> Each day, there are 86,400 of which unit of time?And the answer: seconds.Photo credit: Getty images/Stock photoReady for some math? There are 60 secondsin each minute, and 60 minutes in each hour. So if you multiply 60 seconds by 60minutes, there are 3,600 seconds in an hour. Multiply that by 24 hours, and youhave 86,400 seconds in a day.Timekeeping has been practiced, revised and altered for as long as humans havebeen around. While imperfect (the Earth's orbit around the sun can actually varyby milliseconds), our systems of keeping time allow us to lead functional,synchronized lives. The first attempts to build a clock can be traced back to 3500 BCE, whenEgyptians used an early form of sundial. As the sun moved through the sky, theshadow of a vertical structure moved to indicate the time of day. However, thesestructures weren't too reliable, since the way the shadow moved changed with theseasons. Later sundials got around this problem by pointing the verticalstructure directly north or south. Until around the 17th century, sundials werethe most popular way to tell time. For centuries, the length of an hour was simply the amount of time betweensunrise and sunset divided by twelve. This meant that an hour in summer was muchlong than that of winter. It wasn't until mechanical clocks were invented thatefforts to standardize time grew in popularity. Mechanical clocks, meanwhile,are based on the idea that you can measure time based on a repeated pattern, aslong as it's consistent enough. As early as 1500 BCE, inventors in China createdwater clocks, which relied upon a similar form of pattern standardization. Othersimilar methods include measuring a candle as it burns, or a rope with knots atspecific points. Pendulums have also played a major role in keeping accurate time. Around 1600,scientist Galileo Galilei discovered that pendulums oscillate at the same rate,regardless of weight. About 50 years later, scientist Christian Huygens appliedthis logic to a clock mechanism, and pendulum clocks spread in popularity. Fromthere, the later discovery of quartz was the final huge advancement in theprecision of timekeeping.Learn more about the fascinating history of timekeeping below.