Today every point on Earth has a day and night of very nearly 12 hours for each. This is called an equinox and since this one occurs in September every year it is called the September Equinox. It is also called the Autumnal Equinox in the northern hemisphere as it marks the onset of Autumn.
Only one other equinox occurs every year and it occurs in March. Obviously this is called the March equinox. It is also referred to as the Vernal Equinox and in the northern hemisphere marks the start of spring.
Equinoxes occur when the tilt of the Earth’s of rotation is at a right angle to the line connection the Earth and Sun. This can only occur twice in a single orbit of the Earth around the Sun (ie one year). At the time of each equinox the Sun lies at the intersection of the plane containing the Earth and Sun and the plane defining the Earth’s Equator.
In the figure on the right the blue plane is defined by the Earth’s equator. The yellow plane is the plane containing the earth and Sun (the line the Sun appears to follow is called the ecliptic). The position of the Sun at an equinox is marked as Autumnal and Vernal Equinoxes on the figure. Today the Sun is at the pointed marked as Autumnal Equinox.
