Big Bear, Polaris and Finding True North

Big Bear, Polaris and Finding True North

Sky stories from the north celestial pole.

For almost everyone, the stars of the night sky are simply ornamentation to be glimpsed during a late-evening walk. Studying the night sky more deeply leads to the subject of astronomy, which most people may consider too esoteric. Somewhere in between is stargazing, which makes the night sky more familiar without being too intimidating. And historically this familiarity helped us navigate across the world’s oceans.

The stars also remind us of our heritage. For millennia, the star patterns have evoked stories of the beginning of time, the beginning of life and the cycle of life, and they’ve provided cues for our ancestors to plant and harvest or to begin migrations.

The northern constellations offer particular insight into the lives of cultures that were more attuned to the natural environment than we are. I like the Mi’kmaq sky story of the Big Bear (Muin) and the hunt by seven birds composed of species familiar with the Mi’kmaq. It is a story of a continuous cycle of life that is tied to the change of the seasons as well as the appearance of the Big Bear in our sky.

The story begins in early spring as Muin leaves his den and is pursued by the seven bird hunters. The appearance of the sky corresponds to a couple of hours before sunrise. During the summer, Muin evades the hunters. In the autumn, some of the pursuing birds disappear below the horizon, but three of the birds continue and one of the hunters — the robin — kills Muin with an arrow. Muin’s life spirit leaves his body, and the hungry robin is bloodied in the feast. Climbing into a tree, the robin shakes off all but the blood on its breast. The spray of red blood then coats the leaves in the forest, with the maple tree receiving most of the blood.

During winter, the bear’s skeleton lies on its back, as it does high in the winter sky at 2 hours before dawn. The spirit of Muin then enters the body of another sleeping bear, and in spring, the cycle continues. For a more complete version of the tale, check out the video below.

Stargazers can also learn from observations of the night sky. Repeated observations show that although the stars move across the sky throughout the night (they move about 1 degree every 4 minutes and shift westward over the year at about 1 degree per day), these movements are predictable, governed by the constant rotation of Earth, and Earth’s orbit around the Sun, respectively.

This is chart of the northern summer sky. Two types of “North Poles” are plotted: the North Celestial Poles if a few planets (Blue) and the “north Pole” of their orbits (Red). 

This becomes graphically obvious in a long-exposure image of the night sky. The daily rotation of Earth about its axis draws the stars into arcs, and at the centre of those arcs is the north celestial pole. One star, Polaris, becomes particularly prominent in these images. “Close” to the actual north celestial pole, Polaris has guided sailors around the world. However, this has not always been the case.

As with a child’s spinning top, Earth’s rotation helps stabilize the orientation of Earth’s axis. But also like the toy, it tends to wobble, although very slowly. Unlike the 1-second-or-so wobble period of the toy, Earth’s period is about 26,000 years. (The precise period depends on the distribution of mass in our planet, which is not known to great precision.) This wobble, called “precession,” causes the stars to apparently shift roughly eastward around the sky. After 13,000 years, the constellations that grace our summer sky will instead be seen in winter.

What does that have to do with true north?

This wobble draws stars past the north celestial pole. So, Polaris is not very “consistent” as the North Star. Polaris has been within 3 degrees of the true north for only 450 years. In 1940, this distance was down to 1 degree, and now in 2021 it is a mere 2/3 degrees — a bit more than the apparent diameter of the moon. Polaris is closing in on the north celestial pole, but it will never quite make it. The closest it will get will be a bit less than the apparent diameter of the full moon around 2100.

Our north celestial pole is just one “pole” in our sky. We can consider others if we take a wider view. The sun and all the planets have rotational axes, but they are tilted in different directions and angles, and therefore have different north celestial poles. The accompanying picture (directional TK) has these plotted, in blue, for the major planets and the sun. You will see that all but two are clustered in the sky. The axes of Earth and Mars are remote from the rest, but only Earth has a bright star to mark its pole. The tilt of both Earth’s and Mars’ poles produce the seasonal changes in weather for each planet.

This is a 30-minute exposure of the northern sky showing the rotation of the celestial sphere around “True North”. The star closest to the centre of rotation is Polaris. It is the end star in the handle of Little Dipper – the stars that arc about ¼ of the field of view to the left and downward. 

If we consider the orbital planes of the planets in the solar system, we can plot those planets’ celestial poles (marked in red, in the picture). They are all well clustered north of the “head” of the constellation Draco, 34 degrees from Earth’s celestial pole. There are no bright stars near their locations, but the south ecliptic pole is near the Large Magellanic Cloud (a satellite galaxy of our own galaxy, the Milky Way), which is not bright, but at least it is notable.

So trying to define “true north” can lead you into a discussion of not just the history of global exploration, but also the exploration of the solar system.

Robert Dick
Robert Dick

One of Canada’s foremost writers and educators on astronomical topics, the Almanac has benefited from Robert’s expertise since its inception. Robert is passionate about reducing light pollution and promoting science literacy. He has been an astronomy instructor for our astronauts and he ensures that our section on sunrise and sunset, stargazing, and celestial events is so detailed and extensive it is almost like its own almanac.

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Posted on Thursday, March 17th, 2022
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