Surface brightness of Mars

What is the surface brightness of Mars? It would be interesting to
compare this figure to that of deep sky objects.

Mark

In article ,
M. Tettnanger wrote:
What is the surface brightness of Mars? It would be interesting to
compare this figure to that of deep sky objects.

Mark

At about 25" diameter, it's about 490 square arc seconds in area,
so one square arc sec is ~6.7 mags fainter than the whole planet.
It's at -2.9 now, so its surface brightness is mag +3.8 per
square arc second. Yeow.

In other words, it's really, really bright!

Stuart

Mark

At about 25" diameter, it's about 490 square arc seconds in area,
so one square arc sec is ~6.7 mags fainter than the whole planet.
It's at -2.9 now, so its surface brightness is mag +3.8 per
square arc second. Yeow.

In other words, it's really, really bright!

Yep..600 watts/sq m solar flux (1370 for earth) and an albedo of 0.22 (0.3
earth). The albedo is comparable to earth..... (Venus 0.8!)

In article , Paul Schlyter wrote:
[...]
Then what about Venus? At superior conjunction it shines at mag -3.5
and has an apparent diameter of 10", which yields an apparent area of
some 78 square arcsec. And that gives a surface brightness of mag
+1.2 per square arcsec.

Oh very nice!

Or the Sun: magnitude -26.7, apparent diameter 1800", apparent area
2.5 million square arcsec, surface brightness -10.6 mag per square
arcsec.

Or take Vega: magnitude 0.0, apparent diameter 0.005", apparent area
1.9E-5 square arcsec, surface brightness -11.2 mag per square arcsec.

Hm! I bet Vega is somewhat smaller than .005, since its surface
brightness should be higher. Its surface temp is supposed to be ~10000K,
while the Sun's is around 6000K or a bit less. So if they're not
too far from being blackbodies in total output,
the ratio of surface brightnesses should be around (10000/6000)^4 or 7.7,
or 2.2 magnitudes. So Vega should be surface brightness -12.4 per
square arc sec, giving it an apparent diameter of .005 / sqrt(2.512^1.2)
or .0029". At ~8 pc that makes it about 3.5 million kilometers in diameter.

Stuart Levy wrote:
In article , Paul Schlyter wrote:
[...]
Then what about Venus? At superior conjunction it shines at mag -3.5
and has an apparent diameter of 10", which yields an apparent area of
some 78 square arcsec. And that gives a surface brightness of mag
+1.2 per square arcsec.

Oh very nice!

Or the Sun: magnitude -26.7, apparent diameter 1800", apparent area
2.5 million square arcsec, surface brightness -10.6 mag per square
arcsec.

Or take Vega: magnitude 0.0, apparent diameter 0.005", apparent area
1.9E-5 square arcsec, surface brightness -11.2 mag per square arcsec.

Hm! I bet Vega is somewhat smaller than .005, since its surface
brightness should be higher. Its surface temp is supposed to be ~10000K,
while the Sun's is around 6000K or a bit less. So if they're not
too far from being blackbodies in total output,
the ratio of surface brightnesses should be around (10000/6000)^4 or 7.7,
or 2.2 magnitudes. So Vega should be surface brightness -12.4 per
square arc sec, giving it an apparent diameter of .005 / sqrt(2.512^1.2)
or .0029". At ~8 pc that makes it about 3.5 million kilometers in diameter.

Beware the funny nature of magnitudes here - Vega's output comes
in a rather different spectral distribution than the Sun's, peaking
in the near UV (exact wavelengths for both peaks depend on whether you
plot intensity versus wavelength or frequency). And the magnitude
scale zero point does funny things in comparing energies at various
wavelengths. Not sheer happenstance that there is a magnitude system
in which Vega has equal magnitudes in all passbands. Vega has
absolute magnitude around +0.6 in the V band, 4.1 mag brighter than
old Sol or a factor of 43 in this wavelength band. This still suggests
a smaller angular diameter than 0.005" - oh, cool, I see there;s
near-IR interferometry by Ciardi et al. giving a limb-darkened
profile with diameter 0.00328". Putting these together in a simple
way gives a surface brightness of V=-12.7 per square arcsecond.
How about that - even higher than the earlier estimate! Now if we

Bill Keel
were to talk about one of those O3 stars in the Tarantula Nebula...