Constellations

Constellations are conventional groupings of Stars, whose position in the night sky is apparently close. They are part of the origins of astronomy.

In the cover image you can see stars constellations. Credit: web “freepik.es”

From the most remote antiquity, in the Mesopotamian, Chinese and Hindu cultures, the sky watchers devised figures formed by groups of stars connecting them with imaginary lines.

The astronomers of those towns grouped the main visible stars in constellations to which they gave allusive names to the imagined figures.

In the Odyssey (9th century BC) Homer mentions the constellation Orion.

Orion stars
Orion constellation. Credit: web “nineplanets.org”

The stars of a constellation are only apparently together.

In reality they can be thousands of light years from each other, although in the night sky they appear to form figures such as a scorpion, a lion, a bull or the belt of a celestial hunter (Orion).

In the second century BC Ptolemy included in his work Almagest a catalog of 1,022 stars, grouped into 48 constellations.

Ptolemy
Ptolemy, German engraving 16th Century. Credit: web “wikiwand.com”

This catalog mentioned only the stars visible from Alexandria, the place from which Ptolemy made his observations.

Ptolemy’s work would have been permanently lost if it had not been recovered by Arab astronomers, who added some stars from the southern hemisphere that Ptolemy did not know.

These 48 constellations were the only ones recognized in the West until the late Middle Ages.

The Chinese constellations are one of the oldest stellar groupings in the world, although very different from the constellations recognized by the International Astronomical Union, which are based on Greek astronomy.

The Incas gave great importance to the constellations, which they associated with the rainy season of interest for agriculture. Its constellations include the Southern Cross and the Centaur.

South Cross
South Cross. Credit: Wikipedia, Akira Fujii/DM1

For them the Milky Way was obscured by sacks of coal. Astronomy played a very important role in the construction of their cities.

Starting in 1928, the International Astronomical Union (IAU) decided to officially regroup the celestial sphere into 88 constellations.

The work was mainly carried out by the Belgian astronomer Eugène Joseph Delporte and published by the UAI in 1930.

The 48 constellations of Ptolemy were included in the UAI list.

Zodiac constellations

Among the best known constellations among these 88, are the 13 of the Zodiac.

The Zodiac is a band of sky located around the celestial equator through which, apparently, the Sun and the planets pass.

Zodiac
Zodiac constellations. Credit: unknown

During the V century a. C., the Zodiac was divided into 12 equal parts (one for each month of the year) to which they gave the name of the closest constellation.

Due to the precession of the equinoxes, the constellation of Ophiuchus has been introduced between Sagittarius and Scorpio, so that during the first half of December, the Sun enters this constellation.

For this reason, for some time, it has been considered the thirteenth constellation of the Zodiac.

The other twelve are: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces.

Astronomical atlas

Starting in the 16th century, when European sailors began exploring the South Seas, they encountered an unknown sky.

In order to be able to guide themselves by means of these new stars, they devised new constellations.

In 1603, the German astronomer Johann Bayer (1572 – 1625) published the first astronomical atlas that covered the two hemispheres. He called his work Uranometry.

In addition to including the 48 constellations of Ptolemy, Bayer added 12 constellations plotted for the southern hemisphere, the only place in the world where they were seen.

Bayer’s atlas was the masterpiece of the time. Remember that the telescope had not yet been invented.

Uranometry
Uranometry. Credit: Johann Bayer, Wikipedia.

The new constellations he described were:

  • Apus, the bird of Paradise;
  • Chamaeleon, the chameleon;
  • Dorado, the fish;
  • Grus, the crane;
  • Hydrus, the male hydra;
  • Indus, the American Indian;
  • Musca, the fly; the turkey;
  • Phoenix, the phoenix;
  • Triangulum Australe, the southern triangle;
  • Tucana, the toucan;
  • Volans, the flying fish.

The exotic names of these new constellations assured them immediate success.

In our days, the constellations have lost the importance they once possessed.

Now professional astronomers refer to objects by their position on the celestial sphere, using the coordinate system.

Generally speaking, only amateur astronomers continue to know and study constellations.

In order to correctly identify the constellations, it is necessary to be able to see the stars that draw their figures.

People who live in cities or neighboring areas see very few of them because light pollution adversely affects the visibility of the dimmest stars.

Orion
Orion constellation is easy to spot Credit: web “earthsky.org/?p=3014”

The Orion constellation is one of the most beautiful and easiest to identify, especially in the southern hemisphere.

The constellation Canis Major

The constellation Canis Major is a set of stars that seem to follow Orion, the hunter.

The brightest star in Can Major is Sirius, the brightest star in the night sky, second only in brightness to the Moon and the planets Venus, Jupiter and Mars.

Canis Major
The constellation Canis Mayor. Credit: web “en.wiktionary.org/wiki/Canis_Major”

Sirius is one of the closest stars to Earth (8.7 light years) and around it another star orbits: a white dwarf called Sirius B.

Can Major has many other stars, among which there is one that has 2 orbiting planets around him. There is also in Can Mayor a binary system made up of two pulsars.

The constellation Gemini

The constellation Gemini (the twins) has two very bright stars: Castor and Pollux.

Geminis
The constellacion Gemini. Credit: web “space.com/16816-gemini-constellation.html”

Castor is orange in color and with a very powerful telescope you can see that it is not a single star, but rather a 6-star system.

The constellation Unicorn

The constellation Unicorn , also called Monoceros, is surrounded by Orion, Gemini and Can Major.

Unicorn comes out at nightfall. To find it, focus in on the bright stars Betelgeuse, Sirius and Procyon.

Unicorn
The constellation Unicorn. Credit: web “earthsky.org/?p=3032”

They make a triangle, which – in the Northern Hemisphere – is sometimes called the Winter Triangle. But these stars can be seen from the Southern Hemisphere, too.

In fact, Orion and the stars near it are even better placed for viewing from the southern part of Earth’s globe.

The most interesting thing about this constellation is what William Herschel and his sister Carolina discovered in 1781: that one of its stars, the beta unicorn, is actually a system of 3 stars that form a triangle and that, according to Herschel’s words, constitute one of the most beautiful visions of heaven.

How to identify constellations

To make successful observations, it is recommended to find a dark place. It is advisable to start with a constellation, which we have seen and recognized forever.

From that constellation, we move our eyes to the constellations that it has next to it, comparing with what we have on a map of the night sky.

Once a constellation has been identified in the night sky, the others are found quite easily.

You have to look in the sky and on the map for the constellations that are next to the ones you are identifying. In a single night you can learn half of the 88 in total.