11.3 Day, Month & Year

Without a single clock, ancient people still measured time — by watching three reliable sky cycles. Each gave us a unit we still use.

The day — from the Sun

The Mean Solar Day The Sun appears to rise in the East, cross the sky, and set in the West each day — really because the Earth rotates on its axis. The average time from the Sun’s highest point one day to its highest point the next is 24 hours, the mean solar day. (The Sun is highest when shadows are shortest.)

Activity 11.3 — Measure a day with a shadow Fix a 1 m stick upright in sunlight. From about 11 a.m., mark the tip of its shadow every minute. The shadow is shortest when the Sun is highest (around local noon). Note that time each day — the gap between the shortest-shadow times on two consecutive days is one solar day, very close to 24 hours.

The month — from the Moon

A Cycle of Phases The Moon takes about 29.5 days (nearly a month) to cycle through all its phases, from one new Moon to the next. This natural cycle is the basis of the month — a longer unit of time than the day.

The year — from the seasons

One Trip Around the Sun The Earth revolves around the Sun in about 365¼ days, going through one full cycle of seasons in that time. This defines the solar year — the largest of the three common units.

Stack these natural rhythms together and you have the skeleton of every calendar ever made — which is exactly what we build next.