Humans have used many devices to measure time. The sundial was one of the earliest and simplest.
devices: dispositivos; sundial: reloj de sol; earliest: más primitivos;

A sundial measures the movement of the sun across the sky each day. It has a stick or other object that rises above a flat surface. The stick, blocking sunlight, creates a shadow. As the sun moves, so does the shadow of the stick across the flat surface. Marks on the surface show the passing of hours, and perhaps, minutes. The sundial works well only when the sun is shining. So, other ways were invented to measure the passing of time.
across: a través de; stick: vara; rises above: se eleva sobre; flat surface: superficie plana; shadow: sombra; as: a medida que; perhaps: tal vez, quizás; works well: funciona bien; shining: brillando;

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.
hourglass: reloj de arena; thin stream: flujo fino; falling sand: arena en caída; is shaped like: tiene la forma de; wide: ancho; to drop: caer; through: a través de; is turned with the upside down: se lo da vuelta;

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.
had developed: había desarrollado; the same: la misma; set by: pautado, fijado;

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

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.
increased: se incrementó; became clear: fue evidente;

In 1884, an international conference divided the world into twenty-four 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.
chosen: elegido; starting point: punto de partida;

The time at Greenwich – as measured by the sun – is called Universal Time. For many years it was called Greenwich Mean Time.
Mean Time: Tiempo Medio;

Some scientists say time is governed by the movement of matter in our universe. They say time flows forward because the universe is expanding. Some say it will stop expanding some day and will begin to move in the opposite direction, to grow smaller. Some believe time will also begin to flow in the opposite direction – from the future to the past. Can time move backward?
matter: materia; flows forward: fluye hacia adelante; to grow smaller: para encogerse; backward: hacia atrás;

Most people have no trouble agreeing that time moves forward. We see people born and then grow old. We remember the past, but we do not know the future. We know a film is moving forward if it shows a glass falling off a table and breaking into many pieces. If the film were moving backward, the pieces would re-join to form a glass and jump back up onto the table. No one has ever seen this happen. Except in a film.
trouble: problema, inconveniente (en); re-join: reagruparse; jump back up: saltar hacia atrás; has ever seen: ha visto jamás (que);

Some scientists believe there is one reason why time only moves forward. It is a well-known scientific law – the second law of thermodynamics. That law says disorder increases with time. In fact, there are more conditions of disorder than of order.
well-known: famosa, bien conocida; in fact: en realidad;

For example, there are many ways a glass can break into pieces. That is disorder. But there is only one way the broken pieces can be organized to make a glass. That is order. If time moved backward, the broken pieces could come together in a great many ways. Only one of these many ways, however, would re-form the glass. It is almost impossible to believe this would happen.
there are many ways: existen muchas maneras (en las que); could come together: podrían unirse; however: sin embargo; almost: casi;

Not all scientists believe time is governed by the second law of thermodynamics. They do not agree that time must always move forward. The debate will continue about the nature of time. And time will remain a mystery.
must: debe; will remain: continuará siendo

Our program was written by Marilyn Christiano and read by Sarah Long and Bob Doughty. I'm Steve Ember. Listen again next week for "Science in the News" in VOA Special English.