Equinox facts for kids

An equinox is a special moment that happens twice a year when the Sun shines directly over the Earth's Equator. On the day of an equinox, the Sun appears to rise exactly in the east and set exactly in the west. These events happen each year around March 20 and September 23.

The word "equinox" comes from the Latin words aequus (equal) and nox (night). This is because on the day of an equinox, daytime and nighttime are almost equal in length everywhere on Earth. They aren't perfectly equal, but they are very close.

The equinoxes mark the beginning of new seasons. In the Northern Hemisphere, the March equinox is the start of spring, while the September equinox is the start of autumn. In the Southern Hemisphere, it's the other way around. Throughout the year, equinoxes alternate with solstices, which are the longest and shortest days of the year.

What Causes an Equinox?

Earth as seen from space during the September 2022 equinox. The line between day and night (the terminator) is straight and connects the North and South Poles.

The reason we have equinoxes and seasons is because of the way the Earth is tilted.

Earth's Tilted Axis

Imagine a line running through the Earth from the North Pole to the South Pole. This is Earth's axis, and it's the line our planet spins around. However, this axis isn't straight up and down compared to our path around the Sun. Instead, it's tilted at an angle.

Because of this tilt, as Earth orbits the Sun, different parts of the planet get more direct sunlight. When the Northern Hemisphere is tilted toward the Sun, it gets more direct light and experiences summer. When it's tilted away, it gets less direct light and has winter.

A Moment of Balance

An equinox happens at the two points in Earth's orbit where the tilt is sideways to the Sun. At this moment, the tilt is neither toward the Sun nor away from it. The Sun's light shines evenly on both the Northern and Southern Hemispheres.

This is the only time when the "terminator"—the line separating day and night—is a straight line that runs from the North Pole to the South Pole. Because the sunlight is spread out so evenly, the length of day and night is nearly the same all over the world.

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  How the Sun lights up the Earth during an equinox. Both halves of the planet get equal light.

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  From Earth, the Sun seems to travel along a path called the ecliptic (red line). The equinox happens when this path crosses the celestial equator (white line).

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  A diagram of Earth's seasons as seen from the north. The equinoxes happen between the solstices.

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  A diagram of Earth's seasons as seen from the south.

When Do Equinoxes Happen?

Equinoxes are important markers for our seasons and calendars.

Spring and Autumn Equinoxes

There are two equinoxes each year:

• The March Equinox happens around March 20. In the Northern Hemisphere, this is called the spring or vernal equinox, and it marks the start of spring. In the Southern Hemisphere, it's the autumnal equinox and marks the start of autumn.
• The September Equinox happens around September 23. In the Northern Hemisphere, this is the autumnal or fall equinox, marking the start of autumn. In the Southern Hemisphere, it's the spring equinox.

Why the Dates Can Change

The date of an equinox isn't always the same. It can vary slightly because our calendar system isn't a perfect match for Earth's orbit. A year on Earth is about 365.25 days long. We use leap years to help our calendar catch up, which can cause the date of the equinox to shift by a day or two.

Long ago, calendars like the Julian calendar weren't as accurate. The date of the spring equinox slowly shifted earlier over hundreds of years. To fix this, the modern Gregorian calendar was created. It has a better leap year rule that keeps the equinoxes happening around the same dates each year.

Why Day and Night Aren't Exactly Equal

This chart shows how many hours of daylight different parts of the world get during the year. Notice that on the equinoxes (around day 80 and 265), the daylight hours are close to 12 for everyone.

Even though "equinox" means "equal night," the day and night on an equinox are not exactly 12 hours long. The day is usually a few minutes longer. There are two main reasons for this.

1. The Sun is a Disc, Not a Point: Sunrise is defined as the moment the top edge of the Sun appears over the horizon, not its center. The same is true for sunset. This means we get a few extra minutes of daylight in the morning and evening.
2. Earth's Atmosphere Bends Light: Our planet's atmosphere acts like a lens and bends sunlight. This effect, called atmospheric refraction, allows us to see the Sun's light even when it is technically just below the horizon.

