Time
6 Minutes
Time for kids: learn how day and night work, early clocks, 24-hour railway time, and AM and PM. A short maths lesson in English.
- How Earth's rotation causes day and night to form a day
- Early sundials, water clocks, sand clocks, and candle clocks
- Calculating 24-hour railway time and understanding AM and PM
What this lesson teaches
A day is the period of time it takes for the Earth to complete one full rotation, during which the side facing the Sun has day and the side facing away has night. To track this movement, a full day is divided into 24 hours, with each hour containing 60 minutes and each minute containing 60 seconds.
How day, night, and years work
When light shines on the Earth like a torch on a ball, the bright side is day and the dark back side is night. As the Earth rotates on its axis, night turns into day and day turns into night to complete one full day. It takes 365 days for the Earth to rotate and orbit the Sun in a fixed path, which equals one full year.
Early clocks and their limits
- Sundials relied on sunlight from sunrise to sunset, but they could not show time after the sun went down.
- Candle clocks helped people measure time, but they failed when strong winds blew them out.
- Water clocks and sand clocks were also used before modern 24-hour clocks were designed.
- Modern clocks divide time into precise minutes and seconds, which are useful for recording sports race records and arrival times for trains and flights.
Understanding AM and PM
Standard clocks only have numbers up to 12, so the 24-hour day is divided into ante meridiem, or AM, and post meridiem, or PM. The hours from 12 at night until 12 in the afternoon are called AM, while the hours from 12 in the afternoon until 12 at night are called PM. For example, 6 in the morning is written as 6 AM, and 6 in the evening is written as 6 PM. This system allows a person to tell the exact time of day using only one watch.
How railway time works
- Railway time uses a continuous 24-hour system to avoid confusion when setting alarms or scheduling trains and flights.
- To convert afternoon and evening times into railway time, add 12 to the regular clock hour.
- For example, 4 o'clock in the evening is 12 plus 4, which is called 16 hours, while 8 o'clock in the evening is 12 plus 8, which is 20 hours.
- Time gaps across noon can be found in steps, such as measuring 6 hours from 6 AM to 12 noon, plus 4 hours from 12 noon to 4 PM, giving 10 total hours.