Why Is Equilux Not on Equinox? #shorts

Why It Matters

Knowing when equilux occurs improves scheduling of events reliant on precise daylight balance and deepens public awareness of atmospheric effects on astronomical observations.

Key Takeaways

• Equinox marks sun crossing celestial equator, not exact day/night equality
• Atmospheric refraction extends daylight, causing earlier equal day/night
• "Equilux" occurs a few days before spring equinox each year
• In Oxford 2024, equilux falls around March 17‑18
• Check local sunrise/sunset times to pinpoint your equilux date

Summary

The video explains why the popular notion that the equinox delivers exactly equal daylight and darkness is a misconception, distinguishing the astronomical event from the phenomenon known as equilux.

An equinox occurs when the Sun crosses the celestial equator, aligning Earth’s tilt at 90° to its orbital plane. In theory this should produce a 12‑hour day and night, but atmospheric refraction bends sunlight, allowing the Sun to be visible even after it has geometrically set.

Because of this refraction, the true moment of equal day and night—equilux—happens a few days before the spring equinox. The presenter cites Oxford’s 2024 data, noting equilux around March 17‑18, while the official equinox falls on March 20.

Understanding the distinction helps viewers accurately track sunrise and sunset times, plan outdoor activities, and appreciate the subtle influence of Earth’s atmosphere on everyday timekeeping.

Original Description

We did it we made it through the dark, cold winter and equinox is right around the corner. The day with equal daytime and nighttime, except that’s not quite true. Equal day and night is known as equilux, not equinox, and usually we don’t quite have equal day and night on the equinox. So what actually is the difference? The equinox marks the switch between which hemisphere is pointing towards the Sun, when the Earth’s axis is at 90 degrees to its orbit around the Sun. So technically that should give us equal day and night if everything is perfectly aligned like that, equilux should be on the same day as the equinox. The reason it’s not is because Earth has an atmosphere, that changes the path of Sunlight through refraction. So much that when you watch the sunset, the sun is technically already below the horizon. But you can still see it because the light is bent round the atmosphere, making it appear like the Sun is higher than it is. What that means is that equilux, with a 12 hour day happens a few days earlier than the equinox when the sun crosses the celestial equator. So while the equinox is on the 20th March this year, here in Oxford, our equilux happens around the 17th and 18th March.

👩🏽‍💻 I'm Dr. Becky Smethurst, an astrophysicist at the University of Oxford (Christ Church). I love making videos about black holes, cosmology, dark matter, the early universe, the James Webb Space Telescope, and the biggest unsolved mysteries in astrophysics. I like to focus on how we know things, not just what we know. And especially, the things we still don't know. If you've ever wondered about something in space and couldn't find an answer online - you can ask me! My day job is to do research into how supermassive black holes can affect the galaxies that they live in. In particular, I look at whether the energy output from the disk of material orbiting around a growing supermassive black hole can stop a galaxy from forming stars.

http://drbecky.uk.com