The blackbody approximation is used for the visible solar spectrum to find an order of magnitude estimate for hc/k.

1.
See Craig F. Bohren, What Light Through Yonder Window Breaks? More Experiments in Atmospheric Physics (Wiley, New York, 1991), pp. 113–120.
2.
William R.
Kuhn
and
Susan E.
Postawko
, “
Solar radiation and the Earth’s atmosphere
,”
Phys. Teach.
26
,
266
273
(
1998
) present detailed graphs and discuss the measured spectrum of the sun.
See also Günter Wyszecki and W. S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae (Wiley, New York, 1982), 2nd ed., p. 6, from which Fig. 1 is taken. I thank Rod Nave for his help in locating and providing this figure. The air mass m is defined as the ratio between the actual path length taken by the solar beam when incident at polar angle θ from the zenith to the analogous path length when the sun is directly overhead, so that m=sec θ; in Fig. 1, θ=60° and m=2.
3.
Mark L.
Biermann
,
Debora M.
Katz
,
Robert
Aho
,
James
Diaz-Barriaga
, and
Jerome
Petron
, “
Wien’s law and the temperature of the Sun
,”
Phys. Teach.
40
,
398
400
(
2002
).
4.
For a discussion of the pitfalls of the different representations, see
Bernard H.
Soffer
and
David K.
Lynch
, “
Some paradoxes, errors, and resolutions concerning the spectral optimization of human vision
,”
Am. J. Phys.
67
,
946
953
(
1999
);
James M.
Overduin
, “
Eyesight and the solar Wien peak
,”
Am. J. Phys.
71
,
216
219
(
2003
);
Geoff
Nunes
, “
Comment
,”
Am. J. Phys.
71
,
519
(
2003
);
and
Mark A.
Heald
, “
Where is the Wien peak?
,”
Am. J. Phys.
71
,
1322
1323
(
2003
). I thank Craig Bohren for pointing out to me the representation-independence of hc/k≈xλ.
5.
See Peter Pesic, “Estimating Avogadro’s number from skylight and airlight,” Eur. J. Phys. (submitted).
For a historical survey of views about sky blue and its physical implications, see Peter Pesic, Sky in a Bottle (MIT Press, Cambridge, MA, 2005).
6.
For more accurate determinations using simple equipment, see
R. E.
Crandall
and
J. F.
Delord
, “
Minimal apparatus for determination of Planck’s constant
,”
Am. J. Phys.
51
,
90
91
(
1982
);
J.
Dryzek
and
K.
Ruebenbauer
, “
Planck’s constant determination from black-body radiation
,”
Am. J. Phys.
60
,
251
253
(
1992
);
and
Graciela
Brizuela
and
Alfredo
Juan
, “
Planck’s constant determination using a light bulb
,”
Am. J. Phys.
64
,
819
821
(
1996
).
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