EQUATOR (Late Lat. _aequator_, from _aequare_, to make equal), in geography, that great circle of the earth, equidistant from the two poles, which divides the northern from the southern hemisphere and lies in a plane perpendicular to the axis of the earth; this is termed the "geographical" or "terrestrial equator." In astronomy, the "celestial equator" is the name given to the great circle in which the plane of the terrestrial equator intersects the celestial sphere; it is consequently equidistant from the celestial poles. The "magnetic equator" is an imaginary line encircling the earth, along which the vertical component of the earth's magnetic force is zero; it nearly coincides with the terrestrial equator. EQUERRY (from the Fr. _ecurie_, a stable, through its older form _escurie_, from the Med. Lat. _scuria_, a word of Teutonic origin for a stable or shed, cf. Ger. _Scheuer_; the modern spelling has confused the word with the Lat. _equus_, a horse), a contracted form of "gentleman of the equerry," an officer in charge of the stables of a royal household. At the British court, equerries are officers attached to the department of the master of the horse, the first of whom is called chief equerry (see HOUSEHOLD, ROYAL). EQUIDAE, the family of perissodactyle ungulate mammals typified by the horse (_Equus caballus_); see HORSE. According to the older classification this family was taken to include only the forms with tall-crowned teeth, more or less closely allied to the typical genus _Equus_. There is, however, such an almost complete graduation from the former to earlier and more primitive mammals with short-crowned cheek-teeth, at one time included in the family _Lophiodontidae_ (see PERISSODACTYLA), that it has now become a very general practice to include the whole "phylum" in the family _Equidae_. The _Equidae_, in this extended sense, together with the extinct _Palaeotheriidae_, are indeed now regarded as forming one of four main groups into which the Perissodactyla are divided, the other groups being the Tapiroidea, Rhinocerotoidea and Titanotheriide. For the horse-group the name Hippoidea is employed. All four groups were closely connected in the Lower Eocene, so that exact definition is almost impossible.