Britannica Sources used in this answer

How can the precession of a satellite orbit reveal Earth’s flattening?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Diagram showing the deflection of the vertical from the geoid to the ellipsoid or spheroid, with labeled lines for geoid, spheroid, rock surface, ocean, and geoid height in a geology context. — geoid Deflection of the vertical from the geoid to the spheroid.

The Earth is not a perfect sphere; its rotation creates an equatorial bulge, resulting in an ellipsoidal shape. This mass distribution significantly influences the gravitational field, which in turn affects the motion of orbiting satellites.12

When a satellite travels in an inclined orbit, it passes over the equatorial region and experiences a gravitational force directed toward the Equator due to the mass of this bulge. This force exerts a torque on the satellite's orbit. Much like the physics of a spinning top or gyroscope, this torque causes the satellite's orbital plane to precess around Earth’s rotation axis.1

This precession is a measurable, secular change in the satellite's path. By observing the rate of this precession with high accuracy, scientists can determine the magnitude of the dynamical form factor, known as $J_{2, o}$ . This factor is directly related to the Earth's flattening, or the degree to which the planet deviates from a spherical shape. Consequently, tracking these orbital perturbations allows researchers to refine our understanding of the geoid and the Earth's precise figure.12