Why Are There 60 Minutes in an Hour?
· 7 min read
An hour has 60 minutes because ancient Mesopotamian mathematicians counted in sixties, and later astronomers used their arithmetic to chop the hour into smaller parts. The system survived for a practical reason: 60 splits into whole numbers more ways than any smaller number does, so halves, thirds, quarters, fifths and sixths of an hour all come out as tidy whole minutes.
That is the short version to give a child who asks why 60 minutes in an hour. The longer version is a good story, and it explains several other things at once — why a circle has 360 degrees, why a minute has 60 seconds, and why the words “minute” and “second” sound like they have nothing to do with time.
The 60 came from Sumer and Babylon
The Sumerians, and after them the Babylonians, wrote numbers in base 60 rather than base 10. Where we have a ones column, a tens column and a hundreds column, they had a ones column, a sixties column and a three-thousand-six-hundreds column.
Nobody knows for certain why they chose 60. The most common suggestion is that it made trade and division easy across communities using different smaller units, and a well-known counting method fits it neatly: use the thumb to count the twelve finger joints on one hand, and the five fingers of the other hand to track each full set of twelve, giving 60 on two hands.
Babylonian astronomers used base 60 to record the positions of stars and planets, and the Greeks took the arithmetic over wholesale for their own astronomy. Ptolemy’s astronomical tables, written in Alexandria in the second century, used sexagesimal fractions throughout, and those tables were copied and translated for well over a thousand years. The number system rode along with them.
The 12 came from Egypt
The 60 is only half of the story, because the hour itself is not Babylonian. Egyptian astronomers divided the night using a set of stars that rose in sequence, with about twelve of them visible over a night, and split the daylight into twelve parts using shadow clocks.
Two accounts are usually given for why twelve. One points to the roughly twelve lunar cycles in a year; the other to the twelve joints countable on one hand. Either way, twelve day parts plus twelve night parts is where the 24-hour day comes from.
One detail surprises most students: for most of history those hours were not all the same length. A daylight hour was one twelfth of however much daylight there happened to be, so summer hours were long and winter hours short. Equal hours only became normal once mechanical clocks made them easy to keep, which is also roughly when the day settled into the fixed 24 that the 24-hour clock and its conversion rules still use.
Why 60 beats 100
Ask a child which is the friendlier number and they will pick 100. Then hand them the actual divisions and watch that change.
| Fraction of an hour | Out of 60 minutes | Out of 100 minutes |
|---|---|---|
| One half | 30 | 50 |
| One third | 20 | 33.33… |
| One quarter | 15 | 25 |
| One fifth | 12 | 20 |
| One sixth | 10 | 16.66… |
| One twelfth | 5 | 8.33… |
Sixty has exactly twelve whole-number divisors: 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30 and 60. One hundred, despite being a bigger number, has only nine: 1, 2, 4, 5, 10, 20, 25, 50 and 100. Sixty is what mathematicians call a highly composite number, meaning no smaller number has as many divisors.
The practical consequence shows up every day. A third of an hour is 20 minutes, a sixth is 10 minutes, and a twelfth is 5 — which is exactly why the numerals on a clock face land on whole five-minute counts and why reading a clock to the nearest five minutes works as a skill at all. In a 100-minute hour, a twelfth would be 8 minutes and 20 seconds, and no clock face would be readable by a seven-year-old.
Where the words “minute” and “second” come from
Medieval scholars working from Ptolemy in Latin called the first sexagesimal division of an hour the pars minuta prima, the “first small part.” The next division down was the pars minuta secunda, the “second small part.”
English kept the first word from one phrase and the second word from the other. So a minute is literally a tiny piece of an hour, and a second is a tiny piece of that — the leftover name of a second-level division. Students who have just met the word “minute” meaning small usually enjoy this one.
The same system gave us the circle. A full turn is 360 degrees, each degree splits into 60 minutes of arc, and each of those into 60 seconds of arc. Latitude and longitude are still written that way on nautical charts.
How the 60 shows up on the clock in front of you
Put a real clock where children can see it and walk through the structure rather than describing it.
- Count the small marks around the rim. There are 60, one for each minute.
- Count the marks between the 12 and the 1. There are four, dividing that space into five one-minute steps.
- The minute hand travels the whole circle in an hour; the hour hand travels one twelfth of it in the same time.
- The 3 is a quarter of the way around, so quarter past is 15 minutes. The 6 is halfway, so half past is 30.
Then hand out paper circles. Folding one in half gives 30 minutes and folding again gives 15, which children expect. Ask them to fold a circle into three equal parts and to work out that each part is 20 minutes, and the arithmetic of 60 stops being a fact to remember. Beginners can do the same circle work through whole-hour and half-hour activities for the youngest learners, while older students turn it into fluency with regular practice on the analog face.
The one time a country tried to change it
Revolutionary France decreed decimal time in 1793: ten hours in a day, 100 minutes in each hour, 100 seconds in each minute. It was official from late 1794 and suspended in April 1795, after only a few months.
It collapsed for ordinary reasons rather than mathematical ones. Every clock in the country would have needed replacing, every schedule rewriting, and people had to keep converting between old and new time to deal with anyone outside France. Decimal time is worth mentioning to students because it shows that the 60-minute hour is a convention that could have gone another way, not a law of nature.
Making the idea concrete for younger students
Children under seven do not need the history, but they do need to feel that an hour is made of equal pieces. Time a 15-minute activity, mark it on a paper circle, and do that four times across a morning until the four quarters close the circle.
Tie the fractions to events with fixed lengths: recess is 15 minutes, so one quarter of an hour; a read-aloud is 20 minutes, so one third. Anchoring durations to routine is the same principle behind clock work built around a picture schedule, which helps any student who finds abstract minutes slippery. English learners need the fraction words as well as the numbers, and a beginner list of English words for times and durations covers “quarter,” “half” and “past” before they are needed in a lesson.
For a group that has enjoyed the history, a good follow-on project is building a base-60 counting hand chart or a paper clock with a working minute scale; there are more hands-on clock projects to build if the making is doing the teaching.
For the student who asks about leap seconds
Some student will eventually ask whether every minute really has exactly 60 seconds. Occasionally it does not. Coordinated Universal Time is kept by atomic clocks, and because the Earth’s rotation drifts slightly against them, a leap second has sometimes been inserted to keep the two in step, making one minute 61 seconds long.
Leap seconds have become rare — none has been added since the end of 2016 — and the international body responsible has agreed to stop using them by 2035. The 60-minute hour itself is not in question. It has outlasted the empire that invented it, a revolution that tried to replace it, and the arrival of clocks accurate to a billionth of a second.
If a student wants to go straight from the history back to the practical skill, work through every part of the dial and how to read it, then run a live bingo round in the browser or print a set of cards so the 60-minute structure gets used rather than just admired.