At the vernal and autumnal equinoxes, Earth's rotation axis is perpendicular to the line joining Earth to the Sun, and so day and night are of equal length.
(Earth's rotation axis; is; perpendicular to the line; L:At the vernal and autumnal equinoxes)
Earth's rotation axis
is
perpendicular to the line L:At the vernal and autumnal equinoxes