# How to measure time

Since ancient times the people are familiar with such regularities that day replaces night, spring – summer, the full moon turns into a narrow sickle. Watching these changes, people learned to predict them, using the simplest devices and calculations. Sand and the sundial measured time of day, watching the moon – defined month, monitoring change of seasons – year.

• - calendar,
• - hours

## Instruction

1. Now, as before, the main unit of counting of time are day which is equal to one cycle of rotation of Earth round its pivot-center. Once they were conditionally divided for 24 hours, hour – on 60 minutes, minute – for 60 seconds. Now to define what is the time, it is enough to look at the watch. Mechanical or electronic, being serviceable and in time verified, they will show you the exact time.

2. However day – not such big unit of time. Watching changes of phases of the moon, people established that the extent of month – 28 days. And, in spite of the fact that this calculation was not absolutely exact, we still use the unit of time produced from this size – week, consisting of 7 days.

3. As division of year for months in our calendar is very conditional, different months have various number of days – from 28 to 31. Year – size also not too exact. We live according to the calendar in which three years from four are equal 365 days, and the fourth, because of the arisen error, – 366.

4. Not so long ago in our country such units of counting of time as a five-years period and decade were accepted. Now use them much less often, however these sizes can be met on pages of textbooks of history where it is told about XX century events.

5. Also in textbooks of history it is possible to meet such sizes as a century, century and the millennium. It is accepted to divide a human civilization which contains only several millennia into these periods of time. Speaking about a human century, mean 70-80 years. And arguing on age of our planet and stars, it is necessary to use such sizes as millions and billions of years.

Author: «MirrorInfo» Dream Team