h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

How did Hipparchus miss Sirius? Surely it was there in 2 BC and visible from Greece? Am I missing something?

I guess he labelled it one.

The 18th century astronomers took an 'average' 1st magnitude star, rather than the brightest first magnitude star...

Have a look at the definiton at http://www.bbc.co.uk/h2g2/guide/A471476

This 2.51 thing is a little bit misguiding: It's simply the effect of the definiton, being equal to 100^(1/5).

I _suspect_ (though I don't know) that Polaris has always been the "anchor" of modern magnitudes, and this led to Sirius being below zero. (I think that in former times, Polaris had 2.0 per definitionem. Today it's slightly different, see above.)

The system is calibrated by Vela. Vela is about 0 magnitude in all passbands of the Johnson magnitude system.
Marcelo Allen

Well, not exactly. The first serious approach was indeed with Polaris, or actually the polar sequence, a set of some 300 stars with well known brightness. Today, they callibrate their systems with new very large brightness catalogues.

I don't know much about this Vega thing, but wasn't it because it's an A0 star and thus 'white by definition'? So, to define a stellar colour.

I have never heard about Polaris being reference for magnitude systems. Always Vega is regarded as the calibrating star.
At least for the photometric system.
Astrônomo

You're right, Vega is also used for calibration of brightness measurents. However, as part of a catalogue. (Remember, Vega is not visible from all points on the globe.)

I offer a link to a pdf on my personal space about this.