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I remember a method to make a reasonable estimate of the magnitude of a given star X by using two other stars of known magnitude as references. The method used evaluation phrases like "the star X and the reference star appear to have the same brightness, and this sensation remains even after prolonged observation" or "X and the other star appear to be the same, but after some time, X appears slightly brighter" or "X and A are completely different in brightness". Each of these phrases had a value (IIRC from 1 to 5) and then with a formula you could estimate the magnitude of X from the magnitude of the reference stars and the values obtained from the comparison phrases.

Do you know the name of the method, and the exact protocol ?

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This method sounds way too complicated to me. I'm an experienced variable star observer, and use the methods recommended by the American Association of Variable Star Observers. The AAVSO publishes detailed charts for thousand of variable stars with comparison stars marked to the nearest tenth of a magnitude. The brightness of the variable is estimated using interpolation between the various comparison stars. One thing in your description is very wrong: you must NEVER stare at a variable star for an extended period, as that will lead to serious overestimation of the brightness of red stars. Complete instructions are available in the AAVSO manual, which can be downloaded free from: http://www.aavso.org/visual-observing-manual

Charts can also be downloaded free from that site.

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it was a historical, rather approximate method. Of course there are better methods today, but I fail to remember the name, and I was curious if someone remember it. –  Stefano Borini Jun 2 '11 at 8:43
    
J. B. Sidgwick, in Observational Astronomy for Amateurs (Faber), describes several methods in detail: Herschel-Argelander Step Method, Pogson's Step Method, Pogson's Mixed Method, and the Fractional Method. –  Geoff Gaherty Jun 3 '11 at 15:37
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