Greetings SeeSat-L'ers, This is actually a response to e-mail, although I'm sure some new subscribers could benefit from the content, so here it is on the list. Apparent magnitude, the brightness of stars and other celestial objects, is measured using a logarithmic scale. It is a function of the object's luminosity (or intrinsic brightness), it's distance from the observer and the amount of light absorption occouring between the object and observer. The latter being caused via interstellar matter in the case of stars, and the earth's atmosphere, dust, clouds etc in the case of artificial satellites. In ancient times celestial objects were ranked in six classes of apparent magnitude. The brightest of these were first magnitude and those just visible to the naked eye were sixth magnitude. This system became inadequate as fainter objects were discovered with the aid of optical instruments such as the telescope. In the 1850's it was proposed that the physiological reponse of the eye to physical stimulus was proportional to the logarithm of that stimulus (Weber-Fechner law). A difference in apparent magnitude of two seperate objects is thus proportional to the difference in the logarithms of their brightness, i.e. to the logarithm of the ratio of their brightness. To make the magnitude scale precise, the English Astronomer N.R. Pogson proposed, in 1956, that a difference of five magnitudes should correspond exactly to a brightness ratio of 100 to 1. (W.Herschel had shown this to be approximately true). Hence two stars that differ by a single magnitude have a brightness ratio of: _____ 5 / : 1 OR 2.512 : 1 \/ 100 This ratio is known as the Pogson ratio. This can now be applied to any magnitude difference as per the following example. A magnitude 2 star (X) is 3 magnitudes brighter than a mag 5 star (Y). i.e. 5 - 2 = 3 the actual brightness difference can now be calculated using the Pogson ratio. 3 (2.512) = 15.85 Hence star X is 15.85 times brighter than star Y. To create an absolute scale, a group of stars near the north celestial pole where then used as standards to define precise measurement of any other object. This group is known as the North Polar Sequence. As technology developed and more precise measuring techniques were brought into play (photo electric plates, CCD's etc) it was found that some stars and indeed other celestial objects were brighter than mag 0. Hence the introduction of negative numbers, the greater the absolute value of the negative number the brighter the object: _ Object X has mag of -4. Object Y has mag of 5. X is 9 Mags brighter than Y. i.e. 5 - 4 = 9 9 (2.512) = 3982.7 Hence object X is 3982.6 times as intense as object Y To give you an idea of the variation of objects across the scale as found in the real world, here are a few approximate magnitudes. Full Moon = -12 The Sun = -27 Venus (at maximum) = - 4.7 Sirius (the brightest star) = - 1.47 Mir (on a good pass) = - 2 Bright Iridium Flare = - 8 Faintest Naked Eye Object = + 6 Faintest Object Hubble Can Detect = +25 If It's the space station you're after it'll be somewhere around mag 2 to mag -1 on fair to good pass at the moment. It of course will only get brighter as it gets bigger. In fact I would even say that it's probable that the ISS will become the 3rd brightest object in our skies. L8r Jason Gibson Melbourne Australia 37.9803 S 145.0623 E 40m -----Original Message----- From: John Burgess <burgess@ixtel.com> To: scribble <scribble@eisa.net.au> Date: Monday, 7 December 1998 22:35 Subject: RE: More #25544 Observation Dear Jason I saw your seesat message and I am very new to all of this. Please could you tell me how magnitude works. I presume it is literally the brightness, but I don't understand whether -7 is brighter than 2 or whether 4 is brighter than 5. If you do not have time to explain I quite understand but maybe you might know of an explanation on the www. Thanks alot. Regards John Burgess World Office (44) 700 7111 001