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Northern Rockies Skies for March

February 26, 2013
Star chart

A monthly look at the night skies of the northern Rocky Mountains, written by astronomers Ron Canterna, University of Wyoming; Jay Norris, Challis, Idaho Observatory; and Daryl Macomb, Boise State University.

Most ancient astronomers, shepherds and farmers knew the night skies as well as they knew the back of their hands.

Every season had its own constellations and every major constellation could be used as a guiding indicator to very prominent stars and celestial features. Recognizing the stars and constellations soon became second nature.

Here are a few guidelines to help you become more aware of the locations of prominent winter/spring constellations and stars. First, you must find Orion, which is so unique in form that it is very difficult to miss. Right after sunset this month, it is located to the south on the meridian about 45 degrees above the horizon.  Remember there are the four stars -- Betelgeuse, Bellatrix, Rigel and Saiph -- surrounding the three major stars of Orion’s belt.

To locate Sirius, draw an imaginary line through the three belt stars to the southeast. In the opposite direction, to the northwest, you will find Aldebaran in Taurus (see map). You also will see Jupiter close to Aldebaran this month.

Now draw an imaginary line from Rigel through Betelgeuse to the northeast and you will come to the twins, Castor and Pollux, in Gemini. An imaginary line from Bellatrix through Betelgeuse to the east will land on Procyon, one of the sun’s nearest neighbors.

Finally, trace an imaginary line from the belt stars through Bellatrix to the north and you will run into Capella, the third-brightest star in the northern celestial hemisphere.

Planet Alert: Besides Jupiter, you can see Saturn, which rises around midnight this month.

Astronomical Instruments: Herschel's 40-foot Telescope

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The great German-English astronomer William Herschel -- discoverer of Uranus and cataloguer of binary stars and nebulae -- built hundreds of telescopes on commission and for his own research. Assisted by his sister, Caroline, he realized his dream of building the world's largest telescope of the time over a four-year period (1785-1789) in Slough, England, located 20 miles west of London.

In that era, telescope sizes were usually designated by their focal length. The 40-foot telescope’s mirror size was 49.5 inches (1.26 meters). The Herschelian design tilted the primary mirror to send light out a side of the top end of a supporting tube, which eliminated light loss incurred by reflection of a secondary mirror that is present in the more familiar Newtonian design. The 40-foot telescope may have been used to discover Enceladus and Mimas, the sixth and seventh Saturnian moons.

Due to its mirror’s properties, not much else was discovered with the 40-footer. Up to that time, objective mirrors of reflecting telescopes were made of speculum metal -- about two-thirds copper and one-third tin -- a brittle alloy that was then polished, resulting in a surface with about 67 percent reflectivity in the visible spectrum. These mirrors, subject to quick tarnishing effects in humid air, required continual repolishing and, therefore, frequent refiguring as well. Additionally, the metal mirrors were thick and suffered differential stresses as they cooled during the night, resulting in highly distorted stellar images.

The 40-footer was the world's largest telescope for 50 years, until it was exceeded by Lord Rosse's 72-inch mirror, the Leviathan of Parsonstown, built by William Parsons in 1845. The 40-footer was dismantled (for fear of structural collapse) in 1839 by Herschel's son, John.

 A 10-foot tube section in the Royal Observatory, Greenwich, remains, and one of two original mirrors is on display in London's Science Museum.

By the mid-1800s, front surface silvering of thinner and lighter glass mirrors was under way. Due to heating and cooling, glass suffers much less distortion than metal. Silver reflects 90 percent in the visible, tarnishes more slowly, and is easily removed from the glass when the time comes to resurface the mirror. Thus, the era of speculum telescope mirrors abruptly ended.

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