Topic 1.5: Using a Star Chart: Find Your Way

This lesson is a how-to. By the end, you should be able to go outside on any clear night with a star chart or an app and name at least a few things you see.

Two Numbers Tell You Where to Look

The simplest way to give directions in the sky is with two angles, measured from where you stand.

• Altitude: how high above the horizon, from 0 degrees (on the horizon) to 90 degrees (straight up, the zenith).
• Azimuth: which compass direction, measured clockwise from north: 0 degrees = north, 90 = east, 180 = south, 270 = west.

A handy ruler is your hand. At arm's length:

• Your fist covers about 10 degrees.
• Your spread hand (thumb tip to pinky tip) covers about 20 degrees.
• Your pinky covers about 1 degree, twice the width of the full Moon.

So "altitude 40 degrees, azimuth 270" means "a bit more than four fists above the western horizon."

Using a Planisphere or an App

1. Set the date and time on the dial (or let the app read your clock and location).
2. Face the direction named on the chart edge. If the chart's edge says "South," hold it up and face south.
3. Hold the chart overhead, or tilt your phone toward the sky. Stars near the edge of the oval are low, near the center are high.
4. Start with something bright (the Moon, a planet, or a very bright star), then use it to find the others.

A First-Night Plan

1. Find north (Polaris, or a compass). Locate the Big Dipper or Cassiopeia.
2. Use the pointer stars to confirm Polaris.
3. Pick the brightest star you can see and use the chart to name it.
4. Look for a planet (a bright, steady "star" near the ecliptic).
5. Spend five minutes just looking. You will see more stars than you did when you started.

Practical Tips

• Check the Moon phase first. A bright Moon washes out faint stars.
• Dress warmly. Even in Alabama, a still night can feel cold after 20 minutes.
• Write down what you saw and the time. Astronomers kept notebooks for 5,000 years.

Worth a Pause

A map does not make the land, but a good map lets you walk through it without getting lost. A star chart does the same for the sky. Jesus said that his followers could read the weather by the sky, "When it is evening, ye say, It will be fair weather: for the sky is red" (Matthew 16:2). If a person can learn to read the sky for weather, learning to read it for stars is a very natural next step.

Practice Problems

1. Define altitude and azimuth.
2. What altitude is the zenith? What altitude is the horizon?
3. What azimuth is due east? Due south?
4. A star is at altitude 30 degrees and azimuth 180 degrees. Where is it?
5. About how many degrees does your fist cover at arm's length?
6. Why should you check the Moon phase before planning a night of stargazing?