The equinox explained

What Is an Equinox?

An equinox happens twice a year — around March 20 (vernal or spring equinox) and around September 22 (autumnal equinox) — when the Sun crosses the celestial equator, the imaginary line that projects Earth's equator onto the sky. This astronomical event gives us a day when daylight and darkness are almost equal across the globe. The word "equinox" comes from Latin *aequus* (equal) and *nox* (night), and while perfect 12-hour balance is rare due to atmospheric refraction, the equinox remains the closest we get.

During an equinox, neither hemisphere is tilted toward the Sun, so sunlight falls evenly on both the Northern and Southern Hemispheres. This balance is the midpoint of Earth's journey between the extremes of the summer solstice and winter solstice. It marks the official start of spring in March and autumn in September, and cultures worldwide celebrate harvest festivals, new beginnings, and the changing seasons.

Why Day and Night Are Nearly Equal

The key to equinox equality lies in Earth's axial tilt. Our planet spins on an axis tilted 23.5° relative to its orbital plane. During solstices, one hemisphere leans toward the Sun, giving long days and short nights, while the other leans away. At an equinox, the tilt is sideways relative to the Sun, so the terminator — the line separating day from night — passes through both poles. This means every location on Earth receives about 12 hours of sunlight and 12 hours of darkness.

If you're curious about the mechanism behind changing daylight, our article on why days get longer and shorter explains the full seasonal cycle. On equinox day itself, the Sun rises due east and sets due west everywhere except at the poles, making it a fantastic day for observing precise compass directions and shadow lengths.

When the Fastest Sunset Changes Happen

One of the most noticeable effects of the equinox is how quickly sunset times shift from day to day. Around the March and September equinoxes, the Sun sets several minutes earlier or later each evening — the fastest rate of change all year. For example, in mid-latitude cities like Chicago, sunset can shift by nearly three minutes per day during equinox weeks. In contrast, around the solstices, sunset times barely change for weeks.

This rapid shift is driven by the steep angle of the ecliptic relative to the horizon at this time of year. If you want to track these accelerating twilight changes for your own location, you can use our widget to see daily sunset times or explore sunset times and seasons for a deeper dive into annual patterns. After the equinox, the days either lengthen (spring) or shorten (fall) at their maximum pace.

Equinox vs. Solstice: Key Differences

While equinoxes bring equal day and night, solstices represent extremes — the longest day (summer solstice) and shortest day (winter solstice). The spring equinox marks the transition from shorter days to longer ones, while the autumnal equinox signals the opposite. The dates of these events shift slightly each year because Earth's orbit takes about 365.25 days, so we add a leap day every four years to keep calendars aligned.

The equinoxes also affect twilight durations. Because the Sun sets perpendicular to the horizon at the equator, twilight is shortest near the equator during equinoxes. But at higher latitudes, the Sun sets at a shallower angle, extending twilight and golden hour. If you're planning an outdoor photoshoot, the equinox window offers reliable light — check out golden hour vs blue hour for timing tips.

How Latitude Affects the Equinox Experience

At the equator, every day of the year is roughly 12 hours long, so the equinox passes almost unnoticed — sunrise and sunset occur around 6 a.m. and 6 p.m. year-round. But the farther you travel from the equator, the more dramatic the equinox becomes. In places like Norway or Alaska, the equinox is a brief pause between months of midnight sun and polar night. You can read more about this in our guides on how latitude affects sunset times and midnight sun explained.

At the poles themselves, the equinox marks the moment the Sun creeps above the horizon for the first time in six months (at the spring equinox) or dips below for a half-year of darkness (at the autumnal equinox). For photographers and travelers, the equinox is an ideal time to visit cities like Reykjavík or Tromsø for balanced daylight and stunning low-angle sunlight.

Equinox Traditions and Cultural Significance

For thousands of years, humans have marked the equinoxes with festivals, rituals, and monuments. The Mayan pyramid at Chichén Itzá casts a serpent-shaped shadow during the spring equinox. In Iran, Nowruz — the Persian New Year — begins exactly at the vernal equinox. Many modern pagans celebrate Mabon (autumn equinox) as a harvest festival, and the spring equinox is often associated with rebirth and new beginnings.

If you're looking for the best places to witness an equinox sunset, consider ancient sites like Stonehenge or modern skyline vistas. Our list of best cities to watch the sunset usa includes locations where equinox sunsets align with streets or landmarks — a phenomenon known as "Manhattanhenge" in New York City, which occurs in late May and mid-July, but the equinox sunsets in Chicago and San Francisco are equally photogenic. For exact equinox timing wherever you are, visit sunset.now and use our city-specific pages, like Chicago.

Frequently asked questions

Does the equinox really have exactly 12 hours of daylight?

Almost, but not precisely. Atmospheric refraction bends sunlight, making the Sun appear above the horizon a few minutes before actual sunrise and after sunset. This adds about 6–8 minutes of extra daylight, so the true 12-hour balance usually occurs a few days before the spring equinox or after the autumn equinox.

Why do sunset times change faster during equinox?

The Sun's path across the sky changes angle most steeply near the equinoxes because the ecliptic (the Sun's apparent path) meets the horizon at a sharper angle. This causes the point of sunset to shift north or south rapidly, changing the time by up to three minutes per day at mid-latitudes.

What is the difference between an equinox and a solstice?

An equinox occurs when the Sun crosses the celestial equator, giving nearly equal day and night. A solstice happens when the Sun reaches its northernmost or southernmost point (the Tropic of Cancer or Capricorn), marking the longest and shortest days of the year respectively.

When is the next equinox?

The spring equinox occurs around March 20 each year, and the autumn equinox occurs around September 22. Exact dates and times vary slightly by year and time zone, so check sunset.now for precise equinox moments in your city.

The equinox explained — sunset.now guide