The first day of spring (vernal equinox) in the northern hemisphere and the first day of fall (autumnal equinox) in the southern hemisphere takes place on March 20, 2017.
On this day, the Sun rises due east and sets due west. It will shine directly on the equator, and there will be nearly equal amounts of day and night throughout the world.
Equinoxes occur twice a year – in March and September – once when the Sun is traveling northwards, and once when it is traveling southwards. The position of the Sun at the moment of the March equinox is used to define the zero point of both right ascension and declination.
Description of relations between Axial tilt (or Obliquity), rotation axis, plane of orbit, celestial equator and ecliptic. Earth is shown as viewed from the Sun; the orbit direction is counter-clockwise (to the left). Credit: Dennis Nilsson
In practice, this is not exactly the case, however, because of a phenomenon called the precession of the equinoxes. Over hundreds of years, the direction of the Earth's spin axis in space changes because it acts like a gyroscope.
This means that the location of the equinoxes creeps across the sky at a rate of around 50 arcseconds each year. Astronomers quote right ascensions and declinations based on the configuration of the Earth's path around the Sun on January 1, 2000. Even in the years that have passed since the year 2000, the precession of the equinoxes has moved them by several arcminutes.
Video courtesy National Geographic
Seeing Equinoxes and Solstices from Space.
At an equinox, the Earth's terminator - the dividing line between day and night - becomes vertical and connects the north and south poles. The featured time-lapse video demonstrates this by displaying an entire year on planet Earth in twelve seconds. From geosynchronous orbit, the Meteosat satellite recorded these infrared images of the Earth every day at the same local time. The video started at the September 2010 equinox with the terminator line being vertical. As the Earth revolved around the Sun, the terminator was seen to tilt in a way that provides less daily sunlight to the northern hemisphere, causing winter in the north. As the year progressed, the March 2011 equinox arrived halfway through the video, followed by the terminator tilting the other way, causing winter in the southern hemisphere - and summer in the north. The captured year ends again with the September equinox, concluding another of billions of trips the Earth has taken - and will take - around the Sun. Credit: NASA, Meteosat, Robert Simmon
Equinox is a phenomenon that can occur on any planet with a significant tilt to its rotational axis. Most dramatic of these is Saturn, where the equinox places its ring system edge-on facing the Sun. As a result, they are visible only as a thin line when seen from Earth.
Featured image credit: Dennis Nilsson