Talk:Magnitude (astronomy)

Absolute scale based on Vega
Article has : "The star Vega is defined to have a magnitude of zero, or at least near. Modern instruments as bolometers and radiometers give Vega a brightness of about 0.03. The brightest star, Sirius, has a magnitude of −1.46. or -1.5".

But "of zero, or at least near" is not a satisfactory definition. Page http://www.astunit.com/faq/uksciastrofaq.htm (reliable) includes "The magnitudes assigned to standard stars are such that they are consistent with each other, and the calibration is based on a star of magnitude 1.0 having an energy of 9.87 × 10-9 W m-2 at the top of Earth's atmosphere", which is a fairly precise definition.

I've not found a definition on an "official" site. 82.163.24.100 (talk) 13:22, 4 April 2010 (UTC)

This user agrees with the concerns raised by 82.163.24.100; additionally, the use of the word "brightness" is unclear in this context, and smacks of a bad translation from scientific terms to layman's terms. Does this refer to luminosity? Flux received by earthbound observers? Give us units!

Some of the confusion about Vega being "near" a magnitude of zero may stem from the use of multiple definitions of the magnitude scale or from the historical development of the magnitude scale. —Preceding unsigned comment added by 128.111.23.221 (talk) 20:49, 17 September 2010 (UTC)

But, more importantly, since when does "energy" have units of W m-2?? I guess I'd argue that calls the source into question.

96.255.159.197 (talk) 02:55, 27 February 2011 (UTC)mjd

Call for Basis of Absolute Magnitude Scale
This article sorely needs a strict definition for the basis of absolute magnitude. A zero absolute magnitude defined by "Near Vega" is actually a bit ludicrous as a basis for an entire branch of science. I can't believe that a definition based on physical SI units does not exist. I don't know enough astronomy to define it but I do know enough math to know it must exist or else the whole scale is meaningless.ArtKocsis (talk) 04:23, 30 September 2013 (UTC)

Merge Apparent magnitude and Absolute scale based on Vega sections?
It ocurred to me only after creating the 'Apparent magnitude' section, and re-reading things after coming back to the article, that perhaps this would be better merged with the section below it. As far as I'm aware apparent magnitudes (as I termed it) are normally found with reference to Vega (defined as having zero magnitude, as mentioned in that section). While discussion is ongoing about the 'Absolute scale based on Vega' section, I didn't want to compound things by just going ahead and pushing the two together straight away. — Preceding unsigned comment added by Newty23125 (talk • contribs) 23:02, 26 February 2011 (UTC)

Irrational Numbers?
In the article the current statement is made:

The use of the 5th root of 100 is difficult in computations as it is an irrational number.

It's only difficult to use irrational numbers in computations if one insists on using them when not required. So, in this case, that statement appears to suggest that the following calculation, where M stands for some magnitude, is required:

relativeBrightness = (2.512...)-M

Where 2.512... is the irrational 5th root of 100, as detailed in the text. But there are two far more preferable forms that don't require irrational numbers at all. The first is the most obvious form from the definition:

relativeBrightness = 100-M/5

The second is more useful for people using hand-held calculators that have built in 10x functions:

relativeBrightness = 10-2M/5

I'll alter the text if that's considered helpful, but I'll leave it for the original author or someone else more interested to add as I don't consider myself a particularly experienced Wiki contributor.

Thanks!

96.255.159.197 (talk) 01:10, 27 February 2011 (UTC)mjd


 * Looks good to me --  Newty   12:40, 27 February 2011 (UTC)

Quotations in background section
I'm all for quotations to give a flavour of the historical development of the subject, but I think entire passages are a little excessive. Perhaps it could be cut down to just the necessary stuff? --  Newty   00:33, 21 December 2011 (UTC)

Absolutely not! That is the first and ONLY explanation of the basis and history of the stellar magnitude system that I have ever seen. I recently read, from cover to cover, an entire college level text book on astronomy and not once was there the slightest mention of the basis of the system nor its history. All the formulations and equations were relative to some mythical base absolute luminosity. Also there was no explanation as to the reason for the counterintuitive progression of magnitudes getting larger as the apparent luminosity got smaller nor even when it began. I was left guessing that the entire system was recently invented as an exponential progression and that astronomers didn't like to deal with negative exponents. Nothing in the book hinted on how wrong I was. That one full quote made it all make sense. In fact, there ought to be an entire article expanding on that very section. It is a great disservice to the curious to keep that history hidden.ArtKocsis (talk) 00:38, 30 September 2013 (UTC)


