Kelvin Kelvin The Kelvin scale is an absolute thermodynamic temperature scale using as its null point absolute zero, the temperature at which all thermal motion ceases in the classical description of thermodynamics. The kelvin (symbol: K) is the base unit of temperature in the International System of Units (SI). Until 2018, the kelvin was defined as the fraction of the thermodynamic temperature of the triple point of water (exactly 0.01 °C or 32.018 °F). In other words, it is defined such that the triple point of water is exactly 273.16 K. On 16 November 2018, a new definition was adopted, in
Absolute magnitude unit (AU) from both the Sun and the observer, and in conditions of ideal solar opposition. In fact, one has to take into account that Solar System bodies are illuminated by the Sun, therefore the magnitude varies as a function of illumination conditions, described by the phase angle. This relationship is referred to as the phase curve. The absolute magnitude is defined for the ideal case of phase angle equal to zero. To convert a stellar or a galactic absolute magnitude into a planetary one, subtract 31.57. A comet's nuclear magnitude () is a different scale and can not be