Seasons and sky
Equilux
Direct answer
Equilux: The Equilux is the date when measured or calculated daylight and night are approximately equal at a particular latitude.
Equilux is the locally dependent date of roughly equal daylight and darkness.
$0 today, then $12/month after 7 days. Cancel anytime.
At a glance
- Found in
- Seasons & Earth Almanac
- Related
- Season Length Asymmetry, Polar Day, Astronomical Season, Equinox
Equilux is the locally dependent date of roughly equal daylight and darkness.
A Sun-Earth orientation for seasonal, atmospheric, tidal, and terrestrial terms.
What does your own chart say about you?
Choose love, work or purpose. Your reading is written from your chart. Your reading uses your own birth details.
Walk through Alex’s whole example first
The Road of AgesSee the turning points of your whole life, from your birth date
See your road's shape
Prefer to explore first?
The Road of AgesThe turning points of your whole life, from your own chart
Walk my road
Free, by email
Your daily Almanac letter
One short letter at 7 each morning: the day’s theme read for your own chart, and one thing to do with it.
The story behind Equilux
The term became useful in modern popular astronomy to distinguish observed light duration from equatorial crossing geometry.
Why Equilux matters
Equilux separates equal observed day length from the equinox instant because refraction and the finite solar disk extend daylight.
How to read Equilux
Equilux speaks to local day-length tables and public explanations of why equinox does not mean exactly twelve visible hours.
Equilux in practice: an example
At many mid-northern latitudes the spring Equilux occurs several days before the March Equinox.
Where readings of Equilux go wrong
A common error is to publish one global Equilux date; latitude, elevation, horizon, and the definition of sunrise all matter.
Sources and editorial method
Research references used for this entry include:
- U.S. Naval Observatory: Length of Day and Night at the Equinoxes - Official explanation of equinox geometry, refraction, and the difference between equinox and equilux.
- U.S. Naval Observatory: Rise, Set, and Twilight Definitions - Official definitions used for sunrise, sunset, refraction, and twilight calculations.
- Source
- The Great Almanac Dictionary
- Category
- Seasons & Earth Almanac
- Last updated
- 2026-09-26
Calculate your local equal-light date
A definition tells you what it is; your own birth chart tells you what it means for you. The Oracle casts your real chart and answers for your love, your work, and your year ahead, in conversation. It opens with the 7-day Seeker trial.
Speak with the OracleRelated terms
More in Seasons & Earth Almanac
From the reference page to your own chart
Every word here
is in your own chart.
- A personal reading about love, work or purpose, written from your chart
- A practical next step to consider
- It opens with 7 days free, with the AI Oracle and daily guidance
$0 today, then $12/month after 7 days. Cancel anytime.
Common questions
What is the Equilux?
The Equilux is the date when measured or calculated daylight and night are approximately equal at a particular latitude.
What do people get wrong about the Equilux?
A common error is to publish one global Equilux date; latitude, elevation, horizon, and the definition of sunrise all matter.
Free, by email
The Almanac, in your inbox at 7
One short letter at 7 each morning: the day’s theme read for your own chart, and one thing to do with it.
