Contrary to popular belief, the lengths of day and night aren’t exactly equal on the equinox. While the equinox is often celebrated as the day when daylight and darkness perfectly balance, this isn’t entirely accurate. The phenomenon that truly represents equal lengths of day and night is known as the “equilux.” The equilux occurs a few days before or after the equinox, depending on your geographical location.
What is equilux?
The equilux is the day when day and night are of equal length. Its exact date varies depending on your location on Earth, but it always occurs before the spring equinox after the autumn equinox. The term ‘equilux’ comes from the Latin words ‘equi,’ meaning equal, and ‘lux,’ meaning light.
Why Aren’t Day and Night Equal on the Equinox?
There are two reasons for this. But, first, what is the equinox?
Equinox definition
The duration of day and night are almost never equal on Earth because our planet’s axis is tilted. That’s why equinoxes and solstices (see notes 1) occur. That’s also why we have seasons.
So, the Sun’s center is usually not on the equator for most of the year. When the center of the Sun’s disc crosses the equator, astronomers define this moment as the equinox. In other words, an equinox is the moment in time when the plane of Earth’s equator passes through the geometric center of the Sun’s disk.
An equinox occurs twice each year, around March 20th and September 23rd. In the Northern Hemisphere, the spring equinox happens annually on March 19th, 20th, or 21st, marking the beginning of astronomical spring.
The word “equinox” comes from the Latin equi, meaning “equal,” and nox, meaning “night.” However, as mentioned above, the lengths of day and night are only approximately equal on the day of an equinox. There are two reasons for this:
- The Sun isn’t just a point in the sky – it’s a disc. Technically, sunrise begins when the upper edge of the Sun meets the eastern horizon, and sunset ends when the upper edge of the Sun sinks below the western horizon. As a result, daytime is slightly longer than nighttime on the day of an equinox.
- Our planet’s atmosphere refracts (bends) sunlight. When light passes from one medium to another, its path changes. As sunlight travels through the vacuum of space and then enters Earth’s denser atmosphere, it bends. Due to this bending, we can see the upper edge of the Sun several minutes before it touches the eastern horizon and several minutes after it has sunk below the western horizon. This effect adds extra daylight during the equinox. To complicate matters further, this bending varies with atmospheric temperature and pressure, so the lengths of day and night on the equinox are only ever approximately the same at any given location on Earth.
So, when are the duration of day and night equal? We have another term for this, believed to have been coined in the 1980s and gaining wider recognition in the 21st century, and it’s called equilux.
As we see above, the duration of daytime is longer than nighttime on the equinox.
Instead, the equilux is when day and night are of equal length, and the exact date of equilux depends on your location around the world. In the United Kingdom, for example, the equilux occurs 2-3 days before the spring equinox and 2-3 days after the autumn equinox.
However, as you get closer to the equator, the equilux date moves further away from the equinox date. For the spring equinox, it shifts to earlier dates, and for the autumn equinox, it shifts to later dates. As you move away from the equator and closer to the poles, the dates of equilux and equinox converge. In the table below, you can see the approximate dates of equilux for various latitudes. This doesn’t apply to the equinox, as your latitude doesn’t affect the equinox date.
Approx. equilux dates by Latitude
Latitude: Equilux date 1 – Equilux date 2
- 60° North: Mar 18 – Sep 25
- 55° North: Mar 17 – Sep 25
- 50° North: Mar 17 – Sep 25
- 45° North: Mar 17 – Sep 25
- 40° North: Mar 17 – Sep 26
- 35° North: Mar 16 – Sep 26
- 30° North: Mar 16 – Sep 27
- 25° North: Mar 15 – Sep 27
- 20° North: Mar 14 – Sep 28
- 15° North: Mar 12 – Sep 30
- 10° North: Mar 8 – Oct 4
- 5° North: Feb 24 – Oct 17
- 0° (Equator): No equal day and night
- 5° South: Apr 14 – Aug 29
- 10° South: Apr 1 – Sep 10
- 15° South: Mar 28 – Sep 14
- 20° South: Mar 26 – Sep 16
- 25° South: Mar 25 – Sep 17
- 30° South: Mar 24 – Sep 18
- 35° South: Mar 24 – Sep 19
- 40° South: Mar 23 – Sep 19
- 45° South: Mar 23 – Sep 19
- 50° South: Mar 23 – Sep 20
- 55° South: Mar 23 – Sep 20
- 60° South: Mar 22 – Sep 20
For a clear visual explanation for the terms, watch the video titled ‘Spring Equinox vs. Equilux’ below.
Notes
- A solstice is an event that occurs when the Sun reaches its most northerly or southerly position relative to the celestial equator on the celestial sphere. Two solstices happen annually: the summer solstice around June 20th-21st, and the winter solstice around December 20th-21st, both in the Northern Hemisphere.
Sources
- Equinox vs equilux – what’s the difference? on the BBC News website
- Equinox on Wikipedia
- “Equilux: When Day and Night are Equal” on the Time and Date website
