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Chapter Introduction

The central region of our Milky Way galaxy shines brightly above statues on Easter Island in the Pacific Ocean. Besides individual stars in the night sky, you can see clusters of stars, glowing clouds of gas, and darker regions of gas and dust obscuring light from stars behind them. We will explore all of these things and more in the chapters of this book.

Discovering the Night Sky

WHAT DO YOU THINK?

What do astronomers define as constellations?
Is the North Star—Polaris—the brightest star in the night sky?
What causes the seasons?
When is Earth closest to the Sun?
How many zodiac constellations are there?
Does the Moon have a dark side that we never see from Earth?
Is the Moon ever visible during the daytime?
What causes lunar and solar eclipses?

Answers to these questions appear in the text beside the corresponding numbers in the margins and at the end of the chapter.

You are studying astronomy at an exciting time as astronomers draw open the curtains of the cosmos. Our understanding of the cosmos (or the universe) and how it evolves is growing as never before. That is due, in large measure, to the immense light-gathering power and sensitivity of modern telescopes. We can now see so far away and, therefore, so far into the past that we see the first stars and the first galaxies as they just began forming more than 13½ billion years ago. We could not see these objects even a decade ago and, likewise, it took just 21 years for astronomers to discover 1000 planets orbiting nearby stars, a feat that would have been impossible 30 years ago.

In this chapter you will discover

how astronomers map the night sky to help them locate objects in it
that Earth’s spin on its axis causes day and night
how Earth’s orbit around the Sun, combined with the tilt of Earth’s axis of rotation relative to its orbit, creates the seasons
that the Moon’s orbit as seen from Earth creates the phases of the Moon and lunar and solar eclipses
how the year is defined and how the calendar was developed