It may seem silly, but everything to do with time

                               ↑which has 생략

is based on the sun coming up and going down.

～에 기초를 두다      전치사 on의 목적어인 동명사

Every time the sun comes up and goes down, we

⇒ Whenever

call it a day. The amount of time between summers

 V   O  O.C         call 의 목적어 : 도치구문

we call a year.

S  V   O.C         

 A long time ago, people divided the day into two

                      = divide A into B

halves called "ante meridian" and "post meridian".

      ↑which were 생략

The meridian is the time when the sun is at its

                       ★관계부사 = at which

highest. Dividing the day in this way helped people

              주어(명사구)            V     O

to plan things to the nearest half day. This is fine

 O.C

when your life is nice and simple. Suppose you

★종속접속사(시간부사절유도)

are a sheep, you could plan your schedule like

this.... 

ex> How about taking a walk in the afternoon?