Star Trails in the Northern Sky

This article was written by Christine (Coutts) Clement for the Thamesville Herald in honour of IYA2009.

It was published on March 18, 2009.

<Star Trails>

This photograph of star trails was taken from the road leading to the top of Mauna Kea, a dormant volcano in Hawaii. Mauna Kea is the site of some of the largest optical telescopes on planet Earth. The photo was compiled from 150 one-minute exposures from a digital camera. It was posted as the NASA Astronomy Picture of the Day on December 20, 2005. Credit & Copyright: Peter Michaud (Gemini Observatory, AURA)

The northern sky is a giant clock. Because of the Earth's rotation, every star traces out a complete circle in the sky over the course of 24 hours. The accompanying picture represents a portion of these circles: about two and a half hours worth.

At the centre of the circles is the North Celestial Pole. You can't see it in the picture because there is nothing there. It's just a reference point in the sky. However, if you look closely at the picture, you will see that there is a star trail very close to the centre. This is Polaris, sometimes called the North star or the pole star.

It is just a coincidence that such a bright star happens to be so close to the celestial pole, but it is a lucky coincidence. If you can observe Polaris, you know which direction is north. You can also determine your latitude. Near the North Pole (latitude 90 degrees), Polaris is directly overhead, but if you proceed south, it becomes lower and lower in the northern sky until at the equator (latitude zero degrees), it is near the horizon. This fact was very useful for explorers like Christopher Columbus when they crossed the oceans. Measuring how high Polaris was in the sky helped them figure out where they were. Travelling in the southern hemisphere was more difficult because there's no pole star in the south.

The picture shown here was taken at a location in Hawaii where the latitude is 20 degrees. Polaris is 20 degrees above the horizon.

Another important feature of the photo is that the lengths of the trails are much shorter for the stars that are closest to Polaris. In fact, some of their circles are so small that they never set below the horizon. These are called circumpolar stars. On the right hand side of the picture, the stars trace out much larger circles so that over the course of 24 hours, they set below the horizon.

Southern Ontario is much further north than Hawaii; our latitude is about 43 degrees. As a result, Polaris appears higher in the sky here. Another consequence of our more northerly location is that there are more circumpolar stars and constellations.

The two most recognizable constellations in the northern sky are Ursa Major (which includes the Big Dipper) and Cassiopeia (which is like the letter "W"). They are both circumpolar in southern Ontario, but not in Hawaii.

However, even though these two constellations always appear in our night sky, they change their positions. Over the course of a night, they move in a counterclockwise direction around Polaris because of the Earth's rotation and they change with the seasons as the Earth moves in its orbit around the Sun.

In March through May, the Big Dipper is almost directly overhead in the evening, but in September through November, it is close to the northern horizon. On the other hand, Cassiopeia is close to the horizon in the evening in April-May, but almost directly overhead in October-November.

The seasonal motions of stars and constellations are the same from year to year. Not so with the planets. Their positions are much harder to predict. Next time we will discuss the motions of the planets.

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