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The use of magnitude as a unit of measurement

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#1 derangedhermit

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Posted 23 January 2013 - 06:44 PM

Does it bother anyone else?

- The scale itself is unique, I think; the result of an approximate back-fitting to a scale of historical (Greek) usage and extension from there. "5th root of 100", Pogson's ratio.

- The scale doesn't match the human visual system (pho- or sco- or in between), nor does it match anything else.

- "Absolute" isn't, and conversion to something that is, isn't as straightforward as it might be.

- Is it used in any other field besides astronomy? (I don't know if professional astronomers use magnitudes - if they don't I'd like to know what they do use.)

The system of measurement of the brightness of something should be as simple and directly related to standard units and the physics of the thing itself as possible.

This has bugged me for a while, and today I "share" my irritation about it.

Lucky you.

#2 Jon Isaacs

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Posted 23 January 2013 - 07:26 PM

Does it bother anyone else?



Not me, I am too old to worry about such things. :)

It's actually quite close to a natural log scale, probably not bad for such things... where there is a wide range of amplitudes.

It's something I understand, that's good...

It's not hard to remember, 5 magnitudes is a factor of 100 in brightness and 2.5 is close enough to work for all situations we find ourselves in.

Jon

#3 Michael2

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Posted 23 January 2013 - 08:46 PM

Is it Metric or Imperial?? :p

Michael

#4 Jon Isaacs

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Posted 23 January 2013 - 08:52 PM

Is it Metric or Imperial?? :p

Michael


Michael:

I am quite sure that Magnitudes are based on the old Greek Whitworth system.

Jon

#5 BillFerris

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Posted 23 January 2013 - 11:36 PM

Does it bother anyone else?


Has he been crank-calling you, too?

Bill in Flag

#6 derangedhermit

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Posted 24 January 2013 - 04:16 AM

Is it Metric or Imperial?? :p

Michael

Ptolemaic. In origin. Or maybe Hipparchian.

"Modern" system, an adjustment to permit accurate, if not entirely intuitive, comparison: Pogsonian.

#7 Tony Flanders

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Posted 24 January 2013 - 05:39 AM

Does it bother anyone else?


It has its problems. But so would any other system that might replace it.

The fundamental issue, of course, is whether to use a linear or logarithmic scale. A linear scale has the great advantage that you can do simple arithmetic on the values -- adding the brightness of two double-star components, for instance.

However, logarithmic scales are usually used for measures like this that routinely vary over a huge range of values. Think, for instance, of decibels for sound or the Richter scale for earthquakes. They also have the virtue of approximately matching human perception.

Magnitudes also have the unusual feature of running backward, using larger numbers for fainter objects. But that turns out to be handy in a field where astronomers are striving to see fainter and fainter objects.

The scale doesn't match the human visual system (pho- or sco- or in between), nor does it match anything else.


Why do you say that? On the contrary, I would say that the close fit between Ptolemy's seat-of-the-pants classifications and a logarithmic scale base 2.51 indicates that it does indeed match human visual perception.

I don't know if professional astronomers use magnitudes


Yes, of course they do.

The system of measurement of the brightness of something should be as simple and directly related to standard units and the physics of the thing itself as possible.


What would you propose? Picoergs per square millimeter? How would you calibrate your instruments in those measures?

Absolute magnitude is a measure of power -- plain enough. But absolute magnitude can't be measured directly. It is computed based on the measurable quantity (relative magnitude) and distance, which is usually not known very accurately.

#8 GeneT

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Posted 24 January 2013 - 04:49 PM

You raise some good points. However, I just got used to it. Anyway, I am now too old for another change. I still can only think in F and not C temps. :grin:

#9 derangedhermit

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Posted 24 January 2013 - 05:48 PM

Why do you say that? On the contrary, I would say that the close fit between Ptolemy's seat-of-the-pants classifications and a logarithmic scale base 2.51 indicates that it does indeed match human visual perception.


Why do you say that? Are you saying that humans perceive a mag 6 star to be 1/5 as bright as a mag 1 star? Using what vision mode? How far off does something have to be for it to be a "close fit" and "match" to you?

What would you propose? Picoergs per square millimeter? How would you calibrate your instruments in those measures?


Pick one or more applicable SI units. Watts. Cd/m2. Lumens. Something along those lines. I'm not sure about your picoergs/mm2 proprosal - maybe you can explain it.

The only instrument I use, besides my eyes, is the SQM. I think it tries to collect and convert photons, and then does some math to produce its display in the desired units. I can conceive one could change the software program to provide the units one wanted for display. I'm pretty confident the device isn't directly collecting magnitudes.






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