Childrens Illustrated Encyclopedia

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522

Hour follows Hour as time passes. Time always

flows steadily in the same direction. Behind us in time lies

the past, which we know. Ahead lies the future, which we

cannot know. we cannot change time, but we can measure

it. People first measured time in days and nights, which they

could easily see and count. They also measured time in

months, by watching the phases of the moon, and in years,

by watching the cycle of the seasons. Today we have clocks

and watches that can measure time in fractions of a second.

In 1905, German physicist

Albert Einstein proposed

the scientific theory of

relativity. This says

that time is not

constant but

that it would pass more

slowly if you could travel

very fast (near the speed

of light) or in strong

fields of gravity. scientists

believe that time may

even come to a stop

in black holes

deep in space.

YEArs And monTHs

A year is based on the time

Earth takes to go once around

the sun, which is 365.26 days.

months vary from 28 to 31 days.

They were originally based on

the time between full moons,

which is about 29.5 days.

Calendars

The date is fixed by the calendar, which contains

12 months with a total of 365 days. Every fourth year is

a leap year, which has one extra day, february 29. leap

years are years that divide by four, such as 2012 and 2016.

The calendar contains leap years because Earth takes

slightly longer than 365 days to go once around the sun.

Prehistoric peoples may have used monuments such as

stonehenge, in southern England (below), to measure

the sun’s position and find the exact length of the year.

The Hindu calendar is based on

lunar months. Diwali, the Festival

of Lights, marks the start of the

new year, which falls in October

or November.

InTErnATIonAl dATE lInE

The western side of the International

date line is one day ahead of the

eastern side. when you cross

the line, the date changes.

TImE zonEs

The world is divided into 24

regions, called time zones, each

with a different time of day. This

was done to avoid having several

time differences within one area

and to ensure that all countries

have noon during the middle

of the day.

2 p.m. in Cairo, Egypt

The International

Date Line is

at 180 degrees

longitude.

9 a.m. in Rio de Janeiro,

Brazil

unITs of TImE

one full day and night is the time

in which Earth spins once. This is divided

into 24 hours: each hour contains

60 minutes, and each minute contains

60 seconds. The Babylonians fixed these

units about 5,000 years ago, using 24 and

60, because they divide easily by 2, 3, and 4.

Sun

Earth

Twice a year

the sun is

over the

equator at

12 noon.

dAYs And nIGHTs

The sun lights up one half of Earth, where it is day.

The other half, away from the sun, is dark, and

there it is night. days and nights come and go

because Earth spins once every 24 hours. But the

day and night may last different lengths of time

because Earth is tilted at an angle to the sun.

The prime meridian is at 0 degrees longitude.

unIvErsAl TImE

The time at the prime meridian is used

as a standard time known as universal Time

(uT) or Greenwich mean Time (GmT).

In reality, Earth is 400 times

farther away from the sun than

it is from the moon.

Moon

Time

3 p.m. in Moscow, Russian Federation

12 noon in London, England

7 a.m. in New York City

Find out more

Clocks and watches

Earth

Einstein, albert

Physics

science

stars

universe

HourGlAss

sand draining

through an hourglass

shows the passing

of time. It takes one

hour for the sand

to run from the top

to the bottom bulb.

Earth spins counterclockwise when

looking down at the North Pole. It goes

clockwise viewed from above the South Pole.

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