The founding of Rome and the year 20001
The sky is divided into 88 constellations and among these some of them are never visible at our latitudes, as Australian circumpolars, but it is not certain that some of them were not already known in Roman and Greek times, because of the well known precession of the equinoxes, only to be "discovered" long afterwards by the intrepid navigators (one name above all: Magellan) who met them during their voyages of discovery of the world.
I was saying that the ancient Greeks and Romans knew the Cross Southern (indeed, this is what Wikipedia says...): actually, in Roman times it was very low on the horizon. I verified this fact thanks to my personal time machine, by means of which I went to my city on the very day it was founded (April 21, 753 BC), in the evening, at 10 p.m., when a very low Southern Cross seemed almost stuck in the ground. With the passing of the years, then, our constellation would be seen less and less, so much so that already in the year of the fire of Rome by Nero (64 AD) of the Southern Cross only the northernmost star, Gacrux, which we will know better later on, had remained visible.
Since I had filled up in the time machine, I took the opportunity to go into the distant future to see if and how the sky would change: why not do things big and select the date for New Year's Eve at the beginning of the twentieth millennium? Soon said, with Stellarium (yes, it's obviously my time machine) it's very trivial to set the date 1-1-20001 and take the picture at 0:0:0. Just towards the south here is this time a Southern Cross on the horizon, actually a bit crooked.
But if we move a little to the West (to the left for those who are not practical...) what do we see? A Scorpio and a Sagittarius also a little deformed... and if we look north, we see the polar star even below the horizon and the Big Dipper also deformed.
What happened? Stellarium broke down? Does our time machine need servicing? No! Don't worry!
In addition to the effect of the already mentioned Precession of the Equinoxes (on which you can find here an article of mine where I talk about it in detail), here is called the so-called motion of the stars, that is the annual displacement (usually very small) that has practically every star on the celestial sphere and due to the star moving in space on its own: this effect (which has nothing to do with precession, which is a characteristic of the Earth and its motion around the Sun and on itself) causes almost all stars to move on the $ celestial sphere$ in a sufficiently large time span. And 18,000 years are such that some stars still imperceptibly others in a striking way find themselves in a different position from those in the vicinity that perhaps moved less.
With all these journeys into the past and the future we have forgotten to see what the Southern Cross looks like now: Stellarium, as a planetarium, shows us once again what this constellation looks like in the sky. It is an inanimate object, a cross in fact, that a little more prosaically could also be seen as a kite: the four main stars are of first and second magnitude and this is the reason why this constellation is well known because it is flashy, even if physically we don't see it in the sky. By the way, the Southern Cross holds the record of being the smallest of the 88 constellations, while in ancient times its stars were part of the Centaur (that we will know soon) and represented one of its clogs.
The name, the story, the myth
Anciently the stars of the Southern Cross were part of the Centaur constellation (Ptolemy catalogued them among the stars included in the Centaur's paws), but it became autonomous with the first sea voyages, perhaps by Lacaille even if it seems that the first detachment there was in 1679 by an astronomer named Augustin Royer.
R.H. Allen, in his Star Names, highlights the course of an older tradition that identified the cross. The Arabic al-Biruni (XI century). reports that from latitude 30° North, in India, a southern group of stars known as Sula, the "crucifixion ray", was visible. This, Allen suggests, could provide the key to the verses of the Divine Comedy in which Dante Alighieri, passing in Purgatory at the entrance to the southern hemisphere, declares: "I turned my right hand, and may/to the other pole, and saw four stars/ never seen outside the first people" (Hymn 1: 22-24).
Now the stars of the Cross are no longer visible in the northern hemisphere, but they were at the latitude of Jerusalem at the time of Christ: "the first people" would therefore be the first Christians, and Dante, aware of the effects of the precession of the equinoxes, would refer to an era without gods later, when the Cross had slowly disappeared at those latitudes.
Although it is the smallest constellation in the entire southern hemisphere, it is also the best known and most admired, ending up on the flags of many countries such as Australia (in Central Australia, the constellation was called the "eagle's trotter") and New Zealand. More similar to a kite rather than a cross, the constellation has brighter stars than its northern alter-ego, the Big Dipper.
I went to visit some astronomer friends Etacruxians and Gacruxians, but it was disappointing to know that our dwarf star knows very little about it since it is very weak. You have to understand them: the Etacruxians have only two stars with negative magnitude (just Gacrux and Canopus with -1) and poor little ones with naked eye (just "eye" since they have only one, lateral, so they always crash) can see up to magnitude 3.5 . Also the Gacruxians have only two very bright stars to see (once again Canopus and a new entry, Miaplacidus, both of the constellation of the Carena), but unlike the polyphemes of η Cru, they have five eyes and seeing that their star is so reddish, looming and bright, they are forced to wear all the day (which lasts 432.5 days today) some very strange "Gacrux glasses". So let's imagine if with these very thick dark lenses they can see a very small star...
All right, I was joking. Did you get the "Gacrux glasses" joke? Yeah?! Then let's move on.
Of well other scope is the star HIP 60308 of yellow-orange colour and with a diameter equal to 130 times that of our Sun: from 10 UA it appears with a disk of 6° and half, much bigger than the 5° and broken of length of the Cross, as we see it in the sky (or better as we see it in the austral hemisphere). I add that this star is found to more than 2000 a.l. so from such a great distance our Sun (of 14 magnitude) is dispersed in a sea of nothing (so many weak stars), cheered, it is made to say, by the presence of Deneb (famous star of the Swan) that shines of magnitude 3.4 . However, let us console ourselves with the fact that the astronomers Hipsessantatrecentoottani know perfectly our dear Sun, having a pair of very sensitive eyes and a beautiful carrot colour, which allows them to see up to magnitude 20.
Representations of the Southern Cross
Let's see together how the constellation was represented in Uranometria
by astronomer Hevelius
and according to Stellarium
It has to be said that there wasn't much to strive to represent a cross, more or less set with precious stones!
This time there are no stamps, but it has to be said that the constellation is exploited a lot in the flags.
Some flags with the Southern Cross
I have collected in a single drawing four states that have the constellation in their flag and to these I have added two regional flags of two neighboring areas, in the extreme south of South America, the Magellan Region in Chile and the most famous but desolate Land of Fire in Argentina, which we see drawn in this map.
I confess that I had never seen the flag of Tierra del Fuego and I find it very nice, with that stylized seagull that divides the two parts, one orange and the other celestial with the Southern Cross in the celestial part.
Another flag that contains the constellation is the green and gold flag of Brazil.
where, together with other stars and constellations, the Southern Cross is placed in the centre of the dark blue circle, but represented specularly. If we ask ourselves why, I could answer like Bob Dylan in one of his mythical songs: "... the answer is blowing in the wind ", which, if you think about it for a moment, is really appropriate for a flag...
Small constellation with few names
Let us now see the few names of the few stars of the Southern Cross: you learn them immediately and it is impossible to forget them.
- Acrux (α Cru): it derives from α crux
- Becrux (β Cru): derived from β crux
- Gacrux (γ Cru): derived from γ crux
A real mystery is why the poor star δ Cru was not called Decrux, while it must be said that Becrux is also called Mimosa.
When can we observe the Southern Cross?
Never, as I said before.
In this case the saying "if Muhammad doesn't go to the mountain, the mountain will go to Muhammad" is valid, where obviously we are the mountain that has to move to the southernmost latitudes in order to see this beautiful constellation. In fact it is enough to go to the south of Egypt to be able to observe it.