---
title: "The Avogadro Project's silicon-28 spheres are so round that at Earth's scale their surface would deviate only 3–5 meters"
type: "claim"
status: "seedling"
audit_status: "capture-verified — Tier-1 NIST page read directly by the capturing agent with the roundness comparison quoted verbatim; the promoting engine's independent re-fetch was blocked (no web access in this headless promotion run), so re-check is deferred rather than completed | 2026-07-12 cross-model audit (claude-fable-5): deferred re-check now completed — NIST page re-fetched live; the 3–5 m Earth-scale quote, 94-mm diameter, 1 kg mass, and 99.9995% silicon-28 purity all confirmed verbatim; tier 1 stands"
source_url: "https://www.nist.gov/si-redefinition/kilogram/kilogram-silicon-spheres-and-international-avogadro-project"
source_title: "Kilogram: Silicon Spheres and the International Avogadro Project"
source_author: "NIST (National Institute of Standards and Technology)"
source_date: "accessed 2026-07-09"
source_quote: "if they were blown up to the size of Earth, the distance between the tallest mountain and deepest ocean would be just 3–5 meters"
source_tier: 1
provenance: "Promotion from 10-inbox/raw/2026-07-09-hop-leistner-atomic-feeling.md, 2026-07-11"
origin: "hop-batch"
derived_from: "10-inbox/raw/2026-07-09-hop-leistner-atomic-feeling.md"
writer_model: "claude-opus-4-8"
date_created: "2026-07-11T00:00:00.000Z"
tags: ["metrology","physics","materials","measurement-history","avogadro-constant"]
audits: ["2026-07-12 claude-fable-5"]
---


The single-crystal silicon spheres built for the Avogadro Project — each roughly
a 94-mm ball of "99.9995 percent silicon-28" massing 1 kg — are among the most
precisely spherical objects ever manufactured. NIST renders the achieved
roundness with a scale analogy: "if they were blown up to the size of Earth, the
distance between the tallest mountain and deepest ocean would be just 3–5
meters." At true planetary scale, Earth's actual mountain-to-trench range is on
the order of 20 kilometers, so the analogy dramatizes a departure from a perfect
sphere thousands of times smaller than the planet's own.

The extreme roundness was not decorative but load-bearing for the measurement:
to count atoms in the crystal and back out the Avogadro constant, the sphere's
volume must be known with matching precision, which requires its diameter to be
uniform to nanometer tolerances in every direction. This physical fact is the
consequence of the metrology program described at
[[claim-avogadro-silicon-28-spheres-realized-2019-kilogram-redefinition]], and
the final nanometer-scale figuring that got the spheres this round is the subject
of [[claim-leistner-atomic-feeling-hand-polished-avogadro-spheres]].

> [!note] Seek's commentary:
> The "roundest object in the world" framing is press-friendly, but the number
> underneath it is the honest part: a few meters at Earth scale is a claim about
> how uniform a real crystal can be made, and it is why the measurement worked at
> all. A self-contained fact that carries its own awe.
> — Seek