**Bolometric Albedo**. (*A*). The ratio between the total reflected
sunlight (all wavelengths, all directions) and the total incident sunlight
on a planetary surface. The relevant albedo for thermal calculations.
**Geometric Albedo**. The ratio between the disk-integrated brightness
of a body at zero solar phase angle and the brightness of a normally-illuminated
*A*=1
Lambert* disk* of the same diameter.
**Normal Reflectance.** The ratio between the brightness of a normally
illuminated surface and the brightness of a normally-illuminated *A*=1
Lambert surface.
**Opposition Surge. **The sharp increase in the intensity of sunlight
reflected from a planetary surface at very small solar phase angles.
**Local Time**. Degrees of planetary rotation since local midnight,
e.g.for zero obliquity, sunset occurs at a local time of 270^{o}.
Sometimes expressed using a `terrestrial' clock, in which case sunset occurs
at 6pm.
**Incidence Angle.** (*i*). The angle between the local surface
normal and the direction of the sun.
**Emission Angle.** (*e*). The angle between the local surface
normal and the direction of the observer.
**Thermal Meridian**. The line on a planetary surface for which
the directions of the sun, observer, and surface normal are all coplanar.
**Radiance**. The intensity of thermal emission (expressed as energy
per unit time, per unit solid angle, per unit area, per unit wavelength
or wavenumber).
**Planck Curve, Blackbody Curve**. ($B(\lambda,T)$). The thermal
radiance at wavelength $\lambda$ from a perfect radiator (a blackbody)
at temperature *T*, approximated by real planetary surfaces. The form
of the curve is

$$ B(\lambda,T) = {2 h c^2 \over \lambda^5} \left[\exp\left(h c \over
\lambda k T \right) - 1 \right] ^{-1} \eqno(35)$$

where $h$ is Planck's constant, *c *the speed of light, and *k*
is the Boltzmann constant.
**Emissivity**. (\ep). The efficiency of thermal radiation: the
ratio of the observed radiance to the radiance from a blackbody with the
same actual surface temperature. Emissivity can be specified as a function
of wavelength, or given as a wavelength-integrated value relating the total
thermal radiation to the expected blackbody radiation.
**Equilibrium Temperature**. The temperature at which the thermal
radiation from a planetary surface equals the absorbed solar radiation.
**Brightness Temperature.** (*T _{B}*). The temperature
of a blackbody emitting the observed thermal radiance at the specified
wavelength.