VOL. 15 July ISSUE YEAR 2014

Off the Beaten Track

in Vol. 15 - July Issue - Year 2014
Stella Polaris
Earths axis precession and wobbling spinning top

Earths axis precession and wobbling spinning top

"The wind lifting his spirits high, royal Odysseus spread sail - gripping the tiller, seated astern -
...sleep never closing his eyes, forever scanning the stars, the Pleiades and the Plowman¡Kand the Great Bear¡Kthe stars the lustrous goddess told him to keep hard to port as he cut across the sea."

With these words written in the eighth century BC, Homer describes the departure of Ulysses from the island of the nymph Calypso in Book V of The Odyssey. It is perhaps the earliest written reference to stars used as an aid to navigation. The "Great Bear" that Ulysses was instructed to keep to his left was the constellation that is also known by its Latin name Ursa Major. The events recounted in Homer's epic poem took place about four centuries before he composed his masterpiece. But what were the stars that guided Ulysses? Did the Pole Star that guides navigators today also show the way to sailors in that distant past?

The Pole Star used by navigators today has been known by various names over the centuries: North Star, Lodestar, Guiding Star and Polaris (from the Latin "stella polaris" = pole star). This star is unique because it is very bright and because the Earth's northern axis points almost directly at it. In other words, Polaris sits right above Earth's north celestial pole. At night Polaris appears to remain in a fixed position while the other stars rotate around it. An observer standing at the North Pole would see the Pole Star directly overhead. Being only 0.7 degrees off the rotational pole of Earth's northern axis, Polaris is especially useful as a navigational tool, as it indicates almost-perfect true north from any vantage point in the northern hemisphere. It's also easy to calculate latitude by simply measuring the angle of the star with respect to the horizon. Navigators in the southern hemisphere are not similarly blessed, for there is no star as bright as Polaris to indicate the position of Earth's southern axis. The closest equivalent is a dim star called Sigma Octantis, at times invisible to the naked eye, also called the South Star.

But was Polaris the star that guided Ulysses about 3,200 years ago? The answer is no, due to a phenomenon called axial precession. Axial precession is a slow and continuous change in the orientation of Earth's rotational axis. Try to imagine a spinning top as it wobbles. In much the same way, Earth's axis "wobbles" as our planet spins through space. Each "wobble" takes about 26,000 years to complete before the axis returns to its original orientation. During this time, the axis points in the direction of different stars, each one of which becomes the "Pole Star" for a certain period of time before passing on the title to its successor.

At around the time that Ulysses was trying to find his way home after the Trojan War, the star Thuban in the constellation Draco was the visible star nearest to the north celestial pole. However, being only one-fifth as bright as Polaris, Thuban was discarded by Greek navigators, who preferred to use stars in the constellation Ursa Major (the Great Bear, also known as the Big Dipper) that were easier to spot, albeit not as close to the north celestial pole. On the opposite shores of the Mediterranean, Phoenician seamen preferred to look for stars in the constellation Ursa Minor (the Little Bear, also known as the Little Dipper), less visible but closer to the north celestial pole.

While Homer was writing The Odyssey, the star Kochab in the constellation Ursa Minor had become the closest to the north celestial pole. However its relatively long distance from the pole made it unsuitable for most navigators, prompting the fourth century BC Greek navigator Pytheas to comment that the north celestial pole was bereft of stars.

During the Roman Empire, the north celestial pole was equally distant from the star Kochab and the star Polaris, also located in the constellation Ursa Minor.

By the fifth century AD, our present Pole Star Polaris had become the most visible and the closest star to the north celestial pole, albeit still about eight degrees off. Polaris will come to within 0.4 degrees of the north celestial pole in the year 2102.

What about the future? In about one thousand years the scepter will pass to the star Alrai and Iota Cephei will have replaced Alrai by about the year AD 5200. After a further five thousand years, the star Deneb will be the Pole Star and Vega will take its place by AD 14000.
When axial precession will have completed its full turn around AD 27800, Polaris will once again be the North Star. What will the human race look like and what planets will it inhabit? Will Polaris still guide us, perhaps in other directions through space?

By Giovanni Gregorat, Contributing Editor MFN

Author: Giovanni Gregorat