The History of Astronomy, Grade 7: Geocentric to Heliocentric
Proxima Centauri, the nearest star to the Sun, was discovered from Johannesburg in 1915 by Robert Innes. He was not at the Johannesburg Planetarium, because it did not exist for another forty-five years.
Innes worked at the Union Observatory in Johannesburg. The planetarium opened in 1960.
It is a South African error about a South African discovery, and it is the one correction on this page that nobody else is going to make.
What Robert Innes actually did, and where
| Who | Robert Thorburn Ayton Innes, a Scottish-born astronomer who ran the Union Observatory in Johannesburg |
|---|---|
| What | Found that a faint red star near Alpha Centauri shared its motion, and realised it was closer to us than anything else known |
| When | 1915 |
| Where | The Union Observatory, Johannesburg. Not the planetarium, which opened in 1960 |
| Why it is still the nearest | At about 4,2 light years, Proxima remains the closest star to the Sun that we know of |
Proxima means "nearest" in Latin, and the name is Innes's. Our Grade 8 page covers how far 4,2 light years really is; this page is about who found it and from where.
Where this fits in the curriculum
| Subject | Natural Sciences |
|---|---|
| Grade | 7 |
| Term | 4 |
| Strand | Planet Earth and Beyond |
| Topic | The historical development of astronomy |
| Status | Class activities. Term 4 carries no formal assessment |
| Marks | The topic revision is 40. Our worksheet is 40 |
Geocentric and heliocentric, and one definition that is wrong
| Model | What is at the centre | Who | When |
|---|---|---|---|
| Geocentric | Earth | Ptolemy | Second century |
| Heliocentric | The Sun | Copernicus | Worked out by 1514, published 1543 |
You will see heliocentric defined as the Sun being at the centre of THE UNIVERSE.
It is the centre of the solar system. The Sun is one star among a few hundred billion in our galaxy, and the universe has no centre that we know of.
That was Copernicus's own assumption in the sixteenth century, and the bullets under the definition usually say so correctly. But the definition box states it as a present-day fact, which is the version a learner copies. And a page or two later the same material correctly credits Copernicus with putting the Sun at the centre of the solar system, so it contradicts itself.
Galileo did not invent the telescope
| What you will read | What is actually the case |
|---|---|
| "Galilei made the first telescope to observe planets and their moons" | The telescope was invented in the Netherlands in 1608. Galileo built his own in 1609 and was the first to use one systematically on the sky, which is a real and large achievement |
| "Galileo was among the first astronomers to use a telescope" | This version is correct, and it usually appears a couple of pages before the one above. The same material says both things |
| Galileo and Saturn's rings | He saw them in 1610 but could not make them out, and described them as ears or companions. Huygens identified them as a ring in 1655 |
The honest version is better for Galileo, not worse. Being the first person to point a new instrument at the sky and publish what was there is a bigger claim than having built the tube.
And one date on Kepler: you may read that he presented all three laws of planetary motion in 1609. The first two were 1609. The third came in 1619. Newton at 1687 is correct.
Calendars, and the oldest mathematical object we have
The Lebombo bone is the best fact in the Grade 7 year and it is South African.
A baboon fibula from the Lebombo mountains, carved with 29 notches, and it is the oldest known mathematical artefact in the world. Twenty-nine is close to the number of days from one New Moon to the next, which is why it is usually read as a lunar tally.
One thing to fix before you hand it out: it is often given as found in "Swaziland". The country has been Eswatini since 2018.
And the Gregorian months do not alternate. You will read that the year divides into twelve months "alternating with thirty and thirty-one days each".
| Month | Days |
|---|---|
| January, March, May | 31 |
| April, June | 30 |
| July AND August | 31 and 31. The alternation breaks here |
| February | 28, or 29 in a leap year |
Two 31-day months back to back, and one month of 28. A class can disprove the sentence with a wall calendar in ten seconds, which makes it a good thirty seconds of a lesson rather than a nuisance.
What checks out: the lunar year at 354,37 days and the synodic month at 29,53 days are both right, and so is Nabta Playa in Egypt as the world's earliest known astronomical site.
The sundial, and the shadow that does not move evenly
You will read that a stick's shadow "moved at a constant rate". It does not, and that is the whole reason a real sundial looks the way it does.
A stick pushed straight into the ground casts a shadow that sweeps quickly near midday and slowly near the ends of the day, so the hour marks cannot be evenly spaced.
The fix is the thing that makes a sundial a sundial: tilt the pointer so that it is parallel to Earth's axis. In Johannesburg that means tilting it about 26° from horizontal, pointing south. Then the shadow does sweep evenly, at exactly 15° an hour, because the pointer is now lined up with the thing Earth is actually rotating around.
This is a free practical and a genuinely good one. Build both: a vertical stick and a tilted pointer, mark the shadow every hour on the same day, and compare the spacing. The vertical one gives uneven marks and the tilted one gives even ones, and the class has just worked out why gnomons are tilted and why the angle equals your latitude.
