SCALE ASTRA covers the same range as Scale Opus 5: about 62 orders of magnitude, from the observable universe down to the Planck length. The reference point is the same too, Gwanghwamun Square in central Seoul. The difference is the structure. Scale is a single HTML file that switches scenes at each step. SCALE ASTRA is built as one 3D coordinate system in which every object has its own position inside its parent space. Galaxies sit inside the universe, the solar system inside the galaxy, Earth inside the solar system. Zooming means moving further and further into that space.
72 destinations, a 53-step main route
The main route has 53 steps: the observable universe, the cosmic web, Laniakea, the Milky Way, the solar system, Earth, East Asia, the Korean Peninsula, Seoul, Jongno-gu, Sejong-daero, Gwanghwamun Square, a person, a hand, skin, a cell, DNA, a carbon atom, a proton, and on down to the Planck length. Add the side destinations and there are 72 in all. Eight section buttons at the bottom of the screen, from 01 Universe to 08 Planck, let you jump straight there.
Below the scale of quarks, there are no confirmed objects. So those sections only carry scale names, like "Unknown territory · 10⁻²¹ m" and "Toward quantum gravity," and nothing that doesn't exist is drawn in.
Planet positions are from JPL Horizons
The positions and orbital elements of the eight planets, Pluto and the Moon come from the NASA/JPL Horizons API. The reference time is fixed at 2026-09-21 00:00 TDB. The orbit viewer lays the ecliptic down as a faint plane, and you can view the orbits from the side, from above, or zoomed in on the inner planets. The planets themselves are drawn at true size, so from a distance where the whole solar system is visible, they're nearly invisible. Instead, a small circle marks each position, with a note on screen that the circle's diameter is not the planet's size.
From Earth on, real terrain and roads
For the Korean Peninsula and the Seoul metropolitan area, elevation comes from 54 terrain tiles (NASA, USGS/Mapzen). Surface imagery is EOX's Sentinel-2 cloudless. In the Seoul section, I drew district boundaries and 14,196 roads from OpenStreetMap, and Gwanghwamun Square is built from raw OSM data. The square's paving and wall materials are CC0 textures from Poly Haven. The Earth surface imagery is the Earth texture from the Three.js examples, used as is; sources are listed in assets/ATTRIBUTION.txt.
What the numbers on screen mean
The number at the bottom is the width of the focal plane perpendicular to the camera. The scale bar is calculated from this width too. If the screen width is W (meters), the internal scale is log₁₀(W), and turning the wheel by the same amount zooms by the same factor anywhere. Going from 1,000 km to 100 km is one step, the same as going from 100 km to 10 km. The number next to an object is that object's typical size, so it's a different value from the screen range. Inside the skin and inside the cell, the front part is cut away or drawn transparent, and it's stated that the colors and layout are a model for explanation.
Study materials page
The same content as the explorer's info panels is collected separately on the study materials page. It has print styles so you can print it on A4 or save it as a PDF, and when printed, the source link URLs are printed in the text.
What I fixed when attaching it to ghmun.com
It was originally a separate project, so the paths broke when I moved it under ghmun.com/scale_astra. When the page opens at /scale_astra without a trailing slash, relative paths resolve from the site root. So the Earth imagery and all 54 terrain tiles returned 404. I changed every file the app loads to an absolute path starting with /scale_astra/, and fixed the paths inside the terrain list and material list JSON as well. The site build script originally copied only the top-level files of an app folder; I changed it to copy the assets and vendor folders whole. The 3D engine, three.js 0.180, is bundled inside the site rather than loaded from a CDN.
How to use it
Drag to rotate the view, and use the wheel or a pinch to zoom in and out. Objects smaller than the screen appear as clickable position markers; tap a marker to move there. Press the ▷ continuous journey button to travel automatically along the route to the next destination. If you've wandered down a side path, use "Back to main route" to return. Downloading the terrain tiles and 3D modules takes a little while the first time.
This version, and the others
This version was made with GPT Astra. The same scale ladder also exists as Scale Opus 5, made with Claude Opus 5; Scale Gemini 3.6 and Scale Gemini 3.8, made with Gemini; and Scale Odyssey Opus 5.5, made with Claude Opus 5.5.