λ | Wavelength | µm | Wavelength of the incident solar radiation |

γ | Angular distance | arc min | Angular distance from the center of the Earth’s shadow |

*R*_{S} | Angular radius | arc min | Angular radius of the Sun |

*R* | Angular distance | arc min | Angular distance from the center of the solar disk |

ɛ_{0} | Angle | arc min | Maximum angle under which the elementary ring at the solar disk is visible from point *E′* (Fig. 1) |

ɛ | Angle | arc min | Angle that defines the position of a certain point at the elementary ring (Fig. 1) |

ρ | Angular distance | arc min | Angular distance of the elementary ring from point *E′* (Fig. 1) |

π_{S} | Parallax of the Sun | arc min | Angle subtended by the Earth’s equatorial radius at the center of the Sun at the mean distance between Earth and the Sun |

π_{M} | Parallax of the Moon | arc min | Angle subtended by the Earth’s equatorial radius at the center of the Moon at the mean distance between the Earth and the Moon |

d*S* | Area | km^{2} | Surface at the lunar plane illuminated by flux of light when no refraction takes place in the Earth’s atmosphere |

d*S′* | Area | km^{2} | Surface at the lunar plane illuminated by flux of light when refraction takes place in the Earth’s atmosphere |

κ_{λ} | Coefficient dependent on the wavelength | Dimensionless | Coefficient that characterizes the luminosity distribution at the solar disk |

*c*_{λ} | Light dispersion | Dimensionless | Coefficient that refers to the light dispersion in the Earth’s atmosphere |

*h*_{0} | Altitude | km | Minimum altitude of the ray in the Earth’s atmosphere |

*h* | Altitude | km | Altitude of the general point on the sunbeam trajectory |

ℜ | Radius of the Earth | km | Radius of the Earth |

*H* | Scale height | km | Altitude up to which a homogeneous molecular atmosphere would extend |

ρ_{G} | Air density | kg m^{−3} | Air density at the ground |

*r* | Particle radius | µm | Radius of the particle |

*r*_{m} | Particle modal radius | µm | Modal radius of the particle size distribution function *f*(*r*) |

*a* | Parameter of the particle size distribution function | Dimensionless | Refers to modified gamma size distribution function *f*(*r*) ∼ *r*^{a}e^{−}^{br} |

*b* | Parameter of the particle size distribution function | µm^{−1} | Refers to modified gamma size distribution function *f*(*r*) *r*^{a}e^{−}^{br} |

*B* | Euler angle | rad | Specify the particle’s orientation with respect to the laboratory reference frame |

Θ | Euler angle | rad | Specify the particle’s orientation with respect to the laboratory reference frame |

Φ | Euler angle | rad | Specify the particle’s orientation with respect to the laboratory reference frame |

*m* | Refractive index | Dimensionless | Complex refractive index of the particle |

*x* | Size parameter | Dimensionless | Size parameter defined as 2π*r*/λ |

*e*_{λ}(γ) | Relative irradiance | Dimensionless | Irradiance of a certain point at distance γ from the center of the Earth’s shadow (including atmospheric effects) |

*E*_{λ}(γ) | Relative irradiance | Dimensionless | Irradiance of a certain point at distance γ from the center of the Earth’s shadow (excluding atmospheric effects) |

*D*_{λ}(γ) | Optical density | Dimensionless | Negative common logarithm of the ratio of irradiances *E*_{λ}(γ) and *e*_{λ}(γ) (Fig. 1) |

*b*_{λ}(ρ, ɛ) | Relative luminosity | Dimensionless | Relative amount of energy the element at the solar surface radiates in unit time |

ω(*h*_{0}) | Angle of refraction | arc min | Refraction of the rays of light passing through the atmosphere horizontally |

*i*_{λ}(*h*) | Apparent zenith angle | rad | Angle subtended at a general point on the trajectory by its radius vector with the ray (Fig. 1) |

*T*^{ref}(*h*_{0}) | Transmission coefficient | Dimensionless | Factor by which the radiation flux is reduced (due to atmospheric refraction) when reaching altitude *h*_{0} |

*T*^{ext}(*h*_{0}) | Transmission coefficient | Dimensionless | Factor by which the radiation flux is reduced (due to atmospheric extinction) when reaching altitude *h*_{0} |

*T*^{air} | Transmission coefficient | Dimensionless | Factor by which the radiation flux is reduced (due to molecular scattering) when reaching altitude *h*_{0} |

*T*^{ozone} | Transmission coefficient | Dimensionless | Factor by which the radiation flux is reduced (due to ozone absorption) when reaching altitude *h*_{0} |

*T*^{aerosol} | Transmission coefficient | Dimensionless | Factor by which the radiation flux is reduced (due to aerosol extinction) when reaching altitude *h*_{0} |

δ_{λ}(*h*) | Aerosol coefficient | Extinction km^{−1} | Fraction of radiant flux lost from a collimated beam per unit thickness of aerosol |

α_{λ}(*h*) | Ozone coefficient | Extinction km^{−1} | Fraction of radiant flux lost from a collimated beam per unit thickness of ozone due to absorption |

β_{λ}(*h*) | Rayleigh coefficient | Extinction km^{−1} | Fraction of radiant flux lost from a collimated beam per unit thickness of air molecules due to scattering |

τ_{λ}(*h*_{0}) | Aerosol optical thickness | Dimensionless | Negative natural logarithm of ratio of incident radiant flux and the flux transmitted through aerosol environment |

*C*_{ext}(λ, *r*) | Extinction cross section | µm^{2} | This quantity equals the total monochromatic power removed by the particle from the incident beam divided by the incident monochromatic energy flux |

*Q*_{ext}(λ, *r*) | Efficiency coefficient for extinction | Dimensionless | Dimensionless quantity defined as a ratio of *C*_{ext}/π*r*^{2} (for spherical particles of radii *r*) |

*f*_{h}_{0}(*r*) | Particle size distribution | µm^{−3} | Columnar aerosol size distribution function at base altitude *h*_{0} |

*S*_{h}_{0}(*r*) | Particle size distribution | µm^{−1} | Columnar surface aerosol size distribution function at base altitude *h*_{0} |

*s*(*r, h*) | Particle size distribution | m^{−2} | Altitude-dependent particle size distribution function |