Talk:Air mass (astronomy)

message: It seems to me that this section is primarily at “seeing” than air mass as such, but I’d rather defer to someone with more direct experience for retitling. JeffConrad (talk) 00:48, 24 June 2011 (UTC)
thread_title: Section ''Air mass and astronomy''

message: Luis, do you have a source for the formula
:X =\sec\, z \,(1 - 0.0012 \sec^2 z) ?
This looks very much like Young and Irvine's 1967 formula
:X = \sec\,z \,(1 - 0.0012 \,(\sec^2 z - 1)),
except that the term in the inner parentheses is missing a "−1" (adding the term doesn't really help the accuracy much). I'm going to add mention of formulae by Hardie, Young and Irvine, Kasten and Young, Rozenberg, and perhaps a couple of others, and I don't want to duplicate mention of Young and Irvine if that is the one you had intended. JeffConrad 08:03, 29 October 2006 (UTC)
thread_title: Airmass Formulae

message: I've added several airmass formulae, as well as numerous references. I have assumed that the first correction to the secant formula intended Young and Irvine's 1967 formula, so I have revised it to that form (there is little practical difference).
The formula for airmass at the horizon was added without citing a verifiable source; I have retained it for now, but am inclined to delete it because it does not seem to agree with the accompanying text: for the standard ground temperature of 290 K, the isothermal scale height is approximately 8500 m; using this value and Earth's mean radius of 6371 km, the calculated airmass is 34.3, vs. the accepted value of 38. I fail to see how this can be described as "very good accuracy."
I did not revise the graph to add plots for the additional formulae; I did revise the graph caption to indicate that not all formulae are included. JeffConrad 02:35, 31 October 2006 (UTC)
thread_title: Edits of 31 October&ndash;1 November 2006

message: Allen's 1976 airmass table isn't really a 'reprint' of Bemporad's 1904 Table XXIII; the latter was far more expansive, giving values for every 0.1° for zenith angles between 60 and 84°, and for every minute of arc for angles between 84° and 89° (the latter the limit of Bemporad's table). Allen credits Bemporad, Schoenberg (1929), and Snell and Heiser (1968) for the data. I am unable to determine where Allen got the values for 89.51° and 90°; however, my ability to pick details from the text of Bemporad and Schoenberg is severely limited because I don't read German. JeffConrad 07:39, 14 May 2007 (UTC)
thread_title:  Allen's airmass table 

message: The formula was originally given in terms of altitude \gamma as
:X = \frac{1} { \sin\, \gamma + 0.50572 \,(\gamma + 6.07995^\circ )^{-1.6364}}\;;
in this article, it is given in terms of zenith angle z = 90^\circ - \gamma. JeffConrad 06:40, 10 July 2007 (UTC)
replies: text: They actually don't, either in their interpolative formula or in their discussion of the airmass integral at the beginning of the article. Adjustment of the integral for a height other than sea level is easy enough—rather than use zero as the lower limit of integration, use an arbitrary height robs for an observer at an arbitrary height (r is the observer's height plus the radius of the Earth), as is done in the formula given here at the end of Refracting radially symmetrical atmosphere. But it's a numerical integration rather than evaluation of a closed-form solution. JeffConrad (talk) 01:53, 17 October 2010 (UTC)
replies: text: The Kasten and young formula in the book Solar Engineering for Thermal processes (3rd Ed) by duffie and beckman , Page 10 has the numerator as exp(-0.0001184h). I have seen this in other books as well. MySchizoBuddy (talk) 00:22, 22 October 2010 (UTC)
replies: text: I don't have that book, so I can't directly comment. This appears to be a density correction for an isothermal atmosphere with a scale height of about 8435 m. I've seen a few sources include such a correction factor with the “Kasten and Young formula”, but Kasten and Young don't mention it in their 1989 article, which I consider the authoritative source on their formula.  The density correction was apparently added by someone else; though it's done fairly commonly for both airmass and refraction, we need to attribute it to someone other than Kasten and Young if we mention it. JeffConrad (talk) 02:15, 22 October 2010 (UTC)

text: Probably a better way of putting it: this density correction is one that could be just as easily applied to any of the interpolative formulas (and to the homogeneous spherical atmosphere) as to Kasten-Young. JeffConrad (talk) 03:17, 22 October 2010 (UTC)
thread_title:  Kasten-Young formula 

message: Normally, format changes to notes and references are discussed on the talk page before changes are actually made. Is there a reason other than personal preference to reduce the font size for notes? Although I agree that it's normal practice in paginated material, the benefit is less obvious when substantive footnotes essentially are endnotes. One negative is that the notes may be more difficult for readers, especially those over 40 or so, to read, especially when viewed on high-resolution LCD monitors. JeffConrad 08:23, 22 September 2007 (UTC)
thread_title: Font size of Notes

