Stellar Navigation and the Star of Bethlehem, with guests Captain Shaun Hyland, Professor Tony Ferrel and Esme Machekizo

Join us on a fascinating journey through the history of navigation, and the tools and techniques that have guided sailors across oceans.

Exploring the history of navigation from ancient times to modern technology, this episode takes us on a journey across the seas and through the skies. From primitive sailing crafts to the development of navigation tools like the compass and sextant, we learn how early sailors relied on celestial bodies to find their way. Delving into the establishment of observatories and the quest for accurate timekeeping, the narrative brings us to the present day, where GPS and electronic aids have transformed navigation. Alongside tales of maritime traditions and astronomical discoveries, the episode celebrates humanity's enduring fascination with the mysteries of the universe.



Primitive Sailing | Coastal Landmarks | Odysseus the First Astro-navigator | The Magnetic Compass | Celestial Navigation | The Quadrant | Navigating South of the Equator | Solar and Lunar Sightings | The Nautical Almanac | The Astrolabe | The Backstaff and the Crossstaff | The Sextant | The Complex Problem of Longitude | The Navigational Chronometer | Noonday Cannon on Cape Town's Signal Hill | Longitude's relationship with Time | The South African Astronomical Observatory | Reverend Fellows | Astronomer Sir Thomas Maclear | The British Admiralty Board of Longitude | Essential Navigational Time Signals | Fellows fires the Pistol | Henderson drops the Ball | Cape Town Harbour Clock Tower | Signal Hill Noonday Cannon | The Solar Transit Telescope | The Maclean Telescope | David Gill | Modern Nautical Navigation | The Cape Town Planetarium | Professor Tony Ferrell | Esme Machekizo | Planetarium Shows in Xhosa | The Meteorites at the South African Museum in Cape Town | Orion | Andromeda Galaxy | What was the Star of Bethlehem?

Video transcription

Human beings have always been curious about what lies beyond the horizon on water, as well as on land. Primitive sailing crafts were used by early civilizations to fish and to make limited journeys. Down rivers or close to the shore were a punting pole, and later, a simple sail would've powered the craft. To know where they were, these first sailors would almost certainly have used identifiable landmarks, but what about the oceans? Those vast watery deserts where there are no visible landmarks?

How does one find one's way over these expanses? We do this by navigation, the science of finding a vessel's exact position, and the art of going safely from one such position to the next. Somewhere, back in time, early sailors would've had to turn their gaze skyward. They would've noticed that a star's position in the sky was connected with their position on earth. The first written account of a sailor who steered by the stars, is the story of Odysseus sailing eastward. He supposedly sat at the helm, not closing his eyes, for 17 days. He kept on course, following the advice of the wise goddess Calypso, who told him to use the Pleiades and the Great Bear as his guiding stars.

In the 12th and 13th centuries, the art of navigation was revolutionised with the advent of the magnetic compass. It was then possible to tell one's direction relative to magnetic north. Using the stars together with the compass and a sea chart, Mariners could begin to reliably steer their course across the oceans. In the 15th century, around the time of the Portuguese exploration, the art of navigation improved with the use of the quadrant.

It was made from brass, and had a sighting edge and a plumb line, and its circumference, marked in degrees, or with prominent locations.

By 1474, the Portuguese had sailed far south and reached the equator, whereupon they would have lost sight of their primary beacon, Polaris, or the star of the North Pole, because it would've disappeared permanently below their horizon. This forced them to turn to the sun and other celestial bodies as a means of navigation. This was far more complex than stellar navigation as the Earth's tilt, and its orbit around the sun, have the effect of moving the sun north and south. This shift accounts for the seasons and meant that the mariner had to consult tables that predicted the sun's position, to allow accurate measurement of latitude.

By the late 15th century, the astrolabe was in use. This instrument was suspended from the thumb, and comprised a massive graduated ring of brass fitted with an alidade, or a sighting rule, pivoted at the center. The degrees were then read off the outer scale. The Mariner had a table of daily solar altitudes for the hour of noon for Lisbon, or any other port of departure. The difference between the published and the observed altitudes of the sun, gave the observer's latitude. In 1590, an English Sea captain, John Davis, developed the back staff from Levi ben Gerson's cross staff.

