Equinox

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

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 in different places on the horizon. They found special days when day and night were almost the same length. These days are called "equinoxes," from Latin words meaning "equal" and "night."

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

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

Effects on satellites

During the equinox, some communications satellites may have trouble sending signals to Earth. This can happen when the Sun shines directly behind the satellite, making TV or phone signals weaker for a short time.

Satellites in geostationary orbit can also have trouble getting power during the equinox. They pass through Earth's shadow longer than usual, so they use their batteries until sunlight comes back.

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. We first saw this 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.