The sundial works well only when the sun is shining. So, other ways were invented to measure the passing of time.

One device is the hourglass. It uses a thin stream of falling sand to measure time. The hourglass is shaped like the number eight --- wide at the top and bottom, but very thin in the middle. In a true "hour" glass, it takes exactly one hour for all the sand to drop from the top to the bottom through a very small opening in the middle. When the hourglass is turned with the upside down, it begins to mark the passing of another hour.

By the eighteenth century, people had developed mechanical clocks and watches. And today, many of our clocks and watches are electronic.

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. This is because time on Earth is set by the sun's position in the sky above.

We all have a 12 o'clock noon each day. Noon is the time the sun is highest in the sky. But when it is 12 o'clock noon where I am, it may be 10 o'clock at night where you are.

As international communications and travel increased, it became clear that it would be necessary to establish a common time for all parts of the world.

In 1884, an international conference divided the world into 24 time areas, or zones. Each zone represents one hour. The astronomical observatory in Greenwich, England, was chosen as the starting point for the time zones. Twelve zones are west of Greenwich. Twelve are east.