Find the Planet


1. A Naked-eye Mars Projector

The starlike image of Mars must be moveable and similar in diameter and hue to the other red stars in the planetarium sky. Many planetarium projectors already include a good Mars projector that can be adjusted by hand. However, if your projector, like ours, produces planets much greater in diameter than the stars, you can use a single slide projector instead for Mars. We use a simple black Kodalith slide with a small orange dot which projects a star similar to the first magnitude stars in the planetarium sky. The single slide projector is moved by hand to produce the two or three relative motions that occur (during the “days”) in the program. You’ll need to make your own “dot” slide.

2. Battery-operated light pointers

These are available from photography stores. It’s good to have at least two of these: one bright and one dim. The easiest way to dim a pointer is to replace one of the batteries with a “dummy” battery consisting of a block of wood, plastic, or metal which has about the same dimensions as a battery, but that provides a short-circuit electrically through the length of the block. With the “dummy” battery installed, the bulb gets current supplied by only one battery, which makes it just the right brightness for pointing out stars and saves battery and bulb life to boot.

It’s also possible to make an inexpensive light pointer out of LED flashlight See the Constellations Tonight News and Updates on the PASS website for details.

Presenter's Script

Could one of these points of light do something different? We have been watching for many minutes now, and we know all the points of light in the sky appear to turn very slowly throughout the night. But there isn’t much happening so far, and no one star stands out yet. Maybe we should go ahead several nights to see if anything has changed.

Assign each object to a section of the audience to watch.

Now study your assigned object carefully. Look at its position compared with the stars around it, so that you will be able to tell if it has moved.

Turn up daylight, turn down stars and planets. Advance Mars about 10 degrees East.

When the lights dim again we will be four weeks in the future, and the stars will be nearly in their same positions. But will any of our orange or red objects be different?

Turn up stars and planets, turn down daylight.

What happened to your point of light?

Go from group to group. Nearly everybody will say theirs moved a little bit.

Well, most of the objects may have moved a little bit. Maybe we need to go ahead another four weeks to tell for sure which one is a planet. Watch your object again.

Turn up daylight, turn down stars and planets. Advance Mars another 10 degrees East, etc.

What happened to your point of light now, eight weeks after we started?

Now a few will have decided theirs did not move, but the Mars group will be sure that theirs did move.

We are not sure about some of the objects but have ruled out some, and one group thinks this one (Mars) definitely did move. Perhaps members of that group would venture a prediction as to where the object will be in another 4 weeks.

Allow a member of the group to use the pointer to show where they think it will move.

Let’s all watch their object very carefully this time and see if it indeed moves against the background of stars.

Repeat 4 week motion sequence a final time.

Did it move? [Yes.] Is it where you predicted it would be?

So we have found the planet Mars by its motion, the same way the ancients did. In fact our word “planet” comes from the Greek word “planetes,” meaning “wanderer.” Because the Earth is turning, all the stars and planets appear to move slowly every night, together. The planets, due to their motion around the Sun, appear to turn a little faster or a little slower than the distant stars. Stars and planets have many other important differences, but we can’t tell without a telescope. The way Mars drifts against the starry background convinced astronomers centuries ago that the planets travel in orbits around the Sun, not around the Earth.

We have found it: that red dot of light is the planet Mars, as people have seen it throughout the ages. We find other celestial objects just as red as Mars in the early spring sky. Here’s Aldebaran in Taurus, for example, and Betelgeuse in Orion — both are red-giant stars (point out the red stars named). Following the curve of the Big Dipper’s handle leads us to Arcturus, in Boötes (point out). Antares, the name of a red-giant star in the summer sky, means “rival of Mars.”