The nodal period (or draconic period) of a satellite is the time interval between successive passages of the satellite through either of its orbital nodes, typically the ascending node. This applies to artificial satellites like weather satellites, and natural satellites like the Moon.
It is distinct from the sidereal period, which measures the period with respect to a fixed background of stars, since the location of a satellites nodes will precess over time. For example, the nodal period of the Moon is 27.2122 days, while the sidereal period is 27.3217 days.
The oblate figure of the Earth has important effects of the orbits of near-Earth satellites. An expression for the nodal period (Tn) of a near circular orbit, such that the eccentricity (ε) is almost but not equal to zero, is the following:
where is the semi-major axis, is the gravitational constant, is a perturbation factor due to the oblateness of the earth, is the inclination, is the radius of the earth and is the argument of the perigee.
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