If the stars are seen to rise perpendicular to the horizon by an observer, he is located on the

An observer at the Equator sees stars rise perpendicular (vertically) to the horizon. At the Equator the celestial equator passes through the zenith, so stellar diurnal paths intersect the eastern and western horizons at right angles and thus appear to rise and set vertically. Texts describing sky motions note explicitly that “stars rise vertically at the East point and set vertically at the West point” for an equatorial observer, and that for an observer on the Earth’s equator the celestial equator is overhead while the celestial pole lies on the horizon, producing the perpendicular rise/set geometry. Hence the Equator is the correct location.