This definition is continues to be used by The International System of Units today. This was the first time that the definition of a second was no longer dependent on solar time. 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". 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. The first fractional sexagesimal place we now call a minute, which comes from a Latin phrase meaning small part. The base-60 system is known as sexagesimal. 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 division of hours into 60 minutes comes from another ancient race - The Babylonians, who used the base-60 system for their astronomical calculations. As result, the hours read from today's clocks are called o'clock (of the clock), rather than of the sun. With the spread of mechanical clocks during the 14th century, hours of equal length were adopted. Evidence for this division of nighttime can actually be found on Egyptian coffin lids from the 2nd millennium B.C. During the summer night, 12 clock stars passed the merkhet. These were specific stars that were spread across the sky. 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.įor 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. They also used an additional hour in the morning for twighlight and a further hour in the evening for dusk. 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. The fact that we have 24 hours in a day dates back to the Ancient Egyptians. Why are there 60 minutes in an hour and 24 hours in a day? Or to convert decimal time to Hours, Minutes and Seconds,input a value in the bottom box, then click the "Convert Decimal Time to Hours, Minutes and Seconds!" button. To decimal time, enter values in the top boxes, then click the "Convert Hours, Minutes and Seconds to Decimal Time!" button. Here's a good link on Khan Academy that explains more about modulo.To convert from Hours Minutes and Seconds (In this way, the MOD function works a bit like a clock. The MOD function takes care of the negative problem by "flipping" negative values to the required positive value. For example, to calculate hours between 9 PM and 3 AM: =MOD(0.125-0.875,1)*24 One elegant way to handle this challenge is to add the MOD function to the formula. When times cross midnight, the problem becomes more tricky, since the end time will often be less than the start time. For example, with start time of 9:00 AM and an end time of 3:00 PM, you can simply use this formula: =(3:00 PM-9:00 AM)*24 To calculate hours between times, you can simply subtract the start time from the end time when both times are in the same day. To convert these fractional values to decimal hours, just multiply by 24. In other words, Excel times are just fractional numbers: Time Excel dates are serial numbers, and a day is equivalent to the number 1.
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