The Bond albedo (or ''spheric albedo'' or ''planetary albedo'' or ''bolometric albedo''), named after the American astronomer
George Phillips Bond
George Phillips Bond (May 20, 1825February 17, 1865) was an American astronomer. He was the son of William Cranch Bond. Some sources give his year of birth as 1826.
His early interest was in nature and birds, but after his elder brother Willia ...
(1825–1865), who originally proposed it, is the fraction of
power in the total
electromagnetic radiation
In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visib ...
incident on an astronomical body that is scattered back out into space.
Because the Bond albedo accounts for all of the light scattered from a body at all
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...
s and all
phase angles, it is a necessary quantity for determining how much energy a body absorbs. This, in turn, is crucial for determining the
equilibrium temperature of a body.
Because bodies in the outer Solar System are always observed at very low phase angles from the Earth, the only reliable data for measuring their Bond albedo comes from spacecraft.
Phase integral
The Bond albedo (''A'') is related to the
geometric albedo (''p'') by the expression
:
where ''q'' is termed the ''phase integral'' and is given in terms of the directional scattered flux ''I''(''α'') into phase angle ''α'' (averaged over all wavelengths and azimuthal angles) as
:
The phase
angle
In Euclidean geometry, an angle is the figure formed by two rays, called the '' sides'' of the angle, sharing a common endpoint, called the ''vertex'' of the angle.
Angles formed by two rays lie in the plane that contains the rays. Angles ...
''α'' is the angle between the source of the radiation (usually the Sun) and the observing direction, and varies from zero for light scattered back towards the source, to 180° for observations looking towards the source. For example, during opposition or looking at the full moon, α is very small, while backlit objects or the new moon have α close to 180°.
Examples
The Bond albedo is a value strictly between 0 and 1, as it includes all possible scattered light (but not radiation from the body itself). This is in contrast to other definitions of
albedo
Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refle ...
such as the geometric albedo, which can be above 1. In general, though, the Bond albedo may be greater or smaller than the geometric albedo, depending on the surface and atmospheric properties of the body in question.
Some examples:
Albedo of the Earth
/ref>
See also
* Albedo
Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refle ...
* Geometric albedo
References
{{reflist
External links
discussion of Lunar albedo
Electromagnetic radiation
Astrophysics
Radiometry
Scattering, absorption and radiative transfer (optics)