The

Mayans —

and

later,

the

Egyptians —

developed

a

solar

calendar

based

on

the

Earth’s

movement

around

the

Sun.

A

solar

calendar

is

based

on

the

solar

year,

which

is

the

time

it

takes

for

the

Earth

to

travel

all

the

way

around

the

Sun.

A

solar

year

is

365.25

days,

or

365

days

and

six

hours

long.

In

45

BC,

the

Romans

started

using

a

solar

calendar.

The

solar

year

is

a

few

hours

longer

than

365

days,

so

the

Romans

added

an

extra

day

every

four

years

to

create

a

‘leap

year’.

They

called

this

the

Julian

calendar,

after

Julius

Caesar,

a

Roman

ruler.

In

1582,

Pope

Gregory

XIII

made

this

calendar

more

accurate.

Most

of

the

world

still

uses

the

Gregorian

calendar

today.