Keeping Time

Summary of Keeping Time

by iHeartPodcasts

50m•January 20, 2026
Listen to Episode

Overview of Keeping Time

This Stuff You Should Know episode (Keeping Time) traces humanity’s evolving relationship with time—from tracking shadows with simple sticks to modern atomic clocks and GPS. The hosts survey major timekeeping technologies, key inventors, cultural uses (prayer times, work schedules, navigation), and how increasingly precise devices reshaped daily life and social organization.

Key topics covered

  • Earliest timekeeping: gnomons and sundials
  • Water clocks, incense/candle clocks, and hourglasses
  • Islamic, Chinese, Greek, and Roman innovations
  • Mechanical clocks: verge-and-foliot escapement, weights, and clock towers
  • Pendulum clocks (Galileo → Huygens → Hooke/Clement) and the addition of minute hands
  • Portable timepieces and the rise of wristwatches
  • Quartz crystals → atomic clocks → GPS timekeeping
  • Cultural impacts: prayer schedules, legal/work time, and the social shift to “clock time”

Timeline of major developments

Ancient to Classical

  • Gnomon (stick in the ground) → simple shadow tracking.
  • Earliest surviving sundial-like instruments from northern China (~2300 BCE).
  • Egyptian shadow clocks (early, T-shaped devices) track daylight hours.
  • Greek sundials (Anaximander, 6th century BCE) developed hourly markings; by ~350 BCE Greeks used seasonal/unequal “hours.”
  • Hemicycle sundials (~280 BCE): carved hemispherical dials that accounted for seasonal sun paths.

Medieval / Islamic / East Asian advances

  • Muslims refined sundials (polar-axis gnomon), developed trigonometric methods, and used timekeeping for prayer.
  • Incense clocks, candle clocks, and water clocks were widely used across Asia; incense clocks could be scented to mark different time segments.
  • Su Song (11th century China) built an elaborate water-driven astronomical clock tower with escapement-like mechanisms.
  • Ismail al-Jazari (12th–13th c., Upper Mesopotamia) created ornate mechanical clocks (e.g., the elephant clock) demonstrating gears, automata, and complex regulation.
  • 13th-century scholars (e.g., Abu al-Hasan al-Marrakushi) advocated for uniform hours, pushing toward standardized timekeeping.

Mechanical clocks and precision (Europe)

  • Early mechanical clock work in Europe attributed to inventors like Gerbert of Aurillac (later Pope Sylvester II, c. 996) — development and adoption took centuries.
  • Verge-and-foliot escapement mechanisms regulated gear-driven clocks (medieval—R. Anglicus references, 1271).
  • Tower clocks (e.g., Tower of the Winds; public clock towers) centralized time for cities and churches—bells marked hours.
  • Pendulum clocks: Galileo identified pendulum isochronism; Christian Huygens built the first practical pendulum clock (1656). Hooke and others improved escapements; William Clement’s longcase clock (c. 1680) popularized the minute hand and the long pendulum case (later “grandfather clock”).

Modern precision and watches

  • Spring-driven portable clocks → pocket watches (15th–16th c.), then wristwatches (popularized in late 19th–early 20th c.; military use in WWI).
  • Rolex Oyster (1926) introduced practical water resistance; chronograph functions and luminous dials developed for military and practical uses.
  • Quartz crystal oscillators (20th century) and atomic clocks enabled extreme precision; atomic time underpins GPS and modern digital devices.

Notable inventions and inventors (highlights)

  • Gnomon / sundials — basic shadow methods (ancient, global).
  • Egyptian shadow clock — early T-shaped device for daytime hours.
  • Anaximander (Greece) — early Greek sundials (6th c. BCE).
  • Hemicycle sundials (Hellenistic era) — seasonally calibrated dials.
  • Zhang Sixun (10th-century China) — used mercury in astronomical mechanisms to avoid freezing and improve reliability.
  • Su Song (11th-century China) — large water-driven astronomical clock tower with escapement-like features.
  • Ismail al-Jazari (12th–13th c.) — complex automata clocks (castle clock, elephant clock).
  • Abu al-Hasan al-Marrakushi (13th c., Morocco) — promoted uniform hours.
  • Verge-and-foliot escapement (medieval Europe) — early mechanical regulation.
  • Christian Huygens (1656) — first practical pendulum clock.
  • William Clement (c. 1680) — longcase “grandfather” clocks and minute hand adoption.
  • Modern era: quartz crystal oscillators → atomic clocks → GPS timing systems.

Cultural uses and societal impact

  • Timekeeping driven by practical needs: agriculture/seasonality, prayer times (Muslim, Christian monastic hours), water allocation schedules, navigation, and military coordination.
  • Church clocks and bells centralized urban time—shifting societies from natural rhythms to standardized “clock time.” Lewis Mumford argued this reorganization was foundational to the modern era.
  • Planetary (astrological) hours in ancient Rome influenced daily rituals and gave rise to the term “horoscope” (hour-marker).

Noteworthy anecdotes and colorful details

  • Incense clocks sometimes used different scents so people could “smell the time”; incense could also act as a timed fuse for alarms.
  • Hourglass likely a late-medieval innovation; earliest known reference ~1338 (Italy). Sand and humidity made them trickier than expected.
  • Elaborate clock towers and automata—examples include the Tower of the Winds and Ismail al-Jazari’s elephant clock—showcase public spectacle and engineering.
  • The phrase “grandfather clock” comes from the 1876 song “My Grandfather’s Clock” by Henry Clay Work.
  • The episode mixes humorous sidetracks (hosts’ jokes and personal watch anecdotes) and reassurances about live-show content being family-appropriate (mostly PG-ish).

Main takeaways

  • Humans moved from observational, environment-linked timekeeping (sun/shadow, seasons) to mechanical regulation and ultimately to atomic precision.
  • Different cultures contributed distinct innovations—Greek, Egyptian, Chinese, Islamic, and European advancements all played key roles.
  • The evolution of timekeeping changed social structures: standardized hours enabled industrial and urban coordination and altered daily life.
  • Many timekeeping technologies evolved both for functional necessity (navigation, prayer, water allocation) and for spectacle/status (ornate clocks, pocket watches).

Further listening / resources recommended by the hosts

  • Stuff You Should Know: episode on atomic clocks (for modern precision and GPS context).
  • Stuff You Should Know: episode on the Swatch or watches (for the digital/mechanical watch transition).
  • Visuals to look up: hemicycle sundial, Egyptian shadow clock, Tower of the Winds, Su Song’s clock tower, Ismail al-Jazari’s elephant clock, verge-and-foliot escapement, anchor escapement/pendulum mechanism.

Note: This summary follows the episode’s narrative and clarifies some historically common names/terms (e.g., Zhang Sixun, verge-and-foliot escapement, Huygens). The episode blends history, technology, and entertaining anecdotes to explain how the way we “keep time” shaped — and was shaped by — human needs and institutions.