Definition:Apsis
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Definition
Let $A$ be a body in orbit $O$ around another body $B$.
Let $\mathbf r$ be the radius of $O$.
An apsis is a point on $O$ at which:
- the rate of change of the length of $\mathbf r$ is zero
and:
- the velocity of $A$ is perpendicular to $\mathbf r$.
Apsidal Distance
The length of the radius of the orbit of an apse is known as the apsidal distance.
Line of Apsides
The direction of the radius of the orbit of an apse is known as the line of apsides.
Also known as
An apsis is also known as an apse.
Some sources use the term apsidal point.
Also see
- Results about apsides can be found here.
Linguistic Note
The word apsis comes from the ancient Greek ἁψίς (hapsís), which means arch or vault.
The plural of apsis (and apse) is apsides, pronounced ap-si-dees.
Sources
- 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): apse (plural apsides)
- 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): apsis (plural apsides)