In the field of astronomy, the horizon axis refers to one of the two axes used to define the equatorial coordinate system. The equatorial coordinate system is a celestial coordinate system that is based on the Earth's equator and the axis of its rotation.

The horizon axis is also known as the east-west axis or the right ascension axis. It is defined as the imaginary line that intersects the Earth's surface at the points due east and due west of the observer. The horizon axis is perpendicular to the polar axis, which is the other axis used to define the equatorial coordinate system.

The horizon axis is used to measure right ascension, which is the coordinate that specifies the angular distance of a celestial object from the vernal equinox (the point where the Sun crosses the celestial equator on its way north in the spring). Right ascension is measured in hours, minutes, and seconds of time, and it ranges from 0 hours to 24 hours.