Safekipedia
Dynamics of the Solar SystemEquinoxesMarch observancesSeptember observances

Equinox

Adapted from Wikipedia · Discoverer experience

A satellite view of Earth showing the moment of the September 2022 equinox, when day and night are nearly equal around the world.

A solar equinox is a special moment when the Sun appears directly above the Earth's equator. This happens because the Earth's rotation axis is tilted neither toward nor away from the Sun. On the day of an equinox, the Sun rises directly in the east and sets directly in the west.

This event occurs twice each year, around 20 March and 23 September. The word "equinox" comes from Latin words meaning "equal night" because, on these days, nighttime and daytime are almost the same length all over the world. However, they are not exactly equal due to the Sun's size and Earth's atmosphere.

In the Northern Hemisphere, the March equinox marks the beginning of spring, and the September equinox marks the beginning of autumn. In the Southern Hemisphere, it is the opposite. Equinoxes are important for understanding the changing seasons and the Earth's relationship with the Sun.

Equinoxes on Earth

Main article: Sun path

See also: Equinox (celestial coordinates)

At Earth's poles the Sun stays near the horizon all day around equinox, marking the change between the half year long polar night and polar day. The picture shows the South Pole right before March equinox, with the Sun appearing through refraction despite being still below the horizon.

Observing the sunrise, people noticed it happens at different spots on the horizon. They found midpoint days when day and night were almost equal. These days are called "equinoxes," from Latin words for "equal" and "night."

In the northern hemisphere, the vernal equinox (around March 20) marks spring's start in many cultures. The autumnal equinox (around September 23) marks autumn's beginning. The equinoxes are the only times when the Sun's path (the solar terminator) is straight across the equator. This means both hemispheres get the same amount of sunlight.

During equinoxes, the Sun rises due east and sets due west for everyone on Earth. This is when the Sun is directly above the equator. The equinoxes happen twice each year because Earth's tilt and orbit cause this alignment only at specific times.

Effects on satellites

During the equinox, some communications satellites can have trouble sending signals to Earth. This happens because the Sun can shine directly behind the satellite, causing too much light and energy to reach the antennas on the ground. This can make TV or phone signals weak for a short time.

Satellites in geostationary orbit can also have trouble getting power during the equinox. They pass through Earth's shadow for a longer time than usual, so they must use their batteries to keep working until sunlight returns.

Equinoxes on other planets

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

Equinoxes can happen on any planet that has a tilted axis. For example, on Saturn, an equinox makes its ring system appear edge-on to the Sun. This was first seen from the Cassini space probe in 2009. During this time, the rings get very little direct sunlight. Saturn's most recent equinox was on 6 May 2025.

Mars also has equinoxes. Its most recent ones were on 12 January 2024, during northern autumn, and on 26 December 2022, during northern spring.

Images

A stunning view of the Crab Nebula, the remains of a star that exploded long ago, showing colorful clouds of gas and light from space.
Illustration showing Earth during the four astronomical seasons as seen from the north.
Diagram showing how the Sun lights up Earth equally during the equinox in March and September.
Diagram showing the path of the Sun across the sky as seen from Earth, helping us understand how the Earth orbits the Sun.
An illustration showing the Earth at the beginning of each of the four astronomical seasons, viewed from space.
An antique marine sandglass used for telling time at sea.
A stunning view of Earth rising over the Moon, taken by astronauts during the Apollo 8 mission in 1968.
Map showing the International Date Line and the 180th meridian in the Aleutian Islands region.
An artist's depiction of HE 1523-0901, one of the oldest known stars in our galaxy, located about 7,500 light-years from Earth.

This article is a child-friendly adaptation of the Wikipedia article on Equinox, available under CC BY-SA 4.0.

Images from Wikimedia Commons. Tap any image to view credits and license.