In historical almanacs the vernal equinox is explained as being 'Mid point between the sun and the heavens when the division between day and night is identical' and, holds true that at the vernal equinox, day and night are roughly the same length.
『暦便覧』に「日天の中を行て昼夜等分の時なり」と記されている通り、春分では昼夜の長さがほぼ同じになる。