EL PASO, Texas -- Less than 42 hours before the start of today's solar eclipse, Dr. David W. Dunham feared he would have to make a break for the sun.
The Johns Hopkins University astronomer and veteran eclipse chaser was surrounded by maps and tuned to the Weather Channel in his frayed hotel room here. At 3 p.m. Sunday, he watched clouds of Pacific moisture swirling into Texas like cream into a coffee cup.
He had come to Texas from the Hopkins Applied Physics Lab in Laurel seeking a good view of today's "annular," or "ring," eclipse, the last solