No one knows when the first calendar was developed. But it seems possible that it was based on lunar months. When people started fanning, the sages of the tribes became very important. They studied the sky and gathered enough information to be able to predict when the seasons would change, and were able to announce when it was time to plant crops. The divisions of time we use today were developed in ancient Babylonia 4,000 years ago. Babylonian astronomers believed the sun moved around the Earth every 365 days. They divided the trip into 12 equal parts, each with 30 days. Then they divided each day into 24 equal parts or hours, and divided each hour into 60 minutes, each minute into 60 seconds. Humans have used many devices to measure time. he sundial was one of the earliest and simplest. However, the sundial worked well 0nly when the weather was fine, so other ways of measuring the passing of time were invented. One device was the hourglass. By the eighteenth century, people had developed mechanical clocks and watches. So we have devices to mark the passing of time, but what time is it now? Clocks in different parts of the world do not show the same time at the same time, because time on Earth is set by the sun's position in the sky above us. As international communications and travel grew, it became clear that a way to establish a common time for 'all parts of the world was needed. In 1884, an international conference divided the world into 24 time zones, each zone represents 0ne hour. The astronomical observatory in Greenwich, England, was chosen as the starting point for the time zones. Twelve zones are west of Greenwich. Twelve arc east. The time at Greenwich measured by the sun is considered by astronomers to be Universal Time, also known as Greenwich Mean Time. What does the passage mainly discuss?