Throughout 2009, the International Year of Astronomy, the citizens of the world are encouraged to discover and enjoy the day and night time skies.
One of the most beautiful constellations is Orion, which is conspicuous in the winter sky. It is named for a gigantic hunter in Greek mythology. Four bright stars, two representing the shoulders and two representing the knees, outline the figure, and in the centre are three "belt" stars with a sword hanging below them.
At the end of February, observers in Ontario see Orion prominent in the south at around 8 p.m., but as the evening progresses it moves westward and sets about an hour after midnight. Stars and constellations rise in the east and set in the west every day, just like the Sun. This daily motion is due to the Earth's rotation on its axis.
However, constellations also change their positions as the seasons progress. If you look for Orion in the early evening at the beginning of May, you will find that it is low in the western sky and will set soon after. In the summer, you won't see it at all because it sets before nightfall. This shift with the seasons occurs because the Sun appears to move around the sky, through the constellations of the Zodiac, over the course of a year. We see only the constellations that are above the horizon during the night. Therefore some constellations are visible in the winter while others are visible in the summer. It all depends on the position of the Sun.
Thanks to the contributions of Galileo and other scientists of the 16th and 17th centuries, we know that the Sun does not really move around the sky. It is just a matter of perspective. As the Earth completes its annual orbit around the Sun, the Sun appears to move.
Nowadays, most of us spend our evenings indoors and are not familiar with the night sky. This has not always been the case. The indigenous peoples in North America had a much better knowledge of the constellations: they didn't have televisions or radios to occupy them at night and their sky was much darker than ours. Furthermore, they didn't have clocks or calendars. Familiarity with the constellations provided a method for keeping track of the time and the progression of the seasons, which was important for agriculture. For example, Aboriginal North Americans knew that when Orion sets in the western sky at dusk, it is time to start planting.
While most constellations can only be observed in certain seasons, there are a few that can be observed throughout the year. We call them circumpolar constellations and we'll talk about them next time.