What is time? This question has intrigued people for centuries. From a philosophical perspective, people have wondered why time only moves forward. From a technological perspective, people have invented a variety of devices designed to keep track of the passage of time.

Measuring Time, a unit on the science and technology of timekeeping, is designed for sixth-graders. It is divided into two sections: "Keeping Time with the Sun and the Moon" and "Investigating Invented Clocks". This organization groups activities involving natural cycles and those involving mechanical methods of keeping track of time. In the first section, students observe the apparent motion of the sun and the recurring cycle of the moon's phases. In the second section, students plan and conduct experiments with some of the principal timekeeping devices developed through the ages, from water clocks to mechanical escapements. Throughout the unit, students read about the history of timekeeping and people's evolving understanding of how to measure time.

In Lesson 1, students participate in a brainstorming session of their ideas about time and timekeeping devices. This is an opportunity for students to share their ideas about time with one another and for the teacher to identify students' interests and to assess their understanding of the subject. Students also estimate the passage of a familiar interval of time--one minute--without using a clock.

Students begin their investigation of natural phenomena used to keep time by constructing their own sun clocks in Lessons 2 and 3. They calibrate these devices by tracing the gnomon's shadow at various times throughout the day. Students also measure the length of the shadow at these times and construct a graph showing the changes.

Lesson 4 presents students with a different type of sun clock--a calendar. Students read about how early civilizations devised calendars based on the counting of sunrises and sunsets and the cycle of the moon's phases. Students also devise their own calendars, which are designed to record the past five days of their own history and to predict the next five days of their future.

In Lessons 5 and 6, students explore the cycle of the moon by making predictions about its phases and then observing them. Many students at this grade level do not understand that the moon's apparent changes involve the relative positions of the observer, the earth, the sun, and the moon. Repeated observation now is important, however, because it provides the experience needed for greater student understanding.

Lesson 7 marks a transition to mechanical means of measuring time. In Lessons 7, 8, and 9, students work on a series of activities in which they construct and regulate sinking water clocks and plan and conduct an experiment with them. Students also read about the history of water clocks.

The cycle of planning and conducting experiments is repeated in Lessons 10, 11, and 12, when students investigate the characteristics of pendulums. In Lessons 13, 14, and 15, students have an opportunity to apply much of what they have learned as they assemble, troubleshoot, and then improve a working clock escapement. The challenge of working with a clock escapement enables students to develop their abilities in solving practical problems.

In Lesson 16, students return to the question of how to estimate the duration of one minute, first considered in Lesson 1. Here, they apply what they have learned to design and construct devices that will help them measure this interval of time.

As students work on these timekeeping devices, they may encounter difficulties and have questions that are not easily answered. In particular, constructing the escapement mechanism provides students with the opportunity to experience firsthand the challenges that scientists and engineers encounter. Throughout the unit, it is important to encourage students toward further experimentation and research as an effective way to broaden their knowledge.