I've read that you can change the longitude of ascending node of a satellite (i.e. changing the orbital plane without changing the inclination) to change its orbital plane by using the torque imparted by the oblateness of Earth. How does this happen exactly? I mean how does the oblateness of Earth cause a torque induced precession of a satellite?

PS. I think I don't understand how torque induced precession works and that's why I am not able to understand this. So if any of you kind gentlefolk are willing to explain, I thank you in advance.

EDIT: I just came across Nodal Precession. And from what I understood that due to Earth's equatorial bulge, the gravitational force vector is offset from its center, thus producing torque. I can visualize how the longitude of ascending node will change, but can't visualise how will the change in the torque caused by slightly offset gravitational force vector will cause it.


1 Answer 1


I'd like to start by quoting from the Wiki page on Nodal Precession you mentioned:

A non-rotating body of planetary scale or larger would be pulled by gravity into a sphere. Virtually all bodies rotate, however. The centrifugal force deforms the body so that it has an equatorial bulge. Because of the bulge the gravitational force on the satellite is not directly toward the center of the central body, but is offset toward the equator. Whichever hemisphere the satellite is in it is preferentially pulled slightly toward the equator. This creates a torque on the orbit. This torque does not reduce the inclination; rather, it causes a torque-induced gyroscopic precession, which causes the orbital nodes to drift with time.

(emphasis mine). Note that this is a small force and the precession is slow compared with orbital speeds. Precession towards (or coasting away from) the equator thus is a change in latitude compared with what would be expected, and this "twist" changes the expected longitude at a given point in time because the satellite's vector velocity changes (I believe in both direction and magnitude, but I'm not certain about the magnitude).

  • $\begingroup$ I'm pretty sure that only the direction changes, not the magnitude, since that would change the period of the orbit. $\endgroup$
    – PM 2Ring
    Aug 10, 2019 at 11:22

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