message: I've moved and renamed this section, and removed some redundant material, as discussed above. I've omitted the value for the solar constant because the article is about airmass, not solar irradiance. I'm not sure the reference to ASTM E490 is relevant either, but I've included it nonetheless. I moved the section because it really isn't the main topic of this article.
I haven't reviewed either ASTM standard beyond the online abstract, so including them here is marginal. In particular, unlike the earlier ASTM E891 and ASTM E892, ASTM G173 makes no reference to airmass 1.5 in the title. Perhaps someone familiar with ASTM G173 could confirm that the standard still refers to AM 1.5 internally.
I've changed the lead section to make clear the difference in terminology between astronomy and fields such as solar energy.
As proposed for splitting, the section was titled Air Mass and Standard Reference Spectra, and it read:
:AM (Air Mass) = 1/cosu , where u is the angle of the sun to the vertical http://www.iuma.ulpgc.es/users/jrsendra/Docencia/dispositivos_optoelectronicos/download/teoria/caracteristicas/paneles_solares.pdf. The air mass is an indication of the path length that solar irradiance travels through the atmosphere http://www.powerfromthesun.net/chapter5/Chapter5Word.htm. The higher the value of airmass, the greater the attenuating effect of the atmosphere.
:* Air Mass Zero (AM 0) or extraterrestrial (there are no effects due to air attenuation immediately outside the earth's atmosphere) reference spectrum (ASTM E-490) has been made to conform to the value of the solar constant accepted by the space community; which is 1366.1 W/m2 For all the section. 
:* In Air Mass 1 solar spectral irradiance, the sun´s position vector is perpendicular to the Earth´s surface http://www.volker-quaschning.de/articles/fundamentals1/index_e.html. Corresponds to the sun being directly overhead.
:* In Air Mass 1.5 solar spectral irradiance. It is commonly used as reference condition in rating fotovoltaic modules .
:* In Air Mass 2 solar spectral irradiance, the path of the sunlight through the atmosphere is twice AM 1 and the sun´s position vector has rotated 60.1º away from it perpendicular position.
Much of the material is redundant in this article, but would not be if this section became an article of its own. Without considerable expansion, it's difficult to justify a separate article, but another possibility might be inclusion in a article relating to solar energy that doesn't currently cover the effects of airmass. If this material is moved elsewhere, a few issues should be considered:
#Values for airmass are given, but the method used to determine them isn't stated. In particular, the zenith angle of 61.1° is inconsistent with the formula . There's probably little practical significance, but the alert reader may notice the discrepancy.
# Some of the NREL references have a lot of useful information, but it appears that the NREL site is being reconfigured, redirecting to http://www.nrel.gov/rredc, so it's not clear how long the current links will work.
-- JeffConrad (talk) 03:08, 11 December 2008 (UTC)
thread_title: Edit of 11 December 2008

message: What's the difference between zenith and altitude?  The wiki article on zenith says that no one ever uses zenith for altitude.  Also, JeffConrad above notes that Young did not use "Zenith" but used "altitude".  Seems like this article might be twisting the truth to force the "zenith" terminology.  Ywaz (talk) 12:45, 22 March 2009 (UTC)
replies: text: The wiki article on zenith doesn't mention altitude, and the redirect from zenith angle makes no statement about using zenith for altitude. Be assured that this article is twisting no truth. Whether “zenith angle” or “altitude” is more appropriate depends on context and the scientific field; zenith angle is probably more commonly used in the context of airmass because airmass increases with zenith angle. I almost always use altitude in my work, but thought it more appropriate to follow convention rather than my personal preference. As to expressing the Kasten and Young formula in terms of zenith angle rather than altitude, it seemed to me that having one formula in terms of altitude while having the rest in terms of zenith angle would invite confusion. “Zenith distance” is probably more common in astronomy than “zenith angle”; I used the latter to make the article more accessible to the average reader, but a case could certainly be made for using the former. JeffConrad (talk) 01:18, 23 March 2009 (UTC)
thread_title:  Difference between zenith and altitude? 

message: The article is claiming (6.07995 + altitude) is equal to (6.07995 - (90-altitude)) where (90-altitude) = zenith.  This is true only if it were converting from a sine to cosine, but it's not.  It's raised to a power.  I'm making the correction and referring to this if anyone disagrees.  I found this out because I created a program a couple of years ago based on this article, only now discovering that this article has been wrong due to bad results from my program. Ywaz (talk) 13:38, 11 March 2012 (UTC)
thread_title:  Kasten and Young error 

message: This expression is hard for me to evaluate, as for a typical atmosphere the middle term would entail taking the exp of a fairly large number when we are near the zenith.  Is it correcltly quoted from the reference?— Preceding unsigned comment added by 67.6.156.137 (talk) 18:40, 1 August 2015 (UTC)
thread_title:  Isothermal Atmosphere 

message: We should remove this "citation needed". Sebagr (talk) 18:00, 20 May 2017 (UTC)
replies: text: Agreed.  I removed the tag, but since a cite was requested, I added a wikilink to the shape of the Earth. Doublesuited (talk) 22:00, 13 June 2017 (UTC)
thread_title:  Citation for "The Earth is not flat"? 

message: I'm trying to vectorize the PNG picture with different formulas. And I have a problem with the Young formula: If I define
z(x) = x/180*pi
young(x) = (1.002432*cos(z(x))*cos(z(x)) + 0.148386*cos(z(x)) + 0.0096467)/(cos(z(x))*cos(z(x))*cos(z(x)) + 0.0149864*cos(z(x))*cos(z(x)) + 0.0102963*cos(z(x)) + 0.000303978) and then plot it (along with other models) I get the picture  where this estimation is way differs, which seems incorrect. Can you please help me to understand what is wrong? Amishaa (talk) 14:09, 17 January 2023 (UTC)
replies: text: Found a mistake: I used 0.0149864 instead of 0.149864. After the fix it fits reasonably well (especially taking into account 34' difference between zeniths and true zeniths angles). Amishaa (talk) 00:33, 18 January 2023 (UTC)
thread_title:  Young (1994) formula seems to be incorrect