Convert Between Hours Minutes & Seconds and Decimal Time

Why are there 60 minutes in an hour and 24 hours in a day?

The fact that we have 24 hours in a day dates back to the Ancient Egyptians.  The Egyyptians actually had 10 hours during the day, and developed a sundial around 1500 B.C which divided the daylight hours into these 10 equal parts.  They also used an additional hour in the morning for twighlight and a further hour in the evening for dusk.  Due to the fact that there is a longer period of sunlight during summmer than there is during winter, the hours during the summer were longer than those during winter.

For the nighttime hours, they used an early astronomical tool called a merkhet to mark the passage of "clock stars" or "decans" across the sky at night.  These were specific stars that were spread across the sky.  During the summer night, 12 clock stars passed the merkhet.  Evidence for this division of nighttime can actually be found on Egyptian coffin lids from the 2nd millennium B.C.

With the spread of mechanical clocks during the 14th century, hours of equal length were adopted.  As result, the hours read from today's clocks are called o'clock (of the clock), rather than of the sun.

The division of hours into 60 minutes comes from another ancient race - The Babylonians, who used the base-60 system for their astronomical calculations.  Although it is not certain why they used 60, it is thought that one reason may be because base-60 makes divisional operations relatively easy due to the fact that 60 is divisible by 2, 3, 4, 5, 6 and 10 etc.  The base-60 system is known as sexagesimal. The first fractional sexagesimal place we now call a minute, which comes from a Latin phrase meaning small part. the second place we know as a second.  The sexagesimal system was also adopted by Greek astronomers who used it to divide the hour and the circle, thus these fractional place names were applied to hours, as well as to degrees for measuring angles.

In 1964, a second was redefined by the International Committee on Weights and Measures as "9,129,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom".  This was the first time that the definition of a second was no longer dependent on solar time.  This definition is continues to be used by The International System of Units today.

The atomic clock is so accurate that if it was left unadjusted, it would eventually drift away from the less precise solar time, in a similar way to how the seasons would drift away from the solar year if no leap years existed.  As a corrective measure, a leap second is added when required, to ensure that solar time and atomic time are kept within 0.9 seconds of each other.

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