Sky Science

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Part 2 - Constellations

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2. Describe the location and movement of individual stars and groups of stars (constellations) as they move through the night sky.

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Long-ago, farmers knew that it was time to harvest their crops when certain stars were seen in the sky. In order to remember which stars are which, they grouped certain stars together to form the basis of an image, like a harp, or a ram, or even a herdsman. These are called constellations. To help remember the various constellations these farmers created wonderful stories, which were very dramatic and sometimes tragic. Farmers weren't the only people to create these stories, there were poets and early astronomers as well, who looked to the night sky and imagined wonderful things.

Over the years, people from around the world made hundreds of pictures in the sky by grouping stars together and connecting them with imaginary lines. These days, astronomers recognize 88 official constellations, and all those other pictures in the sky are now referred to as asterisms. Some asterisms, like the Big Dipper, the Little Dipper, the Summer Triangle, and the Teapot, are well known. While some constellations are rather obscure and not widely known, such as Apus, Grus, and Volans. The main difference between a constellation and an asterism is that a constellation has an officially designated boundary in the sky, whereas an asterism does not. So, although both are essentially just pictures in the sky made by drawing some imaginary lines that connect stars together, constellations are different in that they also include the area surrounding the picture. Hence, you should not think of constellations as being merely pictures in the sky, but rather regions of the sky, which are filled with many interesting objects to study, such as galaxies, black holes, asteroids, comets, nebulae, planets, and of course stars.

The asterism called the Little Dipper is located to the right. The blue lines that connect these stars together form the basis of an image, which in this case is that of a Little Spoon.

The exact same image appears below, but this time it is referred to as Ursa Minor, which is a constellation, and there is also a red boundary line all around. All constellations have a boundary around them. That's what makes them different from asterisms.

The boundary lines make it easier for astronomers to share information with each other because they have all agreed where exactly one constellation ends and the next one begins. So, if a comet were passing through this region of the sky, an astronomer would say that the comet was currently located "in the constellation" of Ursa Minor. By doing so, astronomers from around the world would then know what part of the sky to search in order to see the comet for themselves. Before these constellation boundaries were officially designated, many astronomers were frustrated when trying to spot a newly discovered object in the sky because the existing boundaries were usually rather vague.

The Movement of the Stars

Next time, I'm going to write about how the stars move across the sky. Until then, you can watch the video below and see for yourself how the stars move around in the northern sky, as seen from mid-northern latitudes, which is where most Canadians live. This video shows you the stars moving around the sky for 72 hours (3 days) and I'm sure your teacher can tell you more about it. By the way, this is exactly the view you would have if you were inside the AstroDome Planetarium.

Click the "full-screen" button (bottom right) to get a better view. Teachers may want to do this on the SmartBoard.