The astronomical event, known as an equinox, occurs twice each year, the March equinox (usually March 20) and the September equinox (usually September 22), when the plane of Earth’s equator passes through the center of the sun. On the June solstice, the sun reaches its highest point in the sky for an observer at the North Pole. On the December solstice, the sun reaches its highest point in the sky for an observer at the South Pole. Solstices mark the change from fall to winter or spring to summer.
On an equinox, the Sun shines directly on the equator and the length of day and night is nearly equal – but not quite. They are not exactly equal because of the angular size of the sun and atmospheric refraction. The word equilux is used to mean a day in which the durations of light and darkness are equal to distinguish the day from an equinox.
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.
= The Sun’s position directly overhead (zenith) in relation to an observer.
= The Moon’s position at its zenith in relation to an observer (Moon phase is not shown).
Civil Twilight (lightest shade)
Nautical Twilight (next darker shade)
Astronomical Twilight (next darker shade)
Night, no twilight (darkest shade)
The equinoxes and solstices are directly related to the seasons of the year. In the northern hemisphere, the vernal equinox (March) conventionally marks the beginning of spring while the autumnal equinox (September) marks the beginning of autumn. In the southern hemisphere, the vernal equinox occurs in September and the autumnal equinox in March.
We can even see how the sun illuminates Earth during an equinox thanks to a NOAA satellite.
Earth’s tilted axis causes the seasons. Throughout the year, different parts of Earth receive the Sun’s most direct rays. So, when the North Pole tilts toward the Sun, it’s summer in the Northern Hemisphere. And when the South Pole tilts toward the Sun, it’s winter in the Northern Hemisphere.
Aurora season is winding down in the higher southern latitudes, but there is still time to witness it in person. It varies slightly by location. The Northern Lights season is just getting started. It is from September until March. This is because the aurora can more easily be seen due to longer nights.
Studies have shown that during both the fall and spring equinoxes geomagnetic disturbance and thus aurora are twice as likely as during summer and winter.
For a long time, scientists did not know what made equinoxes special. Using data from space missions such as THEMIS, scientists now know that it is all about geometry. THEMIS showed us that there are special magnetic ropes connecting Earth’s upper atmosphere directly to the sun. During the spring and fall equinox, the geometry of the Earth with respect to the sun is such that its magnetic field is best oriented to connect with the sun. Learn more about aurora season and equinoxes at EarthSky.
Solar activity is increasing in this current solar cycle, so hopefully, there will be more aurora.
We can all look for great photos and videos shared online, even if we can’t be there in person. It’s still a bucket list item for us to see this in real life!
But wait, there’s more!
Earth is not the only place in the solar system that experiences seasons. Other planets also have seasons. You can learn more about planetary seasons from NASA. The same is true for aurora. They also don’t just happen on Earth. Several different planets have aurora but some of the strongest and most spectacular outside of Earth are on the planet Jupiter. Two of NASA’s great space telescopes have observed Jovian aurora, Hubble and Webb. Whether or not other planets also have aurora seasons is a great question. Something for scientists to research further.