Blue Planet Project

This article discusses how the Precession of the Equinoxes changes our Pole Star

In the weather-superstition department, March comes in like a Lamb and goes out like a Lion, or vice-versa. In astronomy, it comes in with a Goat and goes out with a Lion and a Ram - in early March Capricornus the goat rises just before the sun; sunset finds Leo the Lion rising and Aires the Ram setting.

Of course, March also brings us Spring. Each year, the Sun crosses the equator on its way North sometime near the 21st of March. Remember that the point where the Sun's path on the ecliptic crosses the equator is called the first point in Aires, even though it's in Pisces, now, and that the zero meridian is defined as passing through that point, too. The three lines intersect - two by nature and one by scientific definition.

If Earth's axis were not tilted, the ecliptic and the equator would be the same, astronomy would be less interesting, and the North celestial Pole, currently marked by Polaris, would be near the cross I've placed in the dim northern constellation Draco. That point is known as the North Ecliptic Pole, and is the centre of the circle drawn by the North Celestial Pole as it traces its 26,000-year-long precession.

The Earth's precession causes the stars to wander through the seasons, the First Point in Aries to wander along the ecliptic, the celestial coordinates to drift, and the Pole Star to change. Each of the stars circled has been a pole star, and will be again, every 26,000 years or so. You should be able to find Polaris on the map. Moving counter-clockwise from Polaris, you find pole stars gamma, beta and alpha Cephei in Cepheus; Deneb in Cygnus the Swan; Vega in Lyra; Tau Herculis in Hercules; Thuban in Draco; and Kochab, one of the Guardians in the Little Dipper. Between these stars there are no bright stars, so most of the time there is no star near the celestial pole to help with navigation.

That's the situation in the southern hemisphere: no star is now near the southern celestial pole. In fact, its 26,000-year circle takes it past surprisingly few bright stars. Indeed, lore from the time when people were settling the islands of the South Pacific make no reference to navigating by the stars.


Here are a few challenges. You might find it easiest to solve them with a planisphere or star calendars. A little logical thought and some simple geometry will speed you along, too.

  1. Next clear night, go out and spot all the northern pole stars. From our location in southern Ontario, all but one are circumpolar, meaning that they never set. Now, that doesn't mean you can walk out on just any clear night and see them all, just that they do not go below the theoretical horizon. The one that sets is only below the horizon for at most a few hours. Two others just graze the horizon.
  2. Which one is not circumpolar? How long does it set for, and on which night is it down the longest ("night" meaning starting at sunset)? How far north would you have to go for it to be circumpolar?
  3. Which two graze the horizon?

When you think you have the answers,click here: