Lesson 1

As city dwellers, we have become strangers to the night sky. The aim of this session is to eliminate some of the strangeness and to better understand both what we can see and why we can see it. First some preliminaries:

  • Never look through a telescope or binoculars pointed at the sun. In fact, the best rule of thumb is to avoid pointing these instruments at the sun. Looking at the sun through a telescope or binoculars can result in immediate, permanent eye damage.
  • When going out to view at night, be sure to select a safe location. In the city, this will often mean a public area and in the company of friends and family.
  • Dress appropriately for the weather. You will, of course, not be stargazing in the rain or snow, but a clear night can get very cold very quickly. A summer day may be 80°F, but the night can quickly drop to around 50°F which will leave you chilled.

Measuring the Sky

In order to describe what we can see in the night sky, we need a way of telling where things are and how large they are.

How Big

The natural units of measurement are angles. For example, you wouldn't say "the moon was one foot across." Here are some easy angles to start out with.

If you face the setting sun and point one hand to the place on the horizon where the Sun will set and another straight up, the angle between your arms will be 90 degrees.

If you face south and point your left arm due east and your right arm due west, your arms will be 180 degrees apart. And each arm will be 90 degrees away from where you are facing.

Your hand can also serve to approximate angles. Everyone's hands are different sizes, but most people have arms and hands that grow in the same proportions. So,

Apparent Motion

In this context, "apparent" just mean "how it appears." When you are traveling, whether by car, train, boat, or airplane, the landscape around you appears to be passing by. Of course, you realize it is really you who are moving. Similarly, as we watch the sky, the Sun appears to rise in the east, climb higher until around noon, then sink lower and eventually set in the west. Unlike the jolting motion of a car (especially on NYC pothole filled streets!), there is no sense of the Earth moving. For thousands of years, this apparent motion is all people knew; they could not observe the motion of the Earth or the planets and the common sense view was the Earth was not moving. All of that is built into our vocabulary: we say the Sun rises, not "the Earth has rotated so the Sun is in view." Trying to say that every morning would get old very fast!

Not all people believed the Earth was stationary. Some realized that the easiest explanation for the apparent motions was that the Earth was spherical and it rotated on its axis and even traveled around the Sun. But it wasn't until the 1600s that the evidence grew strong enough that this idea took over and even then it was another 100 years before the motion of the Earth around the Sun could be measured well enough for people to be sure.




  • horizon
  • zenith
  • nadir
  • meridian
  • ecliptic
  • zodiac
  • constellation
  • asterism