A long line needs a breath. The CAESURA is a word-break falling INSIDE a foot, near the middle, that splits the line into balanced halves. In the hexameter it usually lands after the fifth half-foot (the ‘masculine’ caesura) or the seventh — the hinge the whole line turns on. Slide the pause to its valid positions and feel the line balance.
A caesura is a sense/word break within a metrical foot (distinct from a diaeresis, which falls between feet). In dactylic hexameter the principal caesura is normally PENTHEMIMERAL (after the 5th half-foot, i.e. in the 3rd foot — ‘masculine’ if after a long) or HEPHTHEMIMERAL (after the 7th); a ‘feminine’ caesura falls after the first short of a 3rd-foot dactyl. It divides the 12 half-feet into balanced cola. A fail-loud self-check throws unless the penthemimeral break sits at half-foot 5 within the 12-half-foot line. ◆ the metrical positions are exact, node-verified.
The named positions (penthemimeral / hephthemimeral) are exact conventions; which one a given line uses, and whether a weak break ‘counts’, is the scholar’s reading. The hinge is real; its placement in a specific line can be argued.