ECLIPTIC
ECLIPTIC
LEO
CANCER
GEMINI
AURIGA
PERSEUS
CANIS MINOR
CANIS MINOR
ORION
LEPUS
ERIDANUS
CETUS
TAURUS
ARIES
PISCES
ANDROMEDA
PEGASUS
CYGNUS
AQUILA
LYRA
AQUARIUS
CAPRICORNUS
SAGITTARIUS
SCORPIUS
VIRGO
LIBRA
BOOTES
HERCULES
OPHIUCHUS
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
orbit 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.
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Although we can’t feel it, the Earth rotates eastward at about 800 miles an 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.
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 p.m.). Turn off the room lights, turn the projection lamp on, and see
what constellations are up!
