A lot happens in 24 hours. Your heart beats around 100,000 times. 150 to 200 species of animals become extinct. Lightning strikes earth 8.6 million times. Astronauts aboard the International Space Station will see 16 sunrises and 16 sunsets. About 367,200 babies will be born and about 151,600 people will die. So yeah, a lot goes on in just 24 hours. But why is a day divided in 24 hours in the first place? Who decided how we would calculate it? Today, time is tucked away in our pockets, just an unlock button away. It drives our technology, with atomic clocks being responsible for precise GPS location services and speedy internet search results. Our ancestors, on the other hand, didn’t have it that easy. Time for them was written in the stars, in the sunrise and sunset, in the moon’s waning and waxing. So how did we go from looking up to celestial bodies to looking down at our phones for time? What other time-keeping methods did ancient humans use before watches and clocks became ubiquitous? And what the heck is an atomic clock? You’re watching Explore Mode and today, we are diving into time, and how humans developed technologies to measure it. Let’s start with celestial bodies. Early civilizations like the Egyptians used the moon’s phases to determine the length of a month. In prehistoric Europe, humans built stone rings called recumbent stone circles that would frame and track the moon in each of its phases. It’s believed they were built for ritualistic purposes. Then we have the creation of the sundial. A device that shows the time using the sun’s shadow depending on its position in the sky. The earliest archeological evidence of a sundial was found in the Valley of Kings in Upper Egypt in 2013 and it dates back to 1500 BCE. A larger version of the sundial is the obelisk, which was used to mark the summer and winter solstices. While Egyptians, Romans and Greeks were using sundials to keep track of time, people in Asia were using candles and incense sticks. Candle clocks worked as follows: Each candle had markings on its sides indicating the passage of a certain period of time so as the candle burned you could calculate how much time had expired. The first reference to candle clocks dates back to 520 AD in a poem written by Chinese thinker You Jiangu, who seemed to use them for study sessions. Similar to candle clocks, early evidence of incense clocks takes us back to China 6th century China to be exact. Different incense sticks would indicate the passing of time depending on how quickly they burned. Some sticks had different aromas for each hour that passed and others had weights attached at different sections that would drop as the incense burned. But incense clocks stuck around well after we had developed watches and pendulum clocks. Until 1924, in Japan, geishas would count how many incense clocks had been burned during their services to charge their customers. Egyptians divided their sundials into 12 parts for each hour of the day from dusk to dawn. Although sundials were quite accurate for their time, they had one very evident disadvantage. They were practically useless during overcast days and at night. But back to the 12 hour day division. So 12 hours for day time, 12 hours of the night time. That’s 24 hours a day, no surprise there. But why then is each hour divided into 60 minutes and each minute into 60 seconds? Well, that has to do with a numerical system that is still used today. Time for an Express Explore Explanation. Start the Clock. The Sexagesimal system takes us back to ancient Mesopotamia. It is a numerical system based on the number 60 that was developed by the Sumerians and later adopted by the Babylonians. They developed it by dividing parts of their fingers into single units, and units of 12. Here’s how it worked. Take a look at the palm of your hands. Notice that each of your fingers is divided into three sections. Using their thumbs to count, the Babylonians realized they could count up to 12 for each section on each finger of one hand: 3 x 4=12. The five fingers on the other hand each represented a dozen. So 12 x 5=60. Also, 60 is divisible by 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, and 30, making it an easy number to split for a civilization with no calculators. This system is used nowadays to divide time, angles, geographical coordinates and it is the mathematical base for astronomical coordinate systems. In order to bypass the “no sunlight” conundrum, the Egyptians needed another method of timekeeping and so, they developed the water clock. Here’s how they worked: Egyptians would fill a bowl-shaped container with water. Said water had a spout at the bottom which would allow for water to flow out into another container. It had 12 markings on the inside, each line representing the passing of an hour. As the water flowed out, they would compare the water level with the markings to determine how much time had passed. The problem with this type of water clock was that it didn’t provide an accurate representation of the passage of time. You see, as the container emptied out, pressure of the water would lessen, making the water run more slowly by the end. Enter Ctesibius, an inventor and mathematician from Alexandria, and the man now regarded as the inventor of the first real clock. Between 270 BC and AD 500, he figured that instead of measuring time based on the outflow of water, he would measure the consistent inflow of it. It worked pretty much in the same way except that instead of measuring how much a container had emptied, he would measure the rise of the water level as the liquid steadily flowed into the container. Unbeknownst to Ctesibius, he had created the first, albeit quite rudimentary, mechanical clock. For many years, the perfected water clock was the most accurate timekeeping device. That is until proper mechanical clocks appeared. The first people to build large mechanical clocks were Catholic monks. They needed precise timekeeping devices to schedule chores and prayers within the monastery. According to historian Thomas Woods, the first recorded clock was built by the future Pope Sylvester II for the German town of Magdeburg, around the year 996. Then came the inventions of Christian Huygens, a Dutch physicist, mathematician, astronomer, and inventor who, using theories from none other than Galileo Galilei, created the technology behind the pendulum clock and the springs in the pocket watch. Pocket watches were the first to be mass-produced allowing everyone and their mother to be able to keep time in their pockets. Clocks today are more precise than they have ever been. Seconds are no longer measured with the swing of a pendulum. They are measured with crystals and atomic particles. Time for an Express Explore Explanation, start the clock. With pendulum clocks, one swing of the pendulum equates to one second. This is how you calculated time. But pendulums depend on a lot of external factors to maintain a constant swing, so a more precise form of frequency was needed to create more accurate clocks. That’s where the quartz comes in. Check your wristwatch or whatever timepeace you own. If it says quartz, it means that your device is powered by a crystal. The quartz is a clear mineral made of silicon and oxygen (SiO2) that vibrates at a very precise frequency whenever it is zapped with electricity. Quartz crystals are fashioned into resonators in the shape of a tuning fork and put inside timepieces where they’re receiving constant electric signals. A piece of quartz resonates at 32,768 pulses per second. Indicating that whenever the quartz reached that amount of pulses, a second had passed. But although quartz clocks are fairly accurate, they’re still not the most precise form of timekeeping. Quartz crystals vibrate at different frequencies depending on whether they’re in warm or cool environment making them gain or lose a few seconds. What can be more precise than the oscillation of a crystal being electrocuted? Atoms. Cesium-133 atoms to be precise. See, Cesium-133 atoms resonate between different energy states at an extremely stable frequency. A quality that is imperative for precise timekeeping. So, in 1967 the National Institute of Standards and Technology declared that the official measurement of a second is equivalent to 9,192,631,770 oscillations of a Cesium atom’s resonant frequency. And that is still the standard used today. Timekeeping devices nowadays not only help us to determine our work schedule or set up our alarms. They are an integral part of the technology we see all around us. GPS satellites have at least two caesium and rubidium atomic clocks onboard in order to calculate the time delay of signals and provide an accurate reading of a location. In December 2018, scientists at the National Institute of Standards and Technology developed two clocks using 1,000 atoms of the element ytterbium in grids of lasers. These clocks are so precise that they can show the effects of gravity on Earth, helping us measure the space-time continuum. So far this is the most precise form of timekeeping, but a new technology might emerge in the next decade, or century or millenia … who knows, only time will tell. Thanks for watching Explore Mode, if you liked this video hit the thumbs up button. If you want to explore even more with us, check out our playlist, there’s plenty to explore there. Before you leave, make sure to hit the subscribe and bell button. See you next week, and in the meantime, keep your explore mode on.