10-03-2007 The planet that wasn’t (article 76)

[Simon Kenny]

The planet that wasn’t (article 76)

In the handle of the star pattern we call the Plough, found overhead during spring, take a look at the 2nd star from the end of the handle. This is the star Mizar, and close by lies another star, Alcor, together forming probably the most famous double star in the entire heavens. Both are visible to the unaided eye, and if you have good eyesight you can easily see both stars as separate suns. A pair of 10x50 binoculars will show them beautifully, and you have them steadily held, (best if on a tripod), you may see a fainter star that when taken all together, form a flattened triangular pattern — see image below. Alcor and Mizar are not true physical companions however. In space they are actually separated by 3 light years, but in a 2-inch diameter or larger telescope you’ll find brighter Mizar is a true double, the first ever discovered, by the Italian astronomer Riccioli, way back in 1650. Mizar and it’s gravitationally bound companion actually form a quadruple star system, visible only in special instruments as the other two stars lie so close to their primary that they cannot be seen optically.

But what is this to do with planets you may be asking? Well let us return to that faint star that formed the flattened triangle with Mirar and Alcor. In 1722 the German mathematician Johann Liebknecht was observing this part of the sky, and thought he saw that star move against the background stars from one night to the next. In his excitement, he concluded that it was not a star but a new planet, and christened it Sidus Ludoviciana (or Ludwig’s Star) after his King. Subsequently, time was to prove his perception as erroneous, but to this day the star is still referred to as Sidus Ludoviciana. One of many interesting footnotes in astronomical history.