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Polaris: Earth's North Star
Many people know of Polaris, the Earth's North Star, Pole Star or Cynosure. This respectably bright second-magnitude star makes it relatively easy for navigators to know latitude here on Earth.
Presuming that you live north of the equator, Polaris is found due north every single night of the year. If you're at the Earth's North Pole, Polaris is at zenith (straight overhead), and if you reside at the equator, the North Star sits on the horizon.
If you're midway between the and North Pole, Polaris shines midway between the horizon and zenith. That's an altitude of 45o and a latitude of 45o. Polaris' altitude - angular distance above the horizon - equals your latitute.
Omicron Draconis: Mercury's North Star
Yes, Mercury has a northern pole star called Omicron Draconis, though it's nearly twelve times fainter than Polaris. Even so, Omicron Draconis is bright enough to see with an unaided eye in a dark sky and we could use this star to navigate on the planet Mercury.
Polaris' magnitude equals 1.97 and Omicron Draconis magnitude equals 4.63. (The lower the magnitude number, the brighter the star.) For instance, Polaris is 2.66 times brighter in magnitude than Omicron Draconis (4.63-1.97=2.66). But 2.66 times greater in magnitude means 11.59 times greater in brightness.
Photo of Mercury: NASA
According to the magnitude scale, a 1st-magnitude star is 100 times brighter than a 6th-magnitude star. In the rather complicated magnitude system, a 5th-magnitude star is 2.51189 times brighter than a 6th-magnitude star, a 4th-magnitude star is 2.51189 x 2.51189 times brighter than a 6th-magnitude star, a 4th-magnitude star is 2.51189 x 2.51189 x 2.51189 times brighter than a 6th-magnitude star, and so on and so forth.
Finding Omicron Draconis
Omicron Draconis, though faint, is quite visible in a dark sky, though more of a challenge to locate than Polaris is. Perhaps your best approach would be to locate the constellation Draco the Dragon's two brightest stars - Rastaban and Eltanin - and then star-hop from there. (See the chart of Draco below, with the arrow drawn from Rastaban to Omicron.) Eltanin and Rastaban represent the Dragon's Eyes and are found near the 1st-magnitude star Vega.
Sky Chart of the Constellation Draco the Dragon
Image credit: Wikipedia. Click here for a larger chart.
Unlike any other solar system planet, Mercury's rotational axis stands perpendicular to its orbital plane. Here, on Earth, our rotational axis tilts 23.5o out of perpendiclar with the ecliptic - Earth's orbital plane. You might expect Omicron Draconis to mark the pole of the ecliptic, but no, that's not the case. Although Mercury stands straight up and down relative to its orbital plane, the inclination of Mercury's orbital plane to the ecliptic is 7o.
The pole of the ecliptic is found close to Omicron Draconis, Mercury's pole star. The Cat Eye Nebula (NGC 6543) almost exactly marks the ecliptic pole, but you need a telescope to see it.
Mercury in Your Sky
To find out when and where to look for Mercury and the other planets in your sky, read EarthSky's guide to the five visible planets.
copyright 2012 by Bruce McClure