Motions of the Night Sky: Script (30-40 minutes)



  1. Latitude: Home
  2. Precession: Current
  3. Set Sun and sky for today's date
  4. Time: About 9:00 pm
  5. Turn off Sun, Moon, planets, NESW lights
  6. Laser pointer
  7. Lights up

Dialog is in normal type, actions/audience parts are in bold italics and indented.

Getting Acquainted

Good morning (or afternoon)!

Introduce yourself. Remind audience that there should be no eating or drinking in the planetarium. Have them put their packs, coats, etc. under their seats.

Today, we are going to concentrate on learning about the motions of the night sky. I have the planetarium set up for tonight at about 9 pm. As I turn down the lights, take note of what the sky looks like.

Start turning down the lights. Stop when you can see a few bright stars, like the Big Dipper, Orion, etc.

This is what the night sky generally looks like from Seattle. Because we have so many street lights, car headlights and so on, many of the stars in the sky can't be seen easily. So let's take a moment to identify a few constellations that are pretty easy to find.

Use the laser pointer to point out the Big Dipper, Cassiopaeia, Orion or Sagittarius, or some other constellation near the ecliptic/celestial equator. This is a good time to give them some interesting information, such as: the middle star in the handle of the Big Dipper is actually four stars; Polaris can be found from the pointer stars on the Big Dipper; Cassiopaeia was the mother of Andromeda, and chained her to a rock for the Kraken to eat, but the Perseus and Pegasus came and rescued her; Orion is the hunter; Betelgeuse is expected to go supernova 'astronomically soon'; Betelgeuse is red and Rigel is blue because they are different temperatures... Anything you know that's interesting.

These constellations will help us to identify particular stars, so that we can more easily observe the motions of the sky over time.

East-West Motion

Let's start with the daily motion of the sky. The great thing about the planetarium is that I can actually control time! Now I'm going to turn on the daily motion of the Earth.

Turn on the diurnal motion.

Use the laser pointer to point out the Eastern horizon.

What direction is this? That's right, this is East. How do you know?

So the stars come up in the East, and go across the sky, and set in the West.

Show this with the laser pointer.

Let's pay attention for a moment to the Big Dipper. I'm going to speed up time, so that we can quickly see what happens in a day.

Speed up the time. Ask them to point at the Big Dipper. (or, if you've already turned the lights all the way down, you can point it out yourself, or give the laser pointer to someone to find it. It depends on the age group, and whether they know each other sufficiently well that they will not be embarrassed...)

What's happening to the Big Dipper that's different from the other stars?

Give them time to observe and think about this. Some groups notice RIGHT AWAY, and some take time... If they are very young, you may need to go point at what regular stars do, and come back to the Big Dipper.

That's right. The Big Dipper doesn't set! It's a circumpolar constellation, which means that it goes around and around the pole, and doesn't rise or set. This makes it EXTREMELY handy for navigating. These stars are circumpolar because we are partway up this spinning ball that we call the Earth. At the North Pole, all stars are circumpolar; at the equator, no stars are circumpolar. In between, some stars are circumpolar.

So here, with the Big Dipper, we can see what's happening over one day in the sky. The sky appears to go around and around. What causes this to happen? Is the sky actually moving?

That's right, the Earth is rotating around on it's axis, so that at some times of day we are looking in one direction in space, and at other timese of day we are looking in a different direction.

Some people have success at this point by asking the group to lay on the floor, and try to imagine that the floor is moving around, not the projector. I've never gotten this to work, but I think that might be because I am running the daily motion too fast. You will know when you get it to work, because people will start to complain that they've got vertigo, or feel sick, or something of that sort!

Sun Along Ecliptic

Now, I'm going to turn on the Sun, so that we can see what it looks like in the sky.

Turn on the Sun, and wait for it to come around, rise, cross the sky, and just before it sets, stop time.

Now I've stopped time again. This is the actual size of the Sun in the sky, which surprised me the first time I saw it. I always thought the Sun was HUGE in the sky, (even though I'd seen it!) because it is so bright. But it actually takes up only 0.5 degrees of sky! (There are about 20,000 square degrees visible at any time from any location.) Now what I'm going to do is turn on the annual motion, so that we can see what happens in the sky over the year.

Turn on the annual motion.

We are now imagining that the Earth is not rotating around it's own axis, but simply revolving around the Sun, so we can see what happens due only to the revolution, not the rotation. The Sun moves relative to the fixed stars over the course of the year. In which direction does it move, East or West?

That's right, over the course of the DAY, the Sun moves towards the West. But during the course of the YEAR, the Sun moves towards the East. So now you can see that the Sun is in a different constellation at different times of year. This means that the stars in that direction can not be observed at that time of year. For example, if the Sun is in the direction of Sagittarius, (in the wintertime) we can't see Sagittarius, since the Sun is so bright. But we CAN see Taurus, Orion, and all the stars on the opposite side of the sky. So at different times of year, different stars can be observed. Can you think of a reason why this might be a problem for astronomers? (Particularly Seattle astronomers!) What about the circumpolar stars? Can they be seen only in certain seasons, or can they be seen at all times of year? If they don't get this right away, you may need to stop the annual motion, and show them the daily motion again at a couple of times through the year.

Motion of Moon

Now, we are going to see what happens to the Moon. First let me adjust the planetarium so that we are at a good phase to observe.

Turn on the Moon; run the Sun around to just before sunset. If you can see the Moon, great. If not, run the annual motion until you get a nice 1st quarter->full Moon. Stop the annual motion!

So here we can see the Sun, and the Moon, nearly on opposite sides of the sky. Let's see what happens over the course of the day.

Run the diurnal motion. Let the Moon run up and over, almost to the Western horizon.

What does the Moon do?

That's right, the same thing as the Sun and the stars. It rises in the East and sets in the West. Why? Because the Earth is rotating on its own axis.

Now let's see what happens over the course of the year.

Turn off the daily motion, turn on the annual motion---SLOWLY! Otherwise the Moon just zips around...

I'm running time MUCH more slowly than before, so that we can see what happens over a month, instead of over a whole year. What's happening to the Moon?

motions of planets (retrograde)

north pole; our latitude; equator; southern hemisphere