Keeping Time with the Skies

Overview

People have measured time by watching the Sun, Moon, and stars. This chapter explains how the motions of Earth and Moon give us day and night, months, seasons, and years, and how calendars are made.

Key concepts

• Earth s rotation on its axis causes day and night.
• Earth s revolution around the Sun causes the year and seasons.
• The Moon s phases repeat about every month.
• Ancient people used sundials and star positions to tell time.
• Calendars are based on the motions of Earth and Moon.

Important terms / formulae

• Rotation: spinning of Earth on its axis (about 24 hours).
• Revolution: Earth moving around the Sun (about 365 days).
• Phases of the Moon: changing shapes of the lit Moon.
• Leap year: a year with 366 days, every four years.

Solved example or key process

As the Earth rotates once in about 24 hours, the side facing the Sun has day and the other side has night. As Earth keeps spinning, places move from day into night and back, which is why the Sun seems to rise in the east and set in the west.

Important questions

1. What causes day and night on Earth?
2. Differentiate between rotation and revolution.
3. How do the phases of the Moon occur?
4. Why do we have a leap year?

Quick revision

Earth s rotation gives day and night; its revolution gives the year and seasons. The Moon s phases mark months. People long used the Sun, Moon, and stars to keep time, and calendars follow these motions.