**Time Measurement & Concepts – Short Form**

**Time Units:**
– Millennium: 1,000 years
– Century: 100 years
– Decade: 10 years
– Year: 365 days (common), 366 days (leap)
– Month: 28-31 days
– Week: 7 days
– Day: 24 hours
– Hour: 60 minutes
– Minute: 60 seconds
– Second: Base unit

**Key Time Concepts:**
– **Aristotle**: Time as measurement of change
– **Newton**: Absolute time flows independently
– **Leibniz**: Time is relative to objects/events
– **Einstein**: Spacetime connection, time relativity

**How We Measure Time:**
– Based on sexagesimal system (base 60)
– 24-hour day from Egyptian sundials & star patterns
– 60 minutes/seconds from Greek astronomy
– Gregorian calendar (1582) improved Julian calendar accuracy

**Timekeeping Evolution:**
– Ancient: Sundials, water clocks, hourglasses
– 1656: First pendulum clock (Christiaan Huygens)
– Modern: Atomic clocks (most accurate, using cesium atoms)

**Interesting Facts:**
– Time moves slower at near-light speeds
– Base 60 simplifies time division (many factors)
– Mechanical clocks made fixed-length hours standard (14th century)