Definition:Apsis

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)