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What's Up In the Sky, March 2010

posted Feb 26, 2010, 11:57 AM by Marvin Stewart
                     History books are filled with the names of the men that have sailed in wooden ships to find out what the world we live in looks like and how big it is. We all remember who they were Columbus, Magellan, Cabot, Amerigo Vespucci, the great explorers of the fifteenth and sixteenth centuries. Besides how big the Earth was the next question that needed to be answered was, where in the universe did the Earth belong, and how big was the universe?  Some time before 230 B C Aristarchus, of Samos, held the idea that the Sun was among the fixed stars and that the Earth circled the Sun once a year while also turning once a day. To prove it he attempted to measure the shift of the stars by parallax. He took readings six months apart and hoped to find a shift in the background stars. It didn't work for him. Centuries later Galileo would admit that the only way to prove the Copernican theory was valid was by parallax. Kepler had even written him a letter urging him to use his telescope to see if he could determine a shift in a star. As a matter of fact Galileo would never have been in trouble with the church if just one star would have appeared to move.
                    The method of finding the parallax of a star hasn't changed since Aristarchus first attempted it. The angle to a star is measured and six months later when the Earth is at the furthest point from that point another measurement is taken. A comparison is made to see if there is a shift of that star against the stars in the background. The European Space Agency launched the Hipparcos satellite in 1989 ( Hipparcos stood for HIgh PResicion PARallax COllecting Satellite). It was in use until 1993. During that time it produced a precision catalog of 100,000 stars.
                    Today parallax is used to measure objects out to 500 parsecs. The idea was always valid it just needed the right tools.
                    The next step in measuring stellar distance was to use the standard candle. We owe the standard candle to Alfred " The Great" ( 871-899) King of Wessex. Alfred noticed that the servants replaced his candles every four hours.. He discovered that a slender candle of 72 penny weight burned at the rate of one inch every twenty minutes. Alfred had his candles marked so he could use them as a clock. They also would burn at the same brightness. By 1900 the hunt was on by astronomers to find a standard candle.  From the astronomy standpoint the idea is simple. Light falls off by the square of its distance. Meaning that a point of light that is double the distance from you has 1/4 the brightness of one close to you, and one three times the distance would have 1/9 the brightness. It then comes down to knowing you are looking at a light, or a star, that have the same brightness. Put another way are both bulbs 100 watt?
                    In 1904 a remarkable woman was doing a very unremarkable job.  Henrietta Leavitt was working as a computer, the term computer was used for the women hired to do calculations and other task that supported the astronomer Pickering at the Harvard Observatory. She was examining glass plates ( in today's terms a photographic negative ) taken of the Small Magellanic Cloud. She discovered that one of the stars on the plate would brighten and dim over a period of four and a half days.  She had just found a standard candle, because she also found that all stars that go from minimum to maximum brightness in the same time period have the same brightness. These stars were called Cepheid variables because the first of its kind was found in the constellation of King Cepheus, and Miss Leavitt would eventually find 1,777 of them with periods of from 1.2 days to the longest 127 days. She also noted that the longer the period the greater the brightness. This method would be used to find the distance to the Small Magellanic Cloud and Hubble would use it to measure the distance to the Andromeda Galaxy.
                  Other methods would also be used to find distance also including RADAR, RR Lyrae stars and Type 1a supernovae. One that stands out as accurate and interesting is the Tully-Fisher relationship. In 1977 Brent Tully and Richard Fisher discovered that all galaxies that rotated at the same speed had the same mass and thus the same brightness. So galaxies themselves could be used as standard candles, pushing the distance we can measure accurately further back into time. Supernovae can be seen over the greatest distance of all but they are also rare.
                 01   Saturn is east of the full Moon about 8 deg.
            02-17   Look for the zodiacal light about an hour and 20 minutes after sunset in the western sky.
                03   Spica above the Moon.at dawn.
                07   Antares about 5 deg west of the Moon at Dawn.
                       Last- Quarter Moon.
                14   DAYLIGHT-SAVING TIME begins at  1 a.m. CDT.
                17   Look for the crescent Moon in the western sky after sunset, it is close to the bright planet Venus.
           21-22   Saturn at opposition.
               23    First- quarter Moon 
           23-25   The Moon in Gemini passes Mars andthe bright stars Pollux and Castor.
               29    Fiull Moon
               30    For the next week look for Mercury low in the west just past sunset near Venus.