year 8 maths

164 Chapter 3 Measurement and Pythagoras’ theorem
3J Time
REVISION
Time in minutes and seconds is based on the number 60 . Other units of time, including the day and year,
are defined by the rate at which the Earth spins on its axis and the time that the Earth takes to orbit the
Sun.
The origin of the units seconds and minutes dates back to the ancient Babylonians, who used a base
60 number system. The 24 -hour day dates back to the ancient Egyptians, who described the day as
12 hours of day and 12 hours of night. Today, we use a.m. ( ante meridiem , which is Latin for ‘before noon’)
and p.m. ( post meridiem , which is Latin for ‘after noon’) to represent the hours before and after noon
(midday). During the rule of Julius Caesar, the ancient Romans introduced the solar calendar, which
recognised that the Earth takes about 365 1 days to orbit the Sun. This gave rise to the leap year, which
4
includes one extra day (in February) every 4 years.
L et’s start: Knowledge of time
Do you know the answers to these questions about time and the calendar?
• When is the next leap year?
• Why do we have a leap year?
• Which months have 31 days?
• Why are there different times in different countries or parts of a country?
• What do bce (or bc) and ce (or ad) mean on time scales?
Key ideas
■ The standard unit of time is the second (s).
■ Units of time include:
• 1 minute (min) = 60 seconds (s)
• 1 hour (h) = 60 minutes (min)
• 1 day = 24 hours (h)
• 1 week = 7 days
• 1 year = 12 months
×24 ×60 ×60
day hour minute second
÷24 ÷60 ÷60
■ a.m. or p.m. is used to describe the 12 hours before and after noon (midday).
■ 24-hour time shows the number of hours and minutes after midnight.
• 0330 is 3:30 a.m.
• 1530 is 3:30 p.m.
■ The ‘degrees, minutes and seconds’ button on a calculator can be used to convert a particular
time into hours, minutes and seconds.
For example: 4.42 hours = 4°25′12″ meaning 4 hours, 25 minutes and 12 seconds.
Cambridge Maths NSW
Stage 4 Year 8 Second edition
ISBN 978-1-108-46627-1 © Palmer et al. 2018
Cambridge University Press
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