Definitions
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. The point of greatest separation between two stars, especially in a binary star situation.
from the GNU version of the Collaborative International Dictionary of English
 n. That point in the orbit of a double star where the smaller star is farthest from its primary.
from The Century Dictionary and Cyclopedia
 n. In astronomy, that part in the orbit of a double star where it is furthest from its primary.
Etymologies
Examples

They had already passed apastron and were downward bound.

The station was outward bound for the apastron of its shortened and cometary path.

In this region, nearing apastron, no unusual velocity change was needed to alter orbit radically.

Nevertheless the ship took days, terrible days to reach apastron.

Now, at apastron, with Vixen's atmosphere to filter its radiation, the sun might almost have been Sol: smaller, brighter, but gentle in a blue sky where tall white clouds walked.

The eccentricity of their orbit approaches in form that of Faye's comet, which travels round the Sun; consequently the stars, when at apastron, are twice their periastron distance.

The pair travel in an orbit from fourteen to fortytwo times the radius of the Earth's orbit; so that when at apastron they are three times as distant from each other as when at periastron.

These orbs complete a revolution in 180 years, and when in apastron are seventeen times more remote from each other than when at periastron.

Any approach to, or recession from this point, must occur simultaneously with each; they must always occupy corresponding parts of their orbits, and be in apastron and at periastron in the same period of time.

Consequently, neither star can approach or recede from this point without the other affecting a similar motion, they must be at periastron and apastron together, and any acceleration or retardation of speed must occur simultaneously with each.
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