Show HN: The current sky at your approximate location, as a CSS gradient

For HTML Day 2025 [1], I made a web service that displays the current sky at your approximate location as a CSS gradient. Colours are simulated on-demand using atmospheric absorption and scattering coefficients. Updates every minute, without the use of client-side JavaScript.

Source code and additional information is available on GitHub: https://github.com/dnlzro/horizon

[1] https://html.energy/html-day/2025/index.html

Show HN: The current sky at your approximate location, as a CSS gradient
sky.dlazaro.ca

> the little-known meta http-equiv="Refresh" HTML tag

Oh, don't mind me, I'll just be over here in the corner laughing ruefully as my bones crumble to dust: back when I started, if you wanted a page to refresh on its own, this was the only way.

Beautiful work! A splendid example of formal minimalism at its best.

how i missed this small hn posts. thanks

Awesome. I remember much earlier in my career I was working on a 3D turn-by-turn navigation software, and one of my tasks was to draw the sky in the background. The more senior guy on the team said, just draw a blue rectangle during the day and a dark gray one at night and call it job done. Of course, I had to do it the hard way, so I looked up the relevant literature on sky rendering based on the environment, latitude, longitude, time of day and so on, which at the time was Preetham[1] ("A Practical Analytic Model for Daylight"), and built a fully realistic sky model for the software. I even added prominent stars based on a hard-coded ephemeris table. It was quite fast, too.

Well, the higher ups of course hated it, they were confused as to why the horizon would get hazy, yellowish, and so on. "Our competitors' skies are blue!" They didn't like "Use your eyes and look outside" as an answer.

Eventually, I was told to scrap it and just draw a blue rectangle :(

All that to say, nice job on the site!

1: https://courses.cs.duke.edu/cps124/fall01/resources/p91-pree...

There's two main reasons for this:

- First and most impactful: as the earth curves down and away from the observer's horizon, your line of sight goes through a thicker slice of the atmosphere.

Looking straight up you might have 100km of atmosphere until space (the distance is made up here, but I'm using the Kármán line as an arbitrary ruler), but looking out towards the horizon (assuming a perfectly spherical Earth), it's much, much more than that 100km, so the light will scatter off of (and/or be filtered by, depending on angle and time of day) more particles in the atmosphere, affecting the colour of the sky.

- The compounding factor here is if there are environmental factors that boost the particle count in the air, and especially particles that'd stay in lower layers of the atmosphere. Where I am, we've been dealing with wildfire smoke of varying strengths for a few weeks. Today's gentle enough, but it's bad enough that my gradient goes from rgb(115, 160, 207) at the top of the sky to rgb(227, 230, 227) at the horizon (which is shockingly accurate).