Re: Star Magnitude: why was Sirius not used for #1?

Hipparchus (160-127BC) Greek, is considered to be the first great observational astronomer of Western Civilization. Among other things he constructed a catalog which listed about 1080 of the brightest stars in the sky by position and also by brightness. He divided the stars into six classes according to their brightness. These classes were known as magnitudes. The twenty brightest stars were collectively called first class stars, the next faintest stars were called second class stars, and so on until the last group of stars which were called sixth class stars. The sixth class stars were the faintest stars that Hipparchus could see. Thus, the sixth class stars were those which were at the limit of Hipparchus's ability to visually see with his eyes. Remember that telescopes were not invented yet!! Ptolemy who lived in the Greek settlement of Alexandria in about the year 140AD improved upon Hipparchus's work, but he also was a visual observer without the aid of optical instruments. Here is the important point to consider when answering your question. No attempt was made to distinguish any one star within the class grouping as being any brighter than any other star within the same grouping!

Then in the nineteenth century, astronomers began making more precise determinations of the relative brightness of the stars and they found that the brightest stars in the night sky were about 100 times brighter than the faintest stars. On Hipparchus's magnitude scale the moderm measurement of 100 times corresponded to the difference between first magnitude and sixth magnitude, so today we consider any difference of five magnitudes to be a difference of 100 times in intensity.

The intermediate magnitudes, then represent differences in intensity of 2.5, because 2.5 to the fifth power = 100, where 5 is the number of magnitude jumps that equals the 100 times difference in intensity.

The stars in Hipparchus's catalog were transfered into the newer, more accurate measurement scale during modern times (where we now have modern instruments that can accurately measure the differences in brightness). A difficulty then presented itself when astronomers began using optical aids. They soon realized that some of the brightest stars were actually MORE than 100 times brighter than the faintest stars. These then became classed as the zero-magnitude stars. Sirius, which is the brightest star in the sky other than thes sun, has an apparant magnitude of -1.4 on the modern scale. Had Hipparchus realized just how bright Sirius really is, perhaps he would have used it as a standard starting value for his scale, but the naked eye is not as discerning as the instruments of the nineteenth century, so we now are accustomed to using a list of positive and negative values today.

It may also be of interest to note that the lower end of human visual ability to see stars has been explored in modern times and several experiments have determined that some people can see fainter stars than sixth magnitude. Some have reported that experienced observers can see stars as faint as 7 1/2 magnitude.