Nodal period

 ( Orbits ) 

The nodal period (or draconic period) of a satellite is the time interval between successive passages of the satellite through either of its , typically the ascending node. This type of orbital period applies to artificial satellites, like those that monitor weather on Earth, and natural satellites like the Moon.

It is distinct from the sidereal period, which measures the period with respect to reference stars fixed stars onto a celestial sphere, since the location of a satellite's nodal precession over time.

For example, the nodal period of the Moon is 27.2122 days

Richard Thompson (2025). 9788120819542, Motilal UK Books of India. ISBN 9788120819542

(one draconic month), while its sidereal period is 27.3217 days (one sidereal month).

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Near-Earth satellites The oblateness figure of the Earth has important effects of the geocentric orbit of near-Earth satellites. An expression for the nodal period () of a near circular orbit, such that the eccentricity () is almost but not equal to zero, is the following:

$$

T_n = \frac{2\pi a^\frac32} {\mu^\frac12} \left( 1 - \frac{3 J_2 \left(4 - 5\sin^2 i\right)}{4\left(\frac{a}{R}\right)^2 \sqrt{1-\varepsilon^2}\left(1+\varepsilon \cos\omega\right)^2} - \frac{3 J_2 \left(1 + \varepsilon\cos\omega\right)^3}{2\left(\frac{a}{R}\right)^2 \left(1-\varepsilon^2\right)^3} \right)

where $a$ is the semi-major axis, $\backslash mu$ is the gravitational constant, $J\_2$ is a perturbation factor due to the oblateness of the earth, $i$ is the inclination, $R$ is the radius of the earth and $\backslash omega$ is the argument of the perigee.

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See also

• Lunar nodal period

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