Analysis: Why day and night aren’t equal length on an equinox

PhD candidate Osnat Katz (UCL Science & Technology Studies) describes how the bending of light and the Sun's size means that even on an equinox, day and night are not exactly equal. Winter in the northern hemisphere and summer in the southern hemisphere are both coming to an end. That means the days and nights are becoming roughly equal in length, and the path the Sun traces across the sky is changing. On a winter day, the Sun is low in the sky, whereas on a summer's day the Sun lies considerably higher. But on a specific day in the spring or autumn, the Sun will be visible directly above the equator, somewhere in the middle of the two arcs traced by the Sun in the summer and winter. This is what's called the equinox, and there are two each year. Around 20 March we have the vernal equinox or March equinox, also known as the spring equinox in the northern hemisphere.