Southern African star lore, which is the richest content in the grade
This material is genuinely good and respectfully handled, and it deserves more lesson time than the page count suggests.
| What | Who | Meaning |
|---|---|---|
| The Pleiades | Swahili Kilimia | "To dig or cultivate." Their rising says it is time to plant |
| The Pleiades | Xhosa isiLimela | Marks the renewal of the year |
| Canopus | Sotho Naka | A seasonal marker |
| The Pointers and the Southern Cross | Venda and Sotho Thutlwa | Giraffes "rising above the trees", which says finish planting by October |
| Orion's sword and belt | Tswana dintsa le Dikolobe | Three dogs chasing three pigs |
| The Milky Way | Molalatladi | Where the lightning rests |
| Venus | iCelankobe | The one that asks for food |
| The Moon | Shaka's Qwayi-Nyanga | Moon-gazers, who watched and reported the phases |
Two naming points worth getting right, because they are people's names for their own sky. You will see "abeSotho" and "abeTswane" used throughout. The usual English forms are Basotho and Batswana. And "Aldeberan" should be Aldebaran.
The Southern Cross, the Pointers and finding south are covered on our Grade 8 looking into space page, along with telescopes and SALT. This page takes the time-keeping half: calendars, the sundial, the ecliptic, and the planting year.
Three label errors on the star chart, and one thing that is not a constellation
| Printed | Correct |
|---|---|
| "Corolla Boreus" | Corona Borealis, and it is a northern constellation, barely visible from South Africa. On a chart of the southern sky it should not be there at all |
| "Piscus Austrinus" | Piscis Austrinus |
| "The Argo" | Split up in the eighteenth century into Carina, Puppis and Vela. It is not a modern constellation |
And one to separate carefully: the zodiac. You will read that it is a circle of twelve constellations. The Sun actually passes through thirteen, including Ophiuchus. The twelve-sign zodiac is an astrological convention, not an astronomical one, and in a topic about the history of astronomy the difference is the point. Some charts even place a yin-yang symbol at the centre of the zodiac diagram, with no line drawn between astronomy and astrology anywhere on the page.
The stargazing advice, on the other hand, is excellent and correct: get to a dark site, use a red light, give your eyes thirty minutes to adapt, go at New Moon, and keep away from the horizon. That is exactly what our Grade 8 page recommends too.
One exam answer that is the wrong object
A question asks for "the path followed by the stars across the night sky", with the ecliptic as the intended answer.
The ecliptic is the path of the SUN against the background stars over a year, and the same material defines it correctly earlier.
The apparent nightly movement of the stars is caused by Earth's rotation. It is not the ecliptic, and a learner who has understood the ecliptic properly will get this question wrong.
If it does not work
| Problem | Cause | Fix |
|---|---|---|
| The sundial hour marks are uneven | The pointer is vertical | Tilt it to your latitude, about 26° in Johannesburg, pointing south. That is the lesson |
| The shadow never appears | Cloud, or the stick is in shade | Pick a clear day and an open patch of ground |
| A learner writes heliocentric means the Sun is the centre of the universe | The definition box | The solar system. That was Copernicus's assumption, not a modern fact |
| A learner credits Galileo with inventing the telescope | One of two contradictory statements | Netherlands, 1608. Galileo built his own in 1609 and was first to use it on the sky |
| The class cannot find Corona Borealis | It is a northern constellation | It is not visible from here. The chart should not list it |
| A learner says there are twelve zodiac constellations | The astrological convention | The Sun crosses thirteen. Ophiuchus is the one left out |
How the 40 marks are made up
| Part | Marks |
|---|---|
| Geocentric and heliocentric, and who proposed what | 10 |
| Calendars, the Lebombo bone and the months | 10 |
| South African astronomy and Robert Innes | 8 |
| The sundial practical | 6 |
| Reading a claim critically | 6 |
Term 4 carries no formal assessment. The topic revision is 40 marks and it adds up correctly. Our worksheet is a separate 40 and the split above is ours.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 40 marks, including the Innes correction and the months that break the pattern
- Marking memorandum, with what to accept and the five answers that look right and score nothing
Related practicals
- Looking into Space, Grade 8. Telescopes, SALT, the Southern Cross and finding south
- Beyond the Solar System, Grade 8. How far 4,2 light years to Proxima really is
- The Moon and the Tides, Grade 7. The 29,5 day cycle the Lebombo bone may be counting
- The Sun and the Seasons, Grade 7. The tilt that decides where your sundial pointer has to aim
What you need to run it
Nothing, and the best practical in the topic is free.
The sundial needs two sticks, a patch of open ground, a sunny day and a clock. Build the vertical one and the tilted one side by side and the whole lesson happens by itself.
If you want something for the wall, we carry single-topic astronomy charts at R85 each, including constellations and the stars. Be warned that we hold these in ones and twos, so check before planning an order around them. They are a nice-to-have here rather than a requirement.
What would actually help this topic is a southern-sky star chart, and we do not stock one. That is an honest gap and it is on our list. Until then, a free planetarium app on a phone, set to red-light mode, does the job better than any printed chart because it knows the date.
The charts are in astronomy and educational wall charts.