What does the Latin phrase 'post meridiem' mean?
AnswerAfter midday
Post means after and meridies means midday, so p.m. covers the hours after noon.
Science · Facts & stats
PM facts & stats
7 fact-checked facts about p.m., clocks and how we tell time, each with the reason behind it. 5 more are in today's round and join this page after it closes.
The basics
The Prime Meridian at 0° longitude runs through which place in London?
AnswerGreenwich
The Prime Meridian was set at the Royal Observatory, Greenwich, in 1884.
Going deeper
How many degrees does the Earth rotate in one hour?
Answer15 degrees
The Earth turns 360 degrees in 24 hours, which is 15 degrees per hour.
What is the part of a sundial that casts the shadow called?
AnswerThe gnomon
The gnomon is the raised edge or rod whose shadow marks the hour.
How many seconds are in one day?
Answer86,400
24 hours × 60 minutes × 60 seconds equals 86,400 seconds.
Expert level
Our 60 minutes and 60 seconds trace back to the number system of which ancient people?
AnswerThe Babylonians
The Babylonians used a base-60 number system that shaped how we divide hours and minutes.
The official definition of the second is based on the vibrations of atoms of which element?
AnswerCaesium
Since 1967 the SI second has been defined by a microwave transition frequency of caesium-133 atoms.
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