 * Please note I didn't suggest removing the "Background" section! Merely that perhaps the extensive quotation provided can be summarised more succinctly instead (at least, that was the intention behind my comment). For me personally - although interesting - a quick sentence or two would be more useful to me than looking through the whole paragraph there. People wishing to read further could follow the citation and read the full book cover-to-cover if they wish. :-) --  Newty   22:58, 17 February 2014 (UTC)

Contradictory Statement Re Approximation vs Precise
The statement "Thus in 1856 Norman R. Pogson of Oxford proposed that a standard ratio of 2.512 (\approx \sqrt[5]{100}) be adopted between magnitudes, so five magnitude steps corresponded precisely to a factor of 100 in brightness." is untrue and self contradictory.

Did Pogson define the ratio as 2.512 or as the 5th root of 100?

If as 2.512, then the "precisely to a factor of 100" is wrong and should read "approximately a factor of 100". If as the 5th root of 100, then the text should read "approximately 2.512" and exactly as the 5th root of 100.ArtKocsis (talk) 04:42, 30 September 2013 (UTC)

Section on Apparent and absolute magnitude
This section reads like an orphan from an earlier version of the article. It says "Apparent magnitude, the brightness of an object as it appears in the night sky. For example, Alpha Centauri has higher apparent magnitude (i.e. lower value) than Betelgeuse, because it is much closer to the Earth."

Do astronomers really talk in this confusing way? It contradicts the language of the final sentence of the next section: "Furthermore, many people find it counter-intuitive that a high magnitude star is dimmer than a low magnitude star."

Surely it makes more sense to say that AC has lower apparent magnitude than B, ie, it appears brighter, because it is much closer to the Earth. It would help if the actual magnitudes of these stars were quoted!

And apparent magnitude is not the brightness of an object at all. It's a measure of the brightness, and a pretty quirky one, at that.

David Brightly 82.13.144.73 (talk) 21:42, 16 February 2015 (UTC)

Manilius invented the scale?
After reading the fifth paragraph of the article I started to doubt about it. There are no sources given and should be deleted by Wikipedia guidelines. Most of the web says that Manilius didn't invent the magnitude scale, the only article where this is given is this. But that is the view of the minority and hasn't been proved yet, thus we should state Hipparchus as the inventor. – AstroFizMat (talk) 09:28, 1 February 2021 (UTC)
 * As nobody commented on my question, I take this as approvement for the correction. – AstroFizMat (talk) 21:46, 8 February 2021 (UTC)

File:Magnitude illustration.svg
The image from seems to assume a linear RGB value, so that 25.5 is 1/10th of the brightness of 255. This is not how sRGB colors work: disregarding the effect of "viewing flare", 25.5 corresponds to an actual output power of  relative to the output power of 255.

(The code goes from #ffxxxx for "255" to #19xxxx for "26", so the color is indeed wrong by assuming linearity.)

So uh, here's what I plan to do.


 * Remove the RGB brightness row altogether.
 * Amend the colors so they are dimmed using the linear rules. Instead of multiplying all three elements x of the RGB color with a factor n, the correct result should be Round(ToSrgb(ToLinear(x / 255) * n) * 255). (ToSrgb being the "forward" gamma correction function in SRGB, ToLinear being the "inverse" gamma correction function in SRGB.)
 * Consider stretching out the scale a bit, now what we know we can fit a range of 0.01 - 1 very comfortably in there. Maybe we can do mag 0, 1, 2, 3, 4, 5, for example.

I've been having issues with sitting in front of a computer and doing stuff lately, so uhhhh no guarantees as to when this will be done. --Artoria2e5 🌉 03:41, 2 May 2024 (UTC)

Historical Inaccuracies
This page claims that "Pogson's system the star Vega was used as the fundamental reference star, with an apparent magnitude defined to be zero". But Pogson's system didn't use Vega as a fundamental reference star with magnitude zero. I've edited the paragraph to remove the wrong claim. See Talk:Apparent magnitude and for more details. Explanaria (talk) 20:50, 23 May 2024 (UTC)