The day, the month and the year

From the Earth, the Sun seems to rise in the east, set in the west and rise again. This happens mainly because the Earth spins (rotates) on its axis. That natural cycle gives us the day.

The average time the Sun takes to go from its highest point in the sky one day to its highest point the next is 24 hours: the mean solar day. How do you find the highest point? A shadow is shortest when the Sun is highest.

Measure a day (Activity 11.3). In a sunny, flat patch of ground, fix a 1 m stick upright. From 11:00 a.m., mark a dot at the tip of its shadow every minute, until about 1:10 p.m. The dots move in and then out again. Find the shortest shadow, work out its time by counting dots, record it in Table 11.2, and repeat on the next few days.

The length of the solar day is the difference between those times on two days in a row. The book's readings were 12:20, 12:20 and 12:19 on 22, 23 and 24 March 2025, giving 24:00 and 23:59. Their average is very nearly 24 hours.

The Moon gives a longer cycle: it takes about 29.5 days, nearly a month, to go through all its phases. That is the basis of the month.

Longer still is the cycle of seasons. The Earth goes round the Sun in nearly 365¼ days, passing through one cycle of seasons: a solar year.