For Geoff Chester, "Summertime" began in January.

And by the time Chester and the others at the National Air & Space Museum's Albert Einstein Planetarium polished the script, put the finishing touches on the homemade special effects and synchronized about 200 slides for their latest production, it was May.

"Summertime," the planetarium's new show, explains many things about the universe, including how different cultures see the constellation we call the Big Dipper: The English believed it to be a farmer's plow, the Chinese imagined it as an emperor and servants, and the Greeks thought it was a bear.

The show, which opened two weeks ago and will run through the summer, also explains why Venus is so bright, why Mercury is difficult to see and which Northern Hemisphere constellations can be seen on a cloudless night.

The sky depicted in "Summertime" is the northern hemisphere on June 21, the astronomical first day of summer. The large projector in the middle of the room -- a Zeiss model VIa, for planetarium followers -- fills the dome with the sun, the moon, five planets and 8,900 stars.

When the Zeiss machine was run manually, putting on a show was awkward. Now that it has been linked to a computer and is controlled automatically, it provides a nearly flawless show 14 times a day. West Germany gave the Zeiss machine to the Air & Space Museum in the mid-1970s. "Originally, the machine cost about $750,000," said Chester, the planetarium's production coordinator. "Today, if you were to walk into your friendly neighborhood Zeiss dealer it would be about $1.8 million. Taxes, tags and license not included."

The planetarium used to pay $2,000 for each lightbulb inside the Zeiss machine, and the bulbs lasted only 750 hours. The bulbs "were literally made by a little old lady in Germany" who produced them by hand, Chester said. However, the planetarium now uses $35 General Electric street lamps that last more than 20,000 hours.

"Summertime" was written by James Sharp, the planetarium chief, and Thomas Callen, the program resource manager.

Ellen Sprouls, one of four staff members at the Einstein Planetarium, researched the many pictures used in the show. She also researched American history since the opening of the Air & Space Museum in 1976, research that was used in the photo montage that forms the last sequence in "Summertime" and includes images from Jimmy Carter to the space shuttle Challenger's final crew.

Sprouls also used images of constellations from a 300-year-old star atlas she found at the U.S. Naval Observatory's library -- Johannes Hevelius' "Prodromus Astronomiae," published in 1690, three years after his death.

"One of the things I've always wanted to do at a planetarium was to project classical constellation figures against the sky," said Chester.

He superimposed the drawings of mythological characters Sprouls found in the book against their proper constellations by synchronizing the figures on an array of projectors used along with the central Zeiss machine to produce a three-dimensional effect.

The effect is ruined if the 12 projectors are moved, and if this hap- The show explains, among other things, why Venus is bright and why Mercury is difficult to see. pened they would have to be painstakingly resynchronized.

In the show's opening sequence, several small fires seen burning in front of a horizon of Greek-style buildings were filmed in Callen's back yard, using a Weber grill and "about a gallon of lighter fluid."

Dan Zirpoli, of the Davis Planetarium in Baltimore, narrates the show.

At one point the audience faces the northern constellations. For the audience's sake, the Zeiss machine makes an unnatural movement to show the southern stars.

"Please don't think that the real sky moves this way," Zirpoli says. "If you ever see it doing so, please don't call us at the Einstein Planetarium . Call the Davis Planetarium in Baltimore."

A false phone number for the Davis Planetarium then flashes on the dome.