By moving a sighting vein up and down a vertical scale, the observer aligned the shadow cast by the vein onto the horizon vein to determine altitude. But the most useful and lasting piece of navigation equipment is the sextant, which is still in use today.

So much for latitude, finding longitude is far more complex. The theory is this, the earth revolves on its axis once every 24 hours. The earth is a sphere and can be divided along its circumference into 360 degrees. This means that the earth turns 15 degrees every hour. If a captain of a ship were to set a clock at the port of departure, say to noon, then sailing east or west, he could observe the local time of noon. The difference in time between this, and the time on the clock, could be calculated and converted into degrees of longitude.

These facts were set out by Gemma Frisius in 1530, but it took almost 200 years before someone invented a clock that could keep time on a rolling ship. This clock was to become the ship's chronometer.


I am out here on the bridge of a very large container ship in the Cape Town Harbor. It's very, very windy indeed, and behind me, you can see the city of Cape Town and Table Mountain, but off to the other side is Signal Hill. Now a very strange thing happens here. Once a day on Signal Hill, they fire a canon at noon. This has become a bit of a tradition in Cape Town, but in fact, long ago, the firing of the noonday canon was very, very important indeed. It signaled a specific time, and that time was vital for the ships. To synchronize their clocks, a very accurate clock was necessary for the navigation of those vessels. The headquarters of the South African Astronomical Observatory is the center of astronomy in South Africa, and it is situated in the leafy Cape Town suburb of Observatory, next to the Liesbeek river.

In 1820, the British admiralty requested that a certain Reverend Fellows come out to South Africa to establish a permanent observatory down here at the Cape of Good Hope. He was a mathematician and an astronomer, and his prime purpose was to very accurately map the brightest stars in the southern hemisphere.

The accurate mapping of the stars in the southern hemisphere was vital for navigational purposes.

The stars of the southern hemisphere were not well mapped or catalogued, and the Royal Observatory in Greenwich, England, felt the need to establish an observatory in southern latitudes. By 1714, the British Admiralty had established a Board of Longitude to investigate new methods of determining longitude. A site was required. The Cape of Good Hope was chosen above Saint Helena and the Ascension Islands, because of its favourable weather conditions, and its status as a colony.


Dave, one thing Fellows was instructed to do, was to provide accurate time signals for the ships in the harbor. How did he actually go about doing that?

Well, fellows used a special high-intensity lantern, and apparently sent signals that way. I'm not sure how often he did it, or how well it worked. His successor, Henderson bought himself a pistol. He'd stand over there on the roof, and at some prearranged time, in the middle of the night, he would fire the pistol. Ships in the harbor, would look with spy glasses for the flash, and they would use that to set their chronometers, so they knew what was going on. Still later, Maclear, who came after Henderson, Henderson got chased away by cobras, set up a time ball. Now that was a large structure with a big black object, which was easily visible from most parts of Table Bay, that would drop, and when it dropped, the ships would set their time to 1 PM in the afternoon.

A series of different time balls were built in the grounds of the observatory, and in 1860, this one built in red brick, which was fashionable at the time, was erected in the harbor grounds as it was closer to the ships. On sighting the signal from the observatory, an operator would release a break, allowing the ball to drop. Later still, the noonday Signal Hill Cannon performed this valuable task.

Dave, so this is the very instrument that was used by Fellows, is a portable transit instrument.

He had a number of portable instruments. He got the government to give him an 1820 Settler's hut for a portable observatory, and he started measuring the positions of southern stars. In fact, even though they were portable instruments, they were pretty good, and he was able to get accuracy, which was at least equal to the best that had been done before. So it was useful.

Dave, the principle operation of this piece of equipment… how does it actually work?

It was designed to be used to measure star positions. The two things that it could measure were the altitude above the horizon, which you'd get from looking through this microscope at this circle, obviously having centered the star over in that eyepiece, and also the compass direction around the horizon called the azimuth, which you've got by turning it around on this axis, and looking through this little microscope, aided by this portable candle holder, which gives you the additional desirable feature of being able to see the scale.

