D ▪ declination - The angular measurement on the sky analogous to north-south latitude here on the surface of the Earth. It is one of two measurements used by astronomers to locate things on the celestial sphere. Right ascension is the other. declination is the analogue to longitude on the surface of the Earth. ▪ differentiation - The process whereby a planet will arrange its interior into layers of different density. It requires that the interior had been molten at some point in the history of the planet. ▪ distance modulus - The numerical difference between an objects apparent magnitude (m) and its absolute magnitude (M). This value is used in the appropriate equation to determine the distance to that object.