In celestial mechanics, the plane of reference is the plane from which orbital elements are defined. Specifically, the inclination and the longitude of the ascending node are the two main orbital elements that are measured with respect to the plane of reference.

Depending on the type of body being described, there are four different kinds of reference planes that are typically used:

• Ecliptic - planets, asteroids, comets, etc.
• Equator of orbited body - satellites with small semimajor axes
• Local Laplace plane - satellites with intermediate-to-large semimajor axes
• Equator of ICRF (celestial equator) - extrasolar objects

On the plane of reference, a zero-point must be defined from which the angles of longitude are measured. When the ecliptic or celestial equator is used, this is defined as the point where the former crosses the latter from South to North, known as the First Point of Aries. The zero-points of the other reference planes are defined as the point where they cross the celestial equator from South to North.