This was pretty much a universal instrument. You could use it as the equivalent of a transit instrument for measuring a star's east-west position. You could use it as the equivalent of mural circle, for measuring its north-south position, for that matter. You could take it out and survey with it. It was fully equipped as the theodolite. This was as good as instruments got. It was very accurate. It was precise, and the only limitation it really had, compared to the permanent instruments Fellows was so eager to use, was that it wasn't physically large enough to get the full accuracy, that full-size instruments could obtain at the observatory.

I took a look at some of the other fascinating instruments.

The McLean Telescope is a 24 inch refactor, and was assembled here in 1898 by David Gill to map the southern skies. It is still used today for observational purposes.


Technology today permits aeroplanes and ocean-going vessels to navigate using satellites, with pinpoint accuracy. I spoke to the captain of a very large container ship about these and other methods of navigation.

Captain, navigational aids go back for centuries. Some of them, you know, really a long way back in history. What type of those old instruments do you still use on onboard a vessel like this?

Well, a vessel of this size, which carries 3,000 TEU [Twenty-foot Equivalent Unit], or containers, and it's 258 meters long, still carries the old sextant and still uses the magnetic compass, and we still have the Polaris, because at the end of the day, if the electronic aids breakdown, you still have to get from your port of departure to your destination and deliver the goods.

Besides the old technology, you no doubt have a lot of modern electronic navigational aids too.

Oh, yes. GPS, which gives you instant positioning, and it's used for mainly fuel saving, I think, is the owner's chief interest in that, and also time saving, and you know, you're never a moment where you don't know where you are, and that's usually linked now into the satellite. So, for distress purposes, and of course, the gyro and the automatic pilot, which have been around a long time, is used extensively, mainly once a day, because the automation has reduced crews, and one needs all this sort of thing.

Captain, the sextant, is it ever used? And, if so, when do you use it?

Oh, yes, it's used extensively. The Chief Officer in his morning watch does the morning stars, and the evening stars, and the second and third officers do their morning sun sightings, and their noon sightings, which of course is very important. Can't do without it.


Cape Town has long been the center of astronomy in South Africa. The city has contributed greatly to our knowledge of the southern skies, and astronomy in general. It's here too, where extremely valuable work is being done amongst the youth to nurture an interest in general science.

I went along to the Cape Town Planetarium, inside the South African Museum in the old company gardens. I joined a group of enthusiastic scholars for a show at the planetarium where I spoke with Tony Ferrell and Esme Machekizo about their work.

This is where people are coming to learn about astronomy, sort of their first step before they enter the planetarium proper. And here we have meteorites. Now the nice thing is you are literally touching something from outer space.

So, Tony is this then an object from the asteroid belt?

This is literally a thing from outside the solar system. This piece of junk here has been in orbit between Mars and Jupiter, and must have been there for most of its life. It's probably the fragment of a larger planetary body because it's mainly iron and nickel. Normally you find that inside planets, but something has broken it up, and something has perturbed it so that it came into the inner solar system, and it collided with the earth.

Esme, tell me about your shows here at the planetarium.

Well, we've recently introduced a show for Xhosa speaking people, and we've had some very good response. We've had three shows so far, and catering to a group of about a hundred and twenty to a hundred twenty five each time. The idea is to introduce at a basic level the science of astronomy, and perhaps science in general, to people who have not had the advantage in their school years, or in in the past history, the recent past history, to be allowed to study sciences. And, we are hoping that as a result of this program, we may be able to produce in our country, some scientists and astronomers of high calibre.

And Tony, how did you put the show together?

Okay, like most of the shows, it operates with a recorded soundtrack. So the scripting of the soundtrack, initially done in English by our teacher, Elsabie Uys, who used to be at here at the planetarium, translated into Xhosa the soundtrack. You then add the visual effects. There are something like towards 50 slide projectors on the go. We can build them up to cover the entire dome, and we can create all sorts of pleasing visual effects to entertain the children.>

The idea was to use images that were comfortable and familiar to particularly young children, so a soccer ball first comes up, but a soccer ball is a sphere and so is the earth, so are the moon and planets, and so on. So, from leading from one familiar object, you can go to the earth, go to gravity. We look at stars, we look at the sun, and gradually we look through the solar system. So, if you can start with an image that's familiar, it's easier to take children who have never learned anything before about astronomy, out into the wonders of space.


Hi and welcome to Sky Watch.

The constellation of Orion dominates the summer night sky. It's rich in detail and worth exploring through your binoculars. Sirius, the dog star lies along an imaginary line extending through Orion's belt, and is the brightest star in the whole night sky.

A rather fascinating object to look for is the Andromeda Galaxy, the most distant, visible object in the night sky. It is about 2.3 million light years from the earth. Like other galaxies, it contains millions of stars. It has a dense nucleus with spiralling arms of stars extending in a disc around it. We are not able to see Andromeda's spiral arms very clearly as its disc is tilted 13 degrees to our line of sight. To the naked eye, it looks like a faint misty patch, so it is best observed on a very dark night, or away from the city lights. Binoculars will enable you to see more detail. Andromeda covers an area of the night sky, some six times the diameter of the full moon.

The galaxy lies in a northerly direction, some 16 degrees west of north. Its angle above the horizon differs according to latitude, and so the further north you are in the country, the better your chances are of finding it.

For viewers in Cape Town, it lies 13 degrees above the horizon, in Bloemfontein, two degrees higher at 15 degrees. If you're in Pretoria or Johannesburg, your chances are better still as it is about 18 degrees above the horizon. Look for it immediately after this program tonight at 8:30, as it soon sets below the horizon. If you don't find it tonight, try and look for it tomorrow at the same time.

I was rather curious to find out what the star of Bethlehem actually was. Our only real source of information is a written account in the Bible by the disciple Matthew, some 80 years after the birth of Jesus Christ. The story goes: Jesus was born at Bethlehem in Judea during the reign of Herod. After his birth, astrologers from the east arrived in Jerusalem asking Where is the child? We observed the rising of his star, and we have come to pay him homage. Herod and the whole of Jerusalem were perturbed by this and called the astrologers to meet with him in private, and ascertained from them the time when the star had appeared. They set out at the king's bidding, and the star which they had seen at his rising, went ahead of them until it stopped above the place where the child lay. If we have any chance of simulating those night skies in the planetarium to look for clues, we would at least need to know the year in which the event took place.

At first glance, this seems to be quite a simple task, but I'm sorry to say that it cannot be determined accurately because it was calculated by a monk in 525 AD who left out four years during the reign of Caesar Augustus. That then places the corrected date somewhere around about 4 BC. At least we have one clue to verify this, Herod was alive at the time. According to Josephus, a Jewish historian, Herod died shortly after an eclipse of the moon before Passover.

Now that's better. At least we can program the planetariums projector to go back in time to look for possible lunar eclipses that may have been the one Josephus was referring to. Would you believe it? Lunar eclipses occurred before Passover in the years 4 BC and 1 BC. Most historians agree that Herod died in 4 BC, so all this seems to tie together quite well.

Whatever the star of Bethlehem was, we can at least narrow down the time of the event somewhere between 7 BC at the very earliest and 2 BC at the very latest. A popular theory is that the star was not actually a star at all, but a conjunction of the wandering stars, the planets. Interestingly enough, there was a conjunction of the planets, Saturn and Jupiter, between May 7 BC and December 6 BC. This is when the planet's orbits line up with one another, when viewed from the earth, making them appear very close together in the sky.

It's not too difficult to imagine the astrologers fascination when these two planets met in the sky, not only once, but three times during that period. Knowing the importance modern astrologists place on such a phenomenon, is it perhaps possible that the Middle Eastern astrologers predicted Christ's birth because of this planetary grouping?

This is doubtful as the planets would never get close enough for it to be mistaken as a bright single star-like object. It is very probable that this very bright star was a supernova, a star on its deathbed, violently gasping its last breath. In 1987, such an event occurred, which could clearly be seen with a naked eye. In Chinese records, there is an account of such a guest star, as they called it, which occurred in 5 BC. The first suggestion that the star of Bethlehem could have been a comet was documented more than 17 centuries ago.

It's easy to see how the astrologers would say that this kind of star pointed the way to the young Jesus. Take a look at modern Christmas cards portraying the star. So, what conclusions can we make in this matter? Well, quite frankly, there is very little else I can add if we want a natural explanation. It's a pity that the ancient texts are so short and don't give us any further scientific clues. All that remains for us is to gaze up at the glittering December sky and ponder the various possibilities.

Until next time, wishing you clear skies.

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