20. A Tale of Three Triangles
Summer and Winter triangles, and Triangulum
Triangles abound in the night sky. They are the simplest shape, after straight lines, to be made from the dots that are the stars, and we can see three of these tonight. Two of them are large and bright, the other small and dim, but even that petite polygon has a tale to tell.
At the start of December, two large and beautiful starry triangles bookend the sky, each with its corners marked by the brightest stars. Even light pollution will not overwhelm them so if you are a city dweller, this is something you can look out for. The first one can be seen in the evening sky but as its name implies, it seems to have overstayed its welcome. This is the Summer Triangle, most associated with balmy summer nights when you can stargaze with little more than a light cardigan and a spray of something to repel the mosquitoes. This is not a constellation proper (as defined by the IAU, who established 88 official constellations, based on mostly those of ancient Greece but also some more modern ones to fill the gaps), but an ‘asterism’, a more informal pattern of stars made either of stars within a constellation, or stars from several-this is the latter. In this case, three bright diamond-like stars: Deneb, Vega and Altair, from the constellations Cygnus the Swan, Lyra the Lyre, and Aquila the Eagle respectively.
These bright stars are filled with lore and tradition across the world, most famously the Greek myths associated with the legendary poet and musician Orpheus and his lyre, or the rather adulterous exploits of Zeus (who disguised himself as both a swan and an eagle), whilst in Japan, Vega and Altair represent the starcrossed lovers Princess Orihime and the cow herd Hikiboshi. They fell in love, but Orihime’s father, the emperor of heaven, disapproved of Hikiboshi as he was a simple farm boy, and separated the princess from her beloved with the Milky Way. But once a year, all the magpies of the world would rise up and make a magical bridge across the Milky Way to unite the lovers once more. A similar story is also told in China.
At this time of year, the Summer Triangle slips away to the west, with Aquila setting not long after sunset and Cygnus moving west and then northwards. Cygnus’s brightest star is Deneb, which means tail in Arabic as it mark’s the swan’s tail. This star is a white supergiant, a huge and extremely hot and bright star, 100 times bigger than the Sun and almost 200,000 times more luminous, located at a distance of between 1400 and 2000 light years away. The light we see from Deneb may well have left during the Roman Empire, or during Anglo Saxon times…compare that with the other stars in the triangle, Vega, 25 light years away and Altair, which is around 17 light years away, meaning Deneb is one of the furthest bright stars we can see with eyes alone.
Later in the evening, another large triangular asterism rises. This is the Winter Triangle, and it consists of the hunter Orion walking his dogs across the sky. The stars there are even more dazzling-because the south-pointing corner is marked by striking Sirius, the brightest star in the night sky, marking the heart of the great dog Canis Major. The eastern (left) corner is Procyon, also known as the lesser dog star, in the tiny Canis Minor constellation, and the western (right) corner is Orion’s shoulder, the startling red Betelgeuse.
These two asterisms have another thing in common, other than being triangles-the Milky Way passes through them. If you are lucky enough to live somewhere truly dark, you can see this shimmering band of galactic starlight, especially striking in the Summer Triangle.
But what of the third triangle? Well, this is a true constellation, and it is one of the tiniest and rather faint, a dark sky is required to see it, but it does look exactly like what it’s supposed to, which is more than can be said for a lot of constellations which require heaps of imagination to visualise. Triangulum might seem like one of those modern constellations added in during more modern times to fill the gaps between the classical constellations, there were moments between the 17th and 19th centuries when astronomers did just that. There is a Triangulum Australis in the southern hemisphere, and that is indeed a modern constellation. But Triangulum (Borealis), in the northern skies, was first thought up by the ancient Greeks and is part of Ptolemy’s star catalogue. ‘A triangle? Were they running out of ideas?’ I hear you cry. ‘Those same Greeks who put a winged horse, a many headed snake and a fish-tailed goat up in the sky? Did they run out of stars to thread together, did their mythic imagination run dry by then?’ well, that would be a natural assumption, but actually, the story of tiny Triangulum is more interesting than one would expect.
Triangulum is above the zodiac sign of Aries, itself a storied constellation, from its astrological significance as the first sign of spring, to its role in one of the most thrilling Greek myths of all-the tale of Jason and the Argonauts, this was the ram that had a golden fleece worth going on a quest for…but back to our triangle.
The Greeks imagined this delta, as they called it, to be associated with the supreme god Zeus, who sometimes had his name spelt with the letter delta rather than zeta. So this was a way for the king of gods to ‘tag’ his name in the sky-as if he didn’t already, with a lot of constellations credited to him! But another explanation linked this starry delta with another one on earth, the Nile delta. The Greeks were in awe of Egypt and its culture, so what better than to pay tribute to the land of the pharaohs and its marshy river mouth, than with a constellation?
The Romans identified another land with those three stars though-the island of Sicily, which is also triangular in shape. Sicily was Rome’s breadbasket, thanks to the intensely fertile volcanic soils around Mount Etna. Grain was grown as well as grapes for wine and fruit and vegetables, and the island was associated with Ceres, goddess of the harvest.
Within this small pattern of stars is a treasure, like a small fuzzy patch. This is M33, the Triangulum galaxy. Not as famous as neighbouring Andromeda, but it is the second closest spiral galaxy to our own, and like Andromeda, it is gravitationally bound to our Milky Way. it can be seen with good binoculars or a telescope, provided skies are dark.
Prompt:
Think of the two starry triangles of summer and winter as representing the past and future. Think back over 2024, or back to the summer solstice at least. What has happened since?
Now think forward into the month or, if you like, the year to come-what would you like to see happen? What are your plans or ideas for the near future?
Finally, think of the smaller triangle as representing the present. As the letter delta means ‘rate of change’ in science and maths, as yourself, what small change can you make now, to make sure that you get the future you are imagining?