Every day when the sun comes up, people know to wake up because of the sunlight or because of their alarm clocks. People decide when to go to school or work, when to eat meals, when to meet friends, when to go home, and when to go to bed. People also know which days to go to school or work, which days are free days, and which days are holidays. In addition, people can describe how old they are, how long ago something was built, invented, said, or decided, and when something will happen in the future. This is all thanks to the human ability to measure time! Then again, how is time measured' How did people come up with ways to measure time' Time measurement is such an important part of everyday life, but people often still ask these same questions.

Measuring Time During the Day

Most people consider a "second" the smallest unit to measure time. People can measure time using units smaller than seconds like microseconds and nanoseconds, but these kinds of units are only important in measuring things like lasers and electricity. For everyday use, the second is really the best way to measure one moment to the next. In order to imagine a second, go to a basketball or football game and watch the clock on the scoreboard. The seconds are the two numbers at the far right, and when the clock is not stopped, they are the numbers that seem to count down the most often. The next unit of measurement is the minute, which is made up of 60 seconds, and then the hour which is made up of 60 minutes. 

Measuring Time During the Year

After that, the day is measured as 24 hours, although a day can also be measured as by the movement of the sun in the sky from daytime to nighttime to daytime again. A week is seven days, and a month is 28-31 days, depending on the month. In olden days, months were measured by the cycle of the moon from new moon to full moon and back to new moon again. In this case, they are called lunar months. Some people in other countries measure time in lunar months which almost always is about 4 weeks. After that, a year is considered 12 months, or 365 days. However, since years were supposed to be measured by the Earth's orbit around the sun, some people noticed that a true year was actually about 365 days plus 6 hours. In order to make up for the 6 hours lost each year, every fourth year people added a day to February. The years when this extra day was added were called leap years.

Measuring Time Beyond a Year

In order to describe events that happened in the past or things that might happen in the far future, people used units of time that equaled a large amount of time in years. After a year, there is a decade which is measured as 10 years. Then there is a century, which is measured as 100 years, or 10 decades. People often describe events in history as happening in a certain century like, for example, planes were invented in the 20th century (1901-2000), or the middle ages started in the 5th century (401-500). Then there is a millennium, which is measured as 1000 years, or 100 decades, or 10 centuries. When the year changed to from 1999 to 2000, people often called this the new millennium.

Measuring Time in the Past

Back in the ancient past, people did not have clocks to tell time. The only way they could tell how much time was left before evening was by watching shadows cast by the sun on trees and rocks. The first "sundials" were probably just made from sticks placed into the ground. When people could build big monuments, the Egyptians and the Babylonians built these tall stone towers called obelisks that acted as a giant sundial. Finally, sundials were made as a round disk with the hours marked, and in the middle was some kind of needle sticking up to cast the shadow. At the same time, when people started growing food instead of hunting, knowing the seasons became extremely important. Ancient scientists would watch the sky and the movements of the sun, moon, and stars to figure out what time of the year it was. This is why many measurements of time now are based on the movement of the sun or the moon. People in ancient Africa and Europe would carve notches into bones to mark each passing of the moon. From this, some of the earliest calendars were made. Another form of measuring time in the past that is different from today was the hourglass which was two glass bulbs connected by a tiny opening between the two bulbs. One bulb was filled with sand, and when the hourglass was turned upside down, the sand would pour into the second bulb, and this marked a certain amount of time passing. 

Measuring Time in the Present

Around the middle ages, someone started making mechanical clocks which are very similar to the clocks that people still use today. A round disk had marks or numbers around the edge to mark the different hours, and the hours were measured with a short needle while the minutes were measured with a long needle. These needles are often called "hands".  Later on, people added a third "hand" that constantly moved, and this measured seconds. However, now-a-days, people also use digital clocks, which just state the time in numbers like 12:34 or 6:52. Usually these clocks will also say A.M. or P.M.. A.M. stands for ante meridiem which is latin for "before noon", and P.M. is post meridiem which means "after noon". Romans in the ancient times also came up with the calendar that most people use today called the Gregorian calendar.  In fact, many of the names for months were named after Roman gods, festivals, kings, or simply Latin words. The days of the week however are named after Germanic gods, with the exception of Sunday, day of the Sun, Monday, day of the Moon, and Saturday, day of Saturn. All across the world, different places use different calendars still, but in the United States and in Europe the Gregorian calendar is still the most popular.

Measuring time has been important for humans since the beginning of history. Knowing when night was coming helped humans stay safe at night, and knowing the seasons helped people prepare for winter and know when to plant in the spring in order to harvest food in the fall. Then people used measuring time for researching, recording history, and doing things in daily life. Now people every day constantly keeping track of time, so much so that people use "atomic" clocks which measure the most accurate time much like this clock. Because people can measure time years, decades, centuries, and even millennia (plural of millennium) from now will be able to know what people have done today and in the past.