If you recall the meanings of apogee and perigee, then you have a hint about apastron and periastron. These terms apply to the situation where one star revolves around another in an eliptical orbit. (virtually all orbits are eliptical.) When the secondary (smaller) star is closest to the primary, it’s referred-to as being in periastron. At its farthest point, it’s at apastron.
Observers of double stars often take note of this, although from our perspective, the change occurs very slowly -- often over hundreds of years. The distance from the primary star to the secondary at the point where the two are farthest apart is referred to as the semi-major axis.
Now here are two related terms - aphelion and perihelion.. You’ve probably figured them out already -- aphelion is when an object is at its farthest point from the Sun, and perihelion is when it’s closest to the Sun.
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