← All notes

Short answer

Every clock has two parts: something that ticks at a steady rate and something that counts the ticks. An atomic clock uses the natural resonant frequency of atoms as the tick. Because all atoms of an element are identical, their ticks are too, unlike pendulums or quartz crystals. [S1]

Key points

• Different atoms have different resonant frequencies, so the number of cycles per second depends on the atom used. [S1]
• The official second is defined by the cesium-133 frequency: exactly 9,192,631,770 cycles. [S2]
• Atomic time reaches everyday life through computers and phones that sync to it. [S1]

Claims

• id: C1 claim: A clock combines an oscillating mechanism and a counter. source: S1 evidence: "A clock is, at its core, a marriage of two components"
• id: C2 claim: No two pendulums or crystals tick exactly alike. source: S1 evidence: "But no two pendulums or crystals are exactly alike"
• id: C3 claim: Each type of atom has a different intrinsic resonant frequency. source: S1 evidence: "each has a different intrinsic resonant frequency"
• id: C4 claim: The second is defined using the cesium frequency. source: S2 evidence: "The second is defined by taking the fixed numerical value of the cesium frequency"

Sources

• [S1] How Do Atomic Clocks Work?. NIST. https://www.nist.gov/atomic-clocks/how-do-atomic-clocks-work . License: US government work, public domain. Accessed 2026-11-04.
• [S2] Definitions of SI Base Units. NIST. https://www.nist.gov/si-redefinition/definitions-si-base-units . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_yyjq6zcd,
  author = {{iinventedlight}},
  title = {{How atomic clocks work}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/atomic-clocks/},
  note = {Permanent ID iil:note:yyjq6zcd. License: CC-BY-4.0.},
}

Markdown

Spotted a mistake? Use the MCP feedback tool or email hello@iinventedlight.com.

For language models and agents: start at /llms.txt. This page in Markdown: /library/notes/atomic-clocks.md.