The Moving Sky
The horizon ring is an important part of your planetarium. It divides the sky into two halves, the visible and the
nonvisible. In the course of 24 hours, all of the objects visible from a given location on Earth seem to rise in the
east and set in the west.
To see all the stars that are visible from the Northern Hemisphere, slowly rotate the light wand to the left
(westward) while the projection lamp is on. Do you notice that some stars around Polaris never set while some of
the stars that can be seen from the Southern Hemisphere (at the opposite pole of the star sphere from Polaris)
never rise in the Northern Hemisphere?
Just for practice, adjust your planetarium to project the stars visible in the evening of March 20, which is about
the first day of spring for us. Which constellations will be on the meridian (the arc spanning across the sky from
pole to pole)? Which constellations will be rising and which will be setting? That’s easy! With the projection lamp
off, set the date selector to March 20, and then set the time ring to the time at which the Sun sets (about 6:30
PM). Turn off the room lights, turn the projection lamp on, and see what constellations are up!
Although we can’t feel it, the Earth rotates eastward at about 800 miles per hour at its surface. The stars, Sun
and moon appear to us to move westward when, in fact, we are the ones that are moving eastward. Because of
this, it seems like any given constellation or star takes about 24 hours to make one round trip around the Earth.
Astronomers, ancient and modern, counted on this 24 hour trip, day after year after century. They agreed to divide
the east-to-west movement of stars into 24 equal parts.
Astronomers picked the spot in the sky where the ecliptic (the path the Sun takes in relation to Earth during
a year) crosses the celestial equator as the Sun heads north, for the point at which the 24 hour celestial cycle
begins. This is the vernal point, the first day of spring in the Northern Hemisphere.
Into the Night With Your Planetarium
Your planetarium can be taken outdoors into the night for use as a 3-D star map. Remove the star sphere and
light wand from the base and hold it in your hand. The brightest stars will glow in the dark on the star sphere
after you have “charged” it by turning the projection lamp on for a few moments. (Don’t look at the star sphere
while charging it, or you will lose your dark adaptation.)
For a more accurate reading, set your planetarium in its base on a table outside. Adjust it for the current date and
time. Use the compass to position your planetarium to point north. Turn the projection lamp on and use a sheet of
white paper for a projection screen. The stars projected onto the paper can guide you to the stars that appear in
the night sky.
The Constellations of the Zodiac
The constellations of the zodiac are the oldest star patterns, with Taurus the bull being the most ancient of them all.
Because of the Earth’s revolution around the Sun once a year, the Sun seems to move against the background stars.
The path the Sun appears to take is called the ecliptic. The zodiacal constellations lay along the ecliptic, which made
them very important star patterns to the ancient peoples who relied on the night sky as their calendar.
While this map of the zodiacal constellations shows the ecliptic as a curved line, your planetarium projects this
path as a great circle around the entire sky. Turn on your planetarium and project the stars onto a wall. Rotate it
slowly so you follow the constellations of the zodiac through one year.
