Some quick facts:
• On Sept. 22/23, 2018, the day and night will be almost equal in most locations. The September equinox is on or around Sept. 22, while the first equinox of the year, the March equinox, takes place on or around March 21 every year.
• It's the end of summer in the Northern Hemisphere. The fall season ends on the December solstice, which marks the beginning of astronomical winter.
• In the Southern Hemisphere, the September equinox is the vernal (spring) equinox.
• Equinoxes are not day-long events, even though many choose to celebrate all day. Instead, they occur at the moment the sun crosses the celestial equator — the imaginary line in the sky above Earth’s equator.
• At this instant, Earth's rotational axis is neither tilted away from nor toward the sun.
• This year the sun crosses the celestial equator from north to south on Sept. 23, at 01:54 Coordinated Universal Time. Because of time zone differences, the equinox will take place on Sept. 22 across the United States.
• The date usually occurs on Sept. 22 or 23, but it can be as early as Sept. 21 and as late as Sept. 24. A Sept. 21 equinox has not happened for several millennia. However, in the 21st century, it will happen twice – in 2092 and 2096. The last Sept. 24 equinox occurred in 1931; the next one will take place in 2303.
• The equinox dates vary because of the difference between how the Gregorian calendar defines a year (365 days) and the time it actually takes for Earth to complete its orbit around the sun (about 365 and 1/4 days).
• This means that each September equinox occurs about 6 hours later than the previous year's September Equinox. This eventually moves the date by a day.
• The term “equinox” comes from the Latin words “aequus,” meaning equal, and “nox,” meaning night. This has led to a misconception that everybody on Earth experiences equal day and night — 12 hours of daylight and 12 hours of night time — on the day of the September equinox. In reality, most places on Earth enjoy more than 12 hours of daylight on this day. This is because of two reasons: the way sunrise and sunset are defined and atmospheric refraction of sunlight.
• The full moon closest to the September equinox, the harvest moon, is astronomically special. This is because the time between one moonrise to another around this period becomes shorter. Known as the Harvest Moon Effect, this phenomenon occurs due to the low angle the moon's orbit around Earth makes with the horizon during this time of year.
• As the September equinox rolls by, the chances to see the aurora borealis display increases for those located at high Northern Hemisphere latitudes. According to NASA, the equinoxes are prime time for the northern lights — geomagnetic activities are twice as likely to take place in the spring and fall than in the summer or winter.
• Many cultures around the world hold feasts and celebrate festivals and holidays to mark the September equinox.