Basic question, but I'm trying to describe a planetary system and coming up short on vocabulary. Do either of the following exist?:

  • A word for the closest pass between two satellites orbiting the same body. "Periapsis" does not seem to be appropriate as it describes a specific point in a single body's orbit. Either a specific word for this phenomenon or a general word for the closest pass between any two bodies would be welcomed.

  • A word for the period between these events, adjacent to the concept of lunation.


3 Answers 3


Here is a partial answer to your question:

A word for the period between two successive events with satellites in the same relative position should be similar to "synodic period".

The synodic period incorporates not only the orbital relation to the parent star, but also to other celestial objects, making it not a mere different approach to the orbit of an object around its parent, but a period of orbital relations with other objects, normally Earth and their orbits around the Sun. It applies to the elapsed time where planets return to the same kind of phenomena or location, such as when any planet returns between its consecutive observed conjunctions with or oppositions to the Sun. For example, Jupiter has a synodic period of 398.8 days from Earth; thus, Jupiter's opposition occurs once roughly every 13 months.


The synodic period is the amount of time that it takes for an object to reappear at the same point in relation to two or more other objects. In common usage, these two objects are typically the Earth and the Sun. The time between two successive oppositions or two successive conjunctions is also equal to the synodic period. For celestial bodies in the solar system, the synodic period (with respect to Earth and the Sun) differs from the sidereal period owing to the Earth's motion around the Sun. For example, the synodic period of the Moon's orbit as seen from the Earth, relative to the Sun, is 29.5 mean solar days, since the Moon's phase and position relative to the Sun and Earth repeats after this period. This is longer than the sidereal period of its orbit around the Earth, which is 27.3 mean solar days, owing to the motion of the Earth around the Sun.



The closest pass between two objects is simply called the "closest approach". And you may be interested in the Time of Closest Approach (TCA) or the Distance of Closest Approach (DCA).

For objects in roughly circular orbits in the same plane (as is likely to be the case for natural satellites), the TCA occurs when there is a syzygy which could be called an opposition or a conjunction (depending on whether you are on the inner satellite, or the outermost one)

Again, assuming roughly circular orbits in the same plane, the time between such syzygies is the synodic period. For objects with eccentric orbits, or in very different planes, you would not necessarily get a regular interval and there is no special term (but I suppose you could invent "Time Between Closest Approaches" (TBCA)


"Periapsis" does not seem to be appropriate as it describes a specific point in a single body's orbit.

While we start by thinking of a central immovable body and tiny essentially massless satellites in orbit around it, we know in our hearts that this is wrong and everybody moves in an orbital system. Even the Sun moves around the solar system barycenter (dancing with Jupiter, Saturn and Neptune mostly) so much that the barycenter is outside of the Sun roughly half the time.

We can look at our Earth-Moon system where they circle around a spot roughly 2/3 of the way out from Earth's center to its surface, or better yet, the Pluto-Charon system.

Wikipedia's Charon gives the following:

A header Another header
Periapsis 19,587 km
Apoapsis 19,595 km
Semi-major axis 19591.4 km (planetocentric)
17181.0 km (barycentric)

The difference of ~2400 km between the barycenter and planet-centered semimajor axis values is more than double Pluto's mean radius of only 1,188.3±0.8 km meaning that the point they orbit is in empty space.

There's nothin wrong with using periapsis and apoapsis in both the limits of a low-mass orbiter and a central body and of two more equally-massed objects.

It's simply important to note how you're using them.


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