Apsis

An apsis (Greek: ἁψίς; plural apsides /ˈæpsᵻdiːz/, Greek: ἁψίδες) is an extreme point in an object's orbit. The word comes via Latin from Greek and is cognate with apse. For elliptic orbits about a larger body, there are two apsides, named with the prefixes peri- (from περί (peri), meaning "near") and ap-, or apo- (from ἀπ(ό) (ap(ó)), meaning "away from") added to a reference to the thing being orbited.

For a body orbiting the Sun, the point of least distance is the perihelion (/ˌpɛrᵻˈhiːliən/), and the point of greatest distance is the aphelion (/æpˈhiːliən/).

The terms become periastron and apastron when discussing orbits around other stars.

For any satellite of Earth including the Moon the point of least distance is the perigee (/ˈpɛrᵻdʒiː/) and greatest distance the apogee.

For any orbits around a center of mass, there are the terms pericenter and apocenter. Periapsis and apoapsis (or apapsis) are equivalent alternatives.

A straight line connecting the pericenter and apocenter is the line of apsides. This is the major axis of the ellipse, its greatest diameter. For a two-body system the center of mass of the system lies on this line at one of the two foci of the ellipse. When one body is sufficiently larger than the other it may be taken to be at this focus. However whether or not this is the case, both bodies are in similar elliptical orbits each having one focus at the system's center of mass, with their respective lines of apsides being of length inversely proportional to their masses. Historically, in geocentric systems, apsides were measured from the center of the Earth. However, in the case of the Moon, the center of mass of the Earth–Moon system, or Earth–Moon barycenter, as the common focus of both the Moon's and Earth's orbits about each other, is about 74% of the way from Earth's center to its surface.

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Linklaters advises Nhood and Ceetrus on their partnership with Apsys aimed at transforming the commercial market in Poland (Linklaters LLP)

Public Technologies 19 Mar 2025

Linklaters has advised Nhood and Ceetrus on their partnership with Apsys, which aims to transform the commercial market in Poland by initiating innovative and sustainable development projects that ...

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Linklaters advises Nhood and Ceetrus on their partnership with Apsys aimed at transforming the commercial market in Poland (Linklaters LLP)

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