Belgian skies, spring, beginning of the evening
Man has always linked the stars to imaginary figures in the heavens. There are eighty eight of these imaginary figures and they are called constellations. The Earth takes a year to revolve around the Sun, and in this time the stars change along with the seasons. Even with the passing of hours, we can see heavenly bodies moving past our eyes and it is the rotation of the Earth that is responsible for this phenomenon. During the night, it looks as if the constellations are moving from east to west, rising and setting on the horizon. That is why the stars that were visible at the beginning of the evening are no longer there at the end of the evening. This is also why the heavens look different with every passing season. Only those constellations near the northern celestial pole may be seen for the whole night and even the whole year.
Let’s try and find some. In the vicinity of large cities there is so much light pollution that only the brightest stars can be seen. The Great Bear looks like a frying pan. It’s easy to find because it’s so big and bright. Far from light pollution, it’s possible to see up to 3000 stars with the naked eye. When it’s darker, it’s possible to see the entire Great Bear. Above the pan, is the Little Bear, whose brightest star is the Pole Star, in the north. Further on is Cassiopeia, easily recognisable by its ‘W’’ shape.
In its path around the Sun in this season, the Earth positions itself in front of some typical summer constellations. The summer triangle is, strictly speaking, not a constellation but an asterism, which is a geometric shape in the heavens that is quite remarkable. The first vertex is Altair, the eye of the constellation of the Eagle. The star, Deneb, is the tail of the Swan, it’s possible to recognize the outstretched wings and the neck quite easily. Between the Swan and Hercules, is Vega, the third vertex of the triangle. Vega is the brightest star in the Lyre, a small harp-like instrument which Orpheus used to try and convince Hades to save his beloved Eurydice from the underworld.
Did you know that Vega will eventually become the Pole Star? Apart from the rotation and revolution of the Earth there is also precessional movement whereby the Earth moves slightly on its axis like a spinning top. The heavens therefore shift imperceptibly and Vega will be in the north in about 12 000 years’ time.
Sometimes it’s possible to see some planets, they move almost imperceptibly among the other light points of the heavenly dome and wander among the constellations of the Zodiac. They look brighter than the stars because they are closer to us. Moreover they don’t twinkle, they are stable strong points of light. Only five planets are visible to the naked eye: Mercury, Venus, Mars, Jupiter and Saturn. Just like the earth, they rotate around the sun. They aren’t always in our field of vision but they also seem to move around among the constellations as the weeks go by. Jupiter and Saturn can be seen during various months for a long period of time while the terrestrial planets traverse various constellations in only a couple of weeks.