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# Measuring time

Stonehenge, built in southern England more than 4000 years ago, may have been built as a gigantic calendar. The Sun rises above a...Read More >>Stonehenge, built in southern England more than 4000 years ago, may have been built as a gigantic calendar. The Sun rises above a stone outside the main stone circle, The Heel Stone, at the Summer Solstice, with the first rays shining directly into the horseshoe-shaped group of stones. From this, some record of the days passing throughout the year could be made—vital for an ancient people with no other means of knowing exactly when to sow or harvest their crops. Understanding the passing of time has always been a key human concern—knowing when to expect animal migrations, when to plant seeds, and when to seek shelter at sunset have been essential to our survival for millennia. The oldest calendars and other timekeeping devices date back thousands of years. The ability to accurately measure time has become increasingly important as human society has developed. Today, timekeeping is so vital that it is coordinated at an international level and we have learnt to count time periods down to the nanosecond (one billionth of a second).

## The time of day

The Earth takes 24 hours, or one day, to complete one spin. While one half of the Earth is lit up by the Sun, where it is...Read More >>The Earth takes 24 hours, or one day, to complete one spin. While one half of the Earth is lit up by the Sun, where it is daytime, the other half remains in night-time darkness. Clocks in any part of the world are set to a time according to the Sun’s position. This is different from one part of the world to the next. So, at any one moment, clocks around the world tell different times. When it is mid-afternoon in Tokyo, Japan, it is evening on the previous day in Hawaii.The Earth takes 24 hours, or one day, to complete one spin. While one half of the Earth is lit up by the Sun, where it is daytime, the other half remains in night-time darkness. Clocks in any part of the world are set to a time according to the Sun’s position. This is different from one part of the world to the next. So, at any one moment, clocks around the world tell different times. When it is mid-afternoon in Tokyo, Japan, it is evening on the previous day in Hawaii.

The oldest calendar yet discovered dates to 8000 BC and was found in Aberdeenshire, Scotland. It consists of 12 pits that were devised to record the lunar months.