Arcturus 3000BCE-by John Hlynialuk

If the cave artist who drew the star patterns on the walls of the Lascaux Cave in France 17 300 years ago had been a more proficient astronomer, we might have long ago learned that constellations change with time. If the positions of the stars had been accurately recorded, we might have seen the changes from then to now. Still, to the artist's credit, the work has lasted 175 centuries, and I wonder if we will still be able to enjoy the Sistine Chapel that far in our future. I do hope so.

Normal human lifetimes are too short to see constellations change, but several thousand years is plenty of time if one looks carefully. In one ancient star catalogue, three bright stars gave up their secret motions in 1718 AD to Edmund Halley (rhymes with "valley"), and therein, gentle reader, lies an interesting tale.

Back in the days when Apple Computers, Inc. still made "Apple" computers, I did some simple programming on an Apple II+ using BASIC. One program showed the seven Big Dipper stars and with successive presses of a key, you could advance time into the future. The two end stars change position most noticeably and the Dipper morphs into a measuring cup or scoop (yes, a Big Scoop!). My program required the values of stellar "proper motions", a discovery that could not have been made without a 2000-year-old star catalogue called "The Almagest". The story involves Edmund Halley, two earlier Greek astronomers, and the most boring job in all of astronomy, recording star positions.

The first detailed star catalogue is attributed to Hipparchus, a Greek living from 190 BC to 120 BC in Niceae, (now Iznik, Turkey). Considered the greatest astronomer of antiquity, he was the first to develop a reliable method of predicting solar eclipses, mastered spherical trigonometry, and used those tools to solve many astronomical problems. His catalogue of 850 stars was precise even by today's standards, although the instruments used were large and required a great deal of skill to use. Taking readings was a tedious job that Hipparchus repeated at least twice for each of the 850 stars. Then, in 150 AD, Claudius Ptolemy, another "positional astronomer", added to that catalogue to create a listing of 1022 stars that was published under the name "The Almagest" ("the greatest"). Neither catalogue had a single star chart, but consisted of boring lists of co-ordinates just like the latitude and longitude of locations on Earth.

It is in the Almagest table of numbers (that only a mathematician would appreciate) that a puzzle was discovered. The stars Arcturus, Sirius and Aldebaran in Edmund Halley's sky (ours too) were not at the coordinates given in the Almagest. Other astronomers had noticed this and in 1718, Edmund Halley, whose mathematical prowess earned him a comet name, verified these star motions, trusting the Almagest enough to attribute the stellar "proper motions" to real displacements and not human error.

Nowadays, astronomers have measured thousands of mostly tiny proper motions of stars that do not change the appearance of constellations. The greatest proper motion (for a star visible to the naked eye) belongs to Alpha Centauri in the southern hemisphere. Its motion would have been noticed even in a short human lifetime, but there were no astronomers working in the southern hemisphere until after Halley's discovery.

Arcturus turns out to be a very interesting star, indeed. It is the closest of about 50 "runaway" stars, moving as a group perpendicularly through the plane of our Milky Way Galaxy at a speed of around 400 000 km/h! The Arcturus Stream is not gravitationally bound to the Milky Way and is just "passing through" en-route to the Virgo Cluster of Galaxies, a much more interesting neighbourhood. So say farewell to Arcturus, the Runaway Star.

Where should you look to see Arcturus, Sirius and Aldebaran, while they are still with us? Two of the three can be seen in the eastern sky before midnight. By 10:30 pm Orion is well up in the east, -just line up the three stars of Orion's Belt and follow it downwards to find the first, Sirius, the brightest star in our sky. To locate Aldebaran, travel the same distance up from the Belt, -it's bright and slightly reddish. The last of the three, Arcturus rises in the east after midnight, and can be found by following the curve of the handle of the Big Dipper, ("arc to Arcturus"). Better do it soon because in 100 000 years, Arcturus will have moved into intergalactic space and faded from view. Even the handle of the Big Scoop won't help you locate it!




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