Most of us in the Northern Hemisphere eagerly await the vernal equinox, or the day every spring when the day and night are finally of equal length. We count down as the long nights of winter wane and the long days of summer approach. Equinox comes from the Latin words aequi, which means "equal," and nox, which means "night."
The astronomical event, known as an equinox, occurs twice each year around March 20th and September 23rd, when the plane of Earth’s equator passes through the center of the sun.The March equinox marks the moment the Sun crosses the celestial equator – the imaginary line in the sky above the Earth’s equator – from south to north and vice versa in September. The equinoxes are the only times when the solar terminator (the dividing line between night and day) is perpendicular to the equator. Then the northern and southern hemispheres are equally illuminated.
The astronomical event, known as an equinox, occurs twice each year around March 20th and September 23rd, when the plane of Earth’s equator passes through the center of the sun.The March equinox marks the moment the Sun crosses the celestial equator – the imaginary line in the sky above the Earth’s equator – from south to north and vice versa in September. The equinoxes are the only times when the solar terminator (the dividing line between night and day) is perpendicular to the equator. Then the northern and southern hemispheres are equally illuminated.
Equinoxes are the only times when the subsolar point is on the equator, i.e. the Sun is exactly overhead at a point on the equatorial line. The subsolar point crosses the equator moving northward at the March equinox and southward at the September equinox.