On Sept. 11, a sundial outside new Freedom High School in Woodbridge will cast a long, thin shadow on a plaque in the ground.

The time the shadow falls on the plaque will correspond to the time American Airlines Flight 77 struck the Pentagon during the terrorist attack in 2001. If all goes according to plan, the shadow will always fall on the same spot, at the same time, on the same day, year after year, as long as the 62-foot sundial endures outside the school.

Sundials, one of the oldest manmade devices used to mark the passage of time, are reasonably accurate, but creating a sundial for such an exacting task as marking 9:37 a.m. on Sept. 11 each year is exceedingly complicated.

At Freedom High School, which opens in September, creating the sundial required a team effort. Forest Park High School science teacher Kenneth Hicks; Walter Sanford, an earth science teacher at Carl Sandburg Middle School in Alexandria; and Robert Kellogg, a sundial enthusiast with advanced degrees in astronomy and physics, came together to make sure the Freedom High School dial would do what its designers intended.

In using the sun and stars to commemorate an event, Kellogg said, area residents are not much different from ancient people who created Stonehenge or built burial caverns that are fully illuminated only during the winter solstice.

"Celestial alignment has meant a lot to us, in our psyche," said Kellogg, who works at Argon Engineering in Fairfax. "There's something in us that looks for the permanence in the structure and the annual renewing."

Freedom High School, located near the Northern Virginia Community College campus in Woodbridge, was under design before the 2001 attack. During the design phase, the architects contacted Kellogg for help with placing the sundial, which was already planned for the site.

After the attack, the School Board voted to make the sundial its Sept. 11 memorial. Principal Dorothy McCabe called on Hicks, the director of the Forest Park planetarium, for help on the placement.

Like all horizontal sundials in the Northern Hemisphere, the triangular centerpiece of the dial, or gnomon, must be aligned to true north. The angle of the gnomon must correspond to the latitude where the dial is located. That ensures it will keep correct time, but such calculations don't go into the placement of a memorial marker.

Adding to the complexity is the "equation of time." As explained on the Web site of the North American Sundial Society, the apparent motion of the sun will cause dials to be as much as 16 minutes fast or slow at various times of the year.

"The idea of telling time by a sundial is extremely hard," Hicks said. "It's not like looking at your watch." So he called in reinforcements.

Sanford is another sundial enthusiast. At his home, he has created a botanical sundial based on theories that flowers open and close about the same time each day.

"I have worked with sundials a fair amount, and all of them have some error built into them," Sanford said. But a large degree of error would be unacceptable for this project. Sanford called on Kellogg, who was summoned to help again three years after originally lending his assistance on the placement.

Working together, the men developed a spreadsheet program that calculated where the memorial plaque should be placed. But adjustments still must be made. For instance, if the site is slightly graded, to prevent pooling of water, the plaque must be moved to account for that. It is yet to be decided whether the shadow cast by the gnomon will point to the six-inch white plaque or completely obscure it. Also, the plaque currently has "9:20" engraved on it, but Hicks said it's more accurate to place the time at 9:37 a.m. and the plaque will be corrected. A few days before dedication, Hicks said, he will likely visit the site to ensure the plaque is where it should be. Kellogg plans to do the same.

The teachers said they hope the sundial can eventually be used as a tool for the students. Because the school is intended to focus on environmental studies, Hicks said perhaps a botanical clock could be incorporated into the site.

"This is an astronomical tool, and we want to use it as a teaching tool for the students," Hicks said.

Kin Brown, left, of Commercial Concrete, and Kenneth Hicks, planetarium director at Forest Park High School, check the blueprints for placing the sundial, which on Sept. 11 will mark the time of the 2001 terrorist attack on the Pentagon. "We want to use it as a teaching tool," Hicks said.The sundial at Freedom High School is a team effort aimed at ensuring that every year on Sept. 11 as long as the sundial stands, it will cast a shadow on a plaque memorializing the exact time of the 2001 Pentagon attack.