The day when day and night are truly closest to being equal is called an equilux. This usually happens a few days before the spring equinox and a few days after the autumn equinox.

Equinoxes Around the World

At the South Pole, the Sun sits on the horizon for the entire day around the equinox. This picture shows the Sun appearing through the bending of light in the atmosphere.

A View from the Poles

The equinoxes are very dramatic at the North and South Poles. At the North Pole, the March equinox marks the end of six months of darkness as the Sun rises for the first time. The Sun will then stay in the sky for the next six months. At the South Pole, the opposite happens: the Sun sets for the first time in six months.

On the day of the equinox, an observer at either pole would see the Sun skim across the horizon for 24 hours, never fully rising or setting.

Cultural Celebrations

For thousands of years, people have used the equinoxes to mark time and celebrate. Many cultures see the spring equinox as a time of rebirth and new beginnings.

• Nowruz, the Persian New Year, is celebrated on the March equinox in countries like Iran and Afghanistan. It is a festival that celebrates the start of spring.
• Many ancient structures were built to align with the Sun on the equinoxes. For example, at Angkor Wat in Cambodia, the Sun rises directly over the main temple on the spring equinox.
• In Japan, the days of the spring and autumn equinoxes are public holidays for people to spend time with family.

Equinoxes on Other Planets

During Saturn's equinox, its rings are edge-on to the Sun and reflect very little light. This image was taken by the Cassini space probe in 2009.

Any planet with a tilted axis has seasons and equinoxes, just like Earth.

A great example is Saturn. Saturn is tilted, and its famous rings orbit around its equator. During Saturn's equinox, which happens about once every 15 years, the rings are tilted perfectly edge-on to the Sun. From far away, they receive so little sunlight that they almost seem to disappear, leaving only a thin, dark line. Saturn's most recent equinox was on May 6, 2025.

Mars also has equinoxes. Its most recent ones were in December 2022 (northern spring) and January 2024 (northern autumn).

How Equinoxes Affect Satellites

The equinoxes can cause some interesting effects for the communications satellites that orbit Earth.

For a few days around each equinox, the Sun passes directly behind geostationary satellites (from our point of view on the ground). The Sun is so powerful that its energy can overwhelm the satellite's signals. This can briefly disrupt services like TV, radio, and internet. This event is called a sun outage.

Also, during an equinox, satellites in orbit above the Equator pass through Earth's shadow for the longest amount of time. When they are in the shadow, they can't use their solar panels and must rely on batteries for power.

Different Names for Equinoxes

You might hear equinoxes called by different names:

• Vernal equinox and autumnal equinox: These are older names. "Vernal" means spring, and "autumnal" means autumn. However, these names can be confusing because when it's spring in the northern half of the world, it's autumn in the southern half!
• March equinox and September equinox: These names are simpler because they just refer to the month they happen in. This avoids confusion about seasons in different parts of the world.
• Northward equinox and southward equinox: These names describe the direction the Sun appears to be moving. The northward equinox (in March) is when the Sun crosses the equator moving north. The southward equinox (in September) is when the Sun crosses the equator moving south. These terms are also useful for talking about other planets!

Related pages

• Solstice

Images for kids

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  Earth at the March 2019 equinox.

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  When Saturn is at equinox its rings reflect little sunlight, as seen in this image by Cassini in 2009.

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  Day arc at 0° latitude (equator) At the equator, the Sun passes directly overhead at noon, so a straight object like a pole would have no shadow.

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  Day arc at 20° latitude The Sun reaches high in the sky, and its path at sunrise and sunset is quite steep.

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  Day arc at 50° latitude The time when the sky is still light before sunrise or after sunset (twilight) lasts almost two hours.

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  Day arc at 70° latitude The Sun doesn't get very high in the sky, and its path is at a shallow angle. Twilight lasts for more than four hours.

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  Day arc at 90° latitude (pole) At the poles, the Sun would appear to stay right on the horizon all day long, if not for the atmosphere bending its light.

See also

In Spanish: Equinoccio para niños