# Nodal period

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.wikipedia
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### Orbital period

periodsynodic periodsynodic
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 seemingly fixed onto a spherical background, since the location of a satellite's nodes precess over time.

### Satellite

satellitesartificial satelliteartificial satellites
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.

### Orbital node

Nodeascending nodenodes
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.

### Weather satellite

satellitemeteorological satelliteWeather
This type of orbital period applies to artificial satellites, like those that monitor weather on Earth, and natural satellites like the Moon.

### Earth

Earth's surfaceterrestrialworld
This type of orbital period applies to artificial satellites, like those that monitor weather on Earth, and natural satellites like the Moon.

### Natural satellite

moonmoonssatellite
This type of orbital period applies to artificial satellites, like those that monitor weather on Earth, and natural satellites like the Moon.

### Moon

lunarthe MoonLuna
This type of orbital period applies to artificial satellites, like those that monitor weather on Earth, and natural satellites like the Moon.

### Fixed stars

fixed starfixedstars
It is distinct from the sidereal period, which measures the period with respect to reference stars seemingly fixed onto a spherical background, since the location of a satellite's nodes precess over time.

### Celestial sphere

celestialcelestial hemispherehemisphere
It is distinct from the sidereal period, which measures the period with respect to reference stars seemingly fixed onto a spherical background, since the location of a satellite's nodes precess over time.

### Nodal precession

precessnodes precessorbital precession
It is distinct from the sidereal period, which measures the period with respect to reference stars seemingly fixed onto a spherical background, since the location of a satellite's nodes precess over time.

### Lunar month

synodic monthsidereal monthanomalistic month
For example, the nodal period of the Moon is 27.2122 days (one draconic month), while its sidereal period is 27.3217 days (one sidereal month).

### Flattening

oblatenessellipticityflattened
The oblate figure of the Earth has important effects of the orbits of near-Earth satellites.

### Figure of the Earth

curvature of the Earthshape of the EarthEarth's curvature
The oblate figure of the Earth has important effects of the orbits of near-Earth satellites.

### Geocentric orbit

GeocentricEarth orbitEarth-orbit
The oblate figure of the Earth has important effects of the orbits of near-Earth satellites.

### Orbital eccentricity

eccentricityeccentriceccentricities
An expression for the nodal period (T n) of a near circular orbit, such that the eccentricity is almost but not equal to zero, is the following:

### Semi-major and semi-minor axes

semi-major axissemimajor axissemi-major axes
where a is the semi-major axis, \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 \omega is the argument of the perigee.

### Perturbation (astronomy)

perturbationsperturbationperturbed
where a is the semi-major axis, \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 \omega is the argument of the perigee.

### Orbital inclination

inclinationinclinedtilted
where a is the semi-major axis, \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 \omega is the argument of the perigee.

### Argument of periapsis

argument of perihelionargument of perigeeargument of pericenter
where a is the semi-major axis, \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 \omega is the argument of the perigee.

### Registration Convention

Convention on Registration of Objects Launched into Outer SpaceConvention on Registration of Launched Objects into Outer SpaceConvention on Registration of Objects Launched into Outer Space Objects

### Lunar node

nodeNorth Nodelunar nodes
Because the orbital plane of the Moon precesses in space, the lunar nodes also precess around the ecliptic, completing one revolution (called a draconic or nodal period) in 18.612958 year.

### Lunar precession

precess18.6 and 8.85 yearslunar apsidal precession
This is the reason that a draconic month or nodal period (the period the Moon takes to return to the same node in its orbit) is shorter than the sidereal month.

### Lunar standstill

lunar minor standstilllunisticemaximum southern moon rise
The Moon's declination also changes, completing a cycle once every lunar nodal period: 27.212 days.

### Molniya (satellite)

MolniyaMolniya 1Molniya-1
Similarly, to ensure the ground track repeats every 24 hours the nodal period needed to be half a sidereal day.

### Molniya orbit

Molniyahighly elliptical orbitMolniya orbit satellite
However, the oblateness of the Earth also perturbs the right ascension of the ascending node (\Omega), changing the nodal period and causing the ground track to drift over time at the rate shown in equation.