays
Everything in space gives off radio waves, and we cannot see them. A radio telescope collects them with a dish instead of a lens.
| No lenses, no mirrors | You do not look through it. A computer turns the signal into a picture |
|---|---|
| A dish | Collects and focuses the radio waves |
| Radio quiet | They must be far from cellphones and radio transmitters, which is why they are in the Karoo |
| An array | Many dishes working together as one instrument, which collects far more than any single dish could |
SALT, and South Africa's telescopes
| What | The Southern African Large Telescope |
|---|---|
| Size | An 11 metre mirror, weighing 82 tons |
| Type | Reflecting |
| Where | Sutherland, in the Karoo, Northern Cape |
| Claim to fame | The largest single optical telescope in the southern hemisphere |
| How faint | It detects objects a billion times too faint for the naked eye |
The radio telescopes, brought up to date:
| What you will read | What is true now | |
|---|---|---|
| Hartebeesthoek | Built in the 1960s in Gauteng | Correct |
| KAT-7 | A seven-dish array in the Karoo | Correct. It was the prototype |
| MeerKAT | "A larger array still being built" | Finished. Inaugurated in 2018, with 64 dishes |
| The SKA | "Should be completed by about 2024" | Construction began at the end of 2022 and it is not finished. First science is expected towards the end of this decade |
Both of those are simply out of date, and a learner who writes "the SKA was completed in 2024" will be marked wrong.
Give the figure with the year attached, the same way we do with moon counts. That is what a scientist does and it is what an examiner should accept.
Why the Karoo
| Clear weather | Very little cloud, most nights of the year |
|---|---|
| Still, dry air | Low humidity, so the air distorts the image less |
| Dark skies | Almost no light pollution |
| Radio quiet | Far from cellphone towers, which is what MeerKAT and the SKA need |
| High and inland | Less atmosphere overhead |
The Karoo was chosen as the national astronomy site after 1972, and SALT, KAT-7, MeerKAT and the SKA are all there.
Safety
- Never look at the Sun through a telescope or binoculars. It causes immediate and permanent blindness. Never look at the Sun at all, directly, through sunglasses, or through a lens or pinhole
- Stargazing happens at night and outdoors. Go in a group, with an adult, and tell someone where you are
- Take a torch with red cellophane over it, so the class keeps its night vision
If it does not work
| Problem | Cause | Fix |
|---|---|---|
| You cannot find the Cross | Wrong direction or wrong season | Face south. Low in winter, higher in summer |
| You found a cross but it looks wrong | It is the False Cross | Look for the two Pointers. No Pointers, wrong cross |
| The faint stars are invisible | Moonlight or streetlights | Go out when the Moon is down, away from lights |
| A learner insists Alpha Centauri is in the Cross | A Key concepts box says so | It is a Pointer, in Centaurus |
| The magnification answers disagree | Millimetres mixed with centimetres | Both numbers in the same unit, then the ratio works |
| A parent asks which telescope to buy | The topic talks about magnification | Aperture, not magnification |
How the 45 marks are made up
| Part | Marks |
|---|---|
| Early astronomy and constellations | 10 |
| South African star lore | 8 |
| Finding the Southern Cross | 9 |
| How telescopes work | 12 |
| Telescopes in South Africa | 6 |
Nothing in this topic is prescribed for formal assessment. The topic revision section is 30 marks. Our worksheet is 45 and the split above is ours.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 45 marks, including the magnification table, the African star names and a four-sighting observation log for the Southern Cross
- Marking memorandum, with what to accept for each answer and the nine answers that look right and score nothing
Related practicals
- Beyond the Solar System, Grade 8. The Milky Way, light years, galaxies, black holes and dark matter
- The Solar System, Grade 8. The Sun, the planets, and a scale model walked out on a field
- Visible Light, Grade 8. Lenses, refraction and reflection, which is how a telescope works at all
What you need to run it, and one thing we do not sell
Both practicals on this page are free. A clear night, a view of the southern horizon, and the recording sheet from the worksheet. A compass helps for finding south on the first night, and after that the Cross does the job itself.
We do not sell telescopes. Not one, at any price.
You are reading a page about telescopes, so it is fair to say so plainly rather than let you search the site. If you want one for a school, our honest advice is the same as the advice above: buy on aperture, not on magnification, and a pair of decent binoculars on a tripod will show a class more than a cheap telescope will.
| Item | Price | What it is for |
|---|---|---|
| Compass, aluminium alloy | R78 | Finding south on the first night |
| Telescopes wall chart | R85 | How the different types work |
| Hubble Space Telescope wall chart | R85 | The best known space telescope |
| Space exploration wall chart | R85 | For the classroom wall |
One honest note on the charts. We hold the astronomy charts in ones and twos, not in depth. If you want a set rather than a single chart, ask us first and we will tell you what we can actually send this week.
The models and charts are in astronomy, and the full chart range is in wall charts.