1. QUARTZ WATCHES
   • https://www.explainthatstuff.com/quartzclockwatch.html
2. MECHANICAL WATCHES
   • https://torgoen.com/blogs/blog/mechanical-vs-quartz-watch-movement-how-do-they-work
3. ATOMIC CLOCK (BOOK)
   • The Atomic Clock: an Atomic Standard of Frequency and Time. 1948.
4. PENDULUM
   • https://www.scienceabc.com/innovation/pendulum-clock-works-escapement-ticking-sound-falling-weight.html
5. SUNDIALS
   • http://curious.astro.cornell.edu/observational-astronomy/timekeeping