To define locations in the sky, we use declination and right ascension.
To define locations, on Earth we use longitude and latitude. Likewise, we divide the continents into countries. In astronomy we do something similar and also divides the sky into regions, as if they were countries: they are the constellations.
Imagine that, as the ancients thought, the sky was a sphere with all astronomical objects attached to it. If we cross the Earth with a stick or shaft that enters through the north pole of the planet and leaves the south pole, and we extend it on both sides in space, this axis will touch the sky in two points: they are the north and south celestial poles . It is as if the celestial sphere revolved around these two poles, although we already know that it is the Earth that moves.
If we stretch the line of the Earth’s equator and stick it to the celestial sphere, we can trace over it the celestial equator. This line will cut the sky in half in the northern and southern hemispheres, just as the equator divides the Earth into two hemispheres.
Now we will draw on the celestial sphere concentric lines parallel to the equator, centered at the poles. These celestial parallels, such as the latitudinal latitude lines, measure at what distance from the equator, either to the north or to the south, there is a point in the sky. A star that is in the celestial equator has 0º of declination. If that star were in the celestial north pole, its declination would be + 90º, and if it were halfway between the equator and the north pole, it would have + 45º.
Each degree of decline has 60 arc minutes (‘) and every minute of arc, it contains 60 arc seconds (“).
Now let’s draw lines in a north-south direction, from the north pole to the south. These are the so-called straight ascension lines, and they measure the position of the celestial objects towards the east or the oste. The celestial equivalent of the Greenwich meridian, with 0º length, is the so-called vernal point: the point of the celestial equator where the Sun meets every year at the time of the spring equinox.
The sky is divided into 24 hours (h) of right ascension, and each of them has 60 minutes (m). Therefore, the vernal point has 0 hours of right ascension (?).
To identify any place on Earth we can give its position in longitude and latitude. But it is easier to imagine where it is if we place it in a specific country. The same happens in heaven. It is convenient to learn to locate some of the most important constellations to be able to orient yourself in the night sky. Thus, the stars will stop appearing as points and begin to be known figures.
Ptolemy , Greek astronomer, listed a total of 48 constellations of vague borders. Currently, we divide the sky into 88 constellations with clear boundaries, as was officially agreed in 1930.
Some constellations are very old, such as Leo and Taurus (Taurus), which were already known by the Sumerians. Orion, the hunter, has been known for thousands of years. The Chaldeans of Mesopotamia called it Tammuz, the Syrians called it Al Jabbar, the Giant. For the ancient Egyptians he represented Sahu, the soul of Osiris. It was the Greeks who gave it its current name in honor of a mythical giant and so-called great hunter.
Other constellations are from the seventeenth and eighteenth centuries. Some were introduced to map the Southern Celestial Hemisphere, which was unknown in Europe before the conquest of America and the era of explorers that began in the sixteenth century. The Southern Cross (Crux), for example, was defined as a constellation for the first time by Andrea Corsali, who sailed to the tropics in 1515 with a Portuguese expedition.
Thus, the constellations are only some figures that we trace in the sky for convenience, linking one star with another as if it were a drawing. Normally, the stars of a constellation are not related to each other in space. For example, Betelgueuse, the star of Orion’s left shoulder, is 427 light years from Earth, while Bellátrix, the right shoulder, is 243 light years away. The stars of the Orion belt are at distances between 800 and 900 light years.
If we could travel to another part of the galaxy and contemplate the stars of Orion, we would not see the same as from Earth. The aliens from other planets will see very different figures, although their constellations include many of the stars we see from here.