Equinox – Definition, Facts, Dates

The equinox marks a significant astronomical event that occurs twice a year, signaling a moment of balance between day and night across the globe. It is when Earth’s tilt perfectly aligns with its orbit around the Sun, creating nearly equal lengths of day and night. The equinox marks a seasonal change, signaling the beginning of spring or autumn.

Key Points

• Definition and Occurrence: An equinox occurs twice a year, around March 20 and September 23, when the Earth’s equator passes directly under the Sun.
• Day and Night Length: While day and night are very nearly equal, daytime is slightly longer than nighttime at the equinox. This is due to atmospheric refraction and the size of the Sun’s disk, extending daylight by a few minutes.
• Significance and Naming: The March equinox marks the start of spring in the Northern Hemisphere (vernal equinox) and autumn in the Southern Hemisphere, while the September or autumnal equinox signals autumn in the Northern Hemisphere and spring in the Southern Hemisphere.
• Science of the Equinox: The equinox happens because Earth’s axis is tilted neither toward nor away from the Sun, distributing sunlight evenly between the hemispheres.
• Effects and Observations: The equinox affects satellite operations, enhances auroras, and provides ideal conditions for observing phenomena like the green flash.

What Is an Equinox?

An equinox occurs when the Earth’s equator aligns with the Sun, making the Sun appear directly overhead at the equator. On the equinox, the Sun rises due east at the equator, and sets due west. This alignment results in nearly equal daylight and darkness across the globe. The word “equinox” derives from the Latin words aequus (equal) and nox (night), reflecting the near balance of day and night.

Frequency and Naming of Equinoxes

Equinoxes happen twice each year, around March 20 and September 23. These dates vary slightly due to Earth’s elliptical orbit and leap years. The equinox names reflect the seasons they herald in the Northern Hemisphere:

• Vernal (Spring) Equinox: Occurs around March 20, marking the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere.
• Autumnal (Fall) Equinox: Occurs around September 23, signaling the start of autumn in the Northern Hemisphere and spring in the Southern Hemisphere.

How the Equinox Works

The equinox occurs because of the Earth’s axial tilt of approximately 23.5 degrees relative to its orbital plane around the Sun. As Earth orbits the Sun, this tilt causes different parts of the planet to receive varying amounts of sunlight, resulting in the changing seasons. During the equinox, Earth’s axis tilt neither toward nor away from the Sun. This position places the Sun directly above the equator, distributing sunlight evenly between the Northern and Southern Hemispheres.

In the days leading up to the equinox, the Sun’s path across the sky gradually shifts northward or southward, depending on the time of year. After the equinox, the Sun continues its apparent journey, leading to longer days or nights as the seasons progress.

Are Day and Night Truly Equal at the Equinox?

Although the equinox is considered the day when day and night are of equal length, this is not entirely accurate. Daytime is slightly longer than nighttime on the equinox due to two main factors:

1. Atmospheric Refraction: Earth’s atmosphere bends sunlight so the Sun appears slightly above the horizon even when it is technically below it. This refraction effect adds several minutes of light to both sunrise and sunset.
2. Solar Disk Size: The Sun is not a point source of light but rather a disk. The moment the upper edge of the Sun appears at sunrise or disappears at sunset extends the length of daylight slightly beyond 12 hours.

These effects make daytime longer than nighttime by a few minutes, even on the equinox.

Equinox Effect on Satellites

The equinoxes impacts some satellites, particularly those in geostationary orbits. Most of the year, the Earth’s tilt keeps a satellite situated above the equator in continuous sunlight. However, during the equinox, satellites experience what’s known as the “eclipse season.” Each eclipse season lasts about 44 days. This is a period when the Earth blocks the Sun from the satellite’s perspective, causing potential power issues because the satellite relies on solar panels for energy. Most satellites rely on lithium ion batteries, similar to the ones in cell phones, that keep them functioning during eclipse season.

Solar interference or a solar outage is another concern near the equinox. This affects ground-based satellite communication systems rather than the satellites themselves. The alignment between the Sun, Earth, and satellites produces intense solar radio noise that overpowers the signals from the satellites. The temporary communication disruptions typically occur a few days before and after the equinox and last around 15 minutes each day when the Sun, satellite, and ground station align. Communication system operators predict interference periods and either reduce service quality or simply schedule maintenance during these times.

Equinoxes on Other Planets

Equinoxes are not unique to Earth; they occur on other planets with tilted axes. For example:

• Mars: Has equinoxes similar to Earth, marking the transition between seasons. Mars’s equinoxes affect the distribution of its famous dust storms.
• Saturn: Experiences dramatic equinoxes approximately every 15 Earth years, when the Sun illuminates Saturn’s rings edge-on. The rings nearly vanish when viewed from Earth.
• Uranus: Due to its extreme axial tilt, Uranus’s equinoxes are rare and significant. Uranus experiences drastic changes in sunlight distribution over its long seasons.

Table of Equinox Dates and Times (UT) from 2019 to 2029

Interesting Equinox Facts

• Equilux: The day when day and night are truly equal is called the equilux. This usually occurs a few days before or after the equinox, depending on the location.
• Fast Sunrise and Sunset: The Sun rises and sets more quickly at the equinox than at other times of the year.
• Polar Light: For a brief period of time on the equinox, both the North and South Poles see daylight.
• Cultural Celebrations: Many cultures celebrate the equinoxes with festivals, such as Nowruz in Iran (Persian New Year) and Ostara, a pagan holiday celebrating spring.
• Green Flash: The phenomenon of the “green flash” is more common during equinox sunsets and sunrises. This is due to the Sun’s alignment and stable atmospheric conditions associated with the start of spring or fall.
• Auroras: The equinoxes enhance the occurrence of auroras. Geomagnetic activity increases due to better alignment between Earth’s magnetic field and solar wind. Also, the alignment means the auroras at the poles are mirror images of each other (conjugate auroras).
• Solar Alignments: Ancient structures like Stonehenge and Chichen Itza align with the Sun’s position during equinoxes.

References

• Abrahamian, David (2018). “How satellites are affected by the spring and autumn equinoxes“. Viasat, Inc.
• Blackburn, Bonnie J.; Holford-Strevens, Leofranc (1999). The Oxford Companion to the Year. Oxford University Press. ISBN 0-19-214231-3.
• Davis, Neil (1992). The Aurora Watcher’s Handbook. University of Alaska Press. ISBN 0-912006-60-9.
• Lakdawalla, Emily (2016). “Oppositions, conjunctions, seasons, and ring plane crossings of the giant planets“. The Planetary Society.