Thenodal period (ordraconic period) of asatellite is the time interval between successive passages of the satellite through either of itsorbital nodes,^[1][2] typically the ascending node. This type oforbital period applies to artificial satellites, like those thatmonitor weather onEarth, andnatural satellites like theMoon.

It is distinct from thesidereal period, which measures the period with respect to reference starsseemingly fixed onto aspherical background, since the location of a satellite'snodes precess over time.^[3] For example, the nodal period of the Moon is 27.2122 days^[4] (onedraconic month), while its sidereal period is 27.3217 days^[5] (onesidereal month).

Theoblatefigure of the Earth has important effects of theorbits of near-Earth satellites.^[6] An expression for the nodal period (T_n) of a near circular orbit, such that theeccentricity (ε) is almost but not equal to zero, is the following:^[7]

${\displaystyle T_n=\frac{2\pi a^{\frac{3}{2}}}{{\mu }^{\frac{1}{2}}}\left(1-\frac{3J_2\left(4-5{\sin }^{2}i\right)}{4{\left(\frac{a}{R}\right)}^2\sqrt{1-{\epsilon }^2}{\left(1+\epsilon \cos \omega \right)}^2}-\frac{3J_2{\left(1+\epsilon \cos \omega \right)}^3}{2{\left(\frac{a}{R}\right)}^2{\left(1-{\epsilon }^2\right)}^3}\right)}$

where${\displaystyle a}$ is thesemi-major axis,${\displaystyle \mu }$ is the gravitational constant,${\displaystyle J_2}$ is aperturbation factor due to the oblateness of the earth,${\displaystyle i}$ is theinclination,${\displaystyle R}$ is the radius of the earth and${\displaystyle \omega }$ is theargument of the perigee.

• Lunar nodal period

• Orbits