An arc of light may gradually enlarge, become more obvious, and may take the form of a definite zigzag pattern, sometimes called a fortification spectrum (i.e. teichopsia, from Greek τεῖχος, town wall), because of its resemblance to the fortifications of a castle or fort seen from above.
An arc of light can grow larger, become more distinct, and form a zigzag pattern, sometimes called a fortification spectrum (i.e. teichopsia, from Greek τεῖχος, town wall), because it resembles the defenses of a castle or fort viewed from above.