# Is albedo ever greater than unity? How is it calculated for various surface diffusivities and specular reflection? [duplicate]

I'm trying to understand how albedo would be calculated for a body with well-characterized surface reflectance properties, so that it's absolute magnitude could be defined. One could argue that this is going backwards (one should measure apparent magnitude, correct for distances and phase angle to extract absolute magnitude, correct for radius to extract albedo) but consider if the body were not smooth and diffusely reflecting.

Imagine two identically sized spherical bodies; one diffuse or matte white and the other somehow covered in liquid mercury and exhibiting specular rather than diffuse reflectivity, with no wind or waves.

Would they both have albedo of unity?

If these two reflective bodies were very irregularly-shaped and had an extremely large, flattened (much larger radius of curvature) face, would its albedo be larger than unity at times?

Here I'm looking for how astronomers and planetary scientists use the term albedo and not just a mathematical hypothesis. There may be ways this is dealt with practically.

• This question has the answers: astronomy.stackexchange.com/questions/20795/… There's Bond Albedo and geometric albedo. Bond albedo is a property intrinsic to a material and not necessarily an astronomy-only feature. Geometric albedo is about how bright a celestial body ought to be assuming certain properties, such as Lambertian scattering of light and a given bond albedo. Feb 4 '19 at 4:48
• I thought only stars had absolute magnitudes Feb 4 '19 at 4:56
• @Ingolifs see this and this and links therein. Any database of asteroids will have their absolute magnitudes, either explicitly, or hidden in a reported size based on an assumed albedo. For cheezy pics see here.
– uhoh
Feb 4 '19 at 5:03
• @Ingolifs I'm going to assume that it's geometric albedo in zephyr's answer that's used in the relationship between absolute and apparent magnitudes and voting to close this as a duplicate to that. Thanks!
– uhoh
Feb 4 '19 at 5:09
• My source was Meeus' Astronomical Algorithms which cited IAU Commission 20 in 1985. I think the best source of explanation is the Muinonen et al 2010 discussing improving the $H,G$ system. There they state $\log_{10}D=3.1236−0.2H−0.5\log_{10}p_V$ where $D$ is equivalent diameter, $H$ is absolute mag (at 0 phase angle) and $p_V$ is the geometric albedo (in V band). It then explains why the $H,G$ formalism is used since asteroid obs. at phase angles <10 degrees are rare Feb 5 '19 at 19:54