That circle (or "longitudinal line") would only share the same length of day and night if it were measured from the center of the Sun crossing the horizon. Thi

That circle (or "longitudinal line") would only share the same length of day and night if it were measured from the center of the Sun crossing the horizon. This is because of another point glossed over the the article. The length of time it takes the sun to traverse the horizon varies and gets longer the further you are from the equator during the equinox (or the further from perpendicular the suns apparent path is with the horizon). This means that on the equinox, days nearer the poles are longer than days at the equator because we measure days as when any part of the sun are over the horizon. This becomes clear when you think about how at the equinox the north and south pole are in the middle of a week or two long sunrise/sunset.