Zenith: a live local-first fixed viewport planetarium

5/15/2026 at 6:24:42 PM

I did a show and tell for an elementary school class of my astronomy hobby with a tracking telescope and sunfilter. One of the best things happened accidentally when the tracking died, but the kids ended up really amazed by just how fast the sun was moving out of view, and getting to manually chase it; otherwise it would have been a cool but fairly boring view

by petee

5/15/2026 at 8:25:35 PM

exactly. this takes me back to the low-tech beginnings

by smorgasborg

5/16/2026 at 12:07:44 AM

When I put together my telescope setup I got a manual equatorial thinking I’d add a clock drive and my next telescope would have a go-to mount. It’s been about 20 years now.

by buescher

5/15/2026 at 4:26:13 PM

This is an interesting concept. I also like the idea of projecting onto a ceiling of a room. It is always surprising the first time you try using an non-tracking telescope to see how fast the earth is turning. This gives you that without a telescope being necessary.

I block location requests, so it's just showing me the default location as Stonehenge. It would be interesting to allow the user to manually add location coords.

by dylan604

5/15/2026 at 4:53:42 PM

I _LOVE_ watching the moon transit my view port in my telescope. Love being reminded of this movement. The bigger planets are fun too.

by leetrout

5/15/2026 at 8:35:51 PM

> It would be interesting to allow the user to manually add location coords

shouldn't be hard. one difference is moving to a much higher/lower lat. to see the difference in angular speed. Where would you want to see?

by smorgasborg

5/15/2026 at 8:42:35 PM

32°

by dylan604

5/15/2026 at 8:55:10 PM

https://smorgasb.org/zenith32/

was quicker to hardcode one, then add the feature

by smorgasborg

5/15/2026 at 9:47:33 PM

that's all sorts of awesome! thanks!

just as part of that hacker spirit, I tried changing the 32 to a different value in the url...:thinking-face:

by dylan604

5/15/2026 at 9:59:17 PM

lol. not a bad idea, but no, that didn't work. encoding latitude as a URL param... could save me from having to make a ui. it'd be something like ../zenith?lat=35.2N

by smorgasborg

5/15/2026 at 10:15:55 PM

and if you wanted to adjust the timezone you could add &lon=

again, admitted scope creep.

I just hate that browsers allow for location data to be used. Sure, there's some cool/fun things that can be done like what you've done, but it's too easy to abuse it. Adding features like this that allow the user to set it, especially if they wanted to see what it looks like from a different location, would be less invasive while adding more features. Just so you know, even if I open gMaps, it guesses my location based on IP because I damn sure don't share with theGoog.

by dylan604

5/15/2026 at 6:23:18 PM

Very cool!

You can do this with the naked eye in an area with tall sharp mountains such as the Alps, Rockies, Andes, etc. at times when the moon is low in the sky.

Move to a position where the moon is partially obscured by a mountain across the valley, and watch. It is surprisingly easy how little walking it can take to find a useful alignment. Then just stand and watch. The effect is amazing, even more powerful than watching it drift out of frame the telescope — it really shifts one's perspective to feeling how the earth moving.

by toss1

5/15/2026 at 6:25:51 PM

It's even easier to see twice a day with sunrise and sunset!

by dylan604

5/15/2026 at 9:57:52 PM

Yes, I've seen that too, and you can get the sensation.

But it is not nearly as vivid a sensation as the moon against a sharp edge of an alpine slope a couple km across a valley (vs all the way to the horizon).

The difference is on the scale of imagining being traveling in a railway car vs actually being in one. Once I saw it, it wasn't unlike being on a smooth Swiss rail just starting to pull out of the station...

by toss1

5/15/2026 at 10:09:24 PM

Well, not everyone lives in mountains, so it was a pretty specific example. You could say the same thing about someone living in a city with tall buildings. You can just stand there and watch the moon climbing from behind them. There's a popular spot in my city that is a good distance from downtown so you see the skyline where photographers will line up to capture the moon rise behind downtown. You can tell the newbies by how casual they are about what they are doing vs the experienced ones that know once it starts it's over in a matter of minutes.

by dylan604

5/15/2026 at 11:00:29 PM

Yes, big city buildings could work very well for that!

by toss1

5/15/2026 at 8:27:32 PM

exactly. that's the only other time you can get that sense. (or moonrise/set)

by smorgasborg

5/15/2026 at 8:20:34 PM

I'm the dev. happy to answer questions.

by smorgasborg

5/15/2026 at 5:26:16 PM

This is fantastic. I love how it makes Earth’s rotation feel immediate and visceral with zero equipment. The controls and overlays are really well thought out. It would be cool to have a search box to jump to a specific star or coordinate, even if most things in the ribbon are obscure.

by a-kgeorge

5/15/2026 at 7:05:40 PM

This is very cool! Will definitely project on the ceiling.

I am struggling a bit with this explanation though:

> ZenithTrack shows a strip of the sky, a thin ribbon, one rice-grain tall, about 2,500 rice-grains long.

What does it mean to say "one rice-grain tall"? Is that angular diameter at arm's length?

by meta-meta

5/15/2026 at 8:26:28 PM

yes. the Field of View is the size of a grain of rice at arm's length. the total "movie" you see is like 2500 of those rice-grains, end to end (earlier in the explainer, it mentions the FoV size)

by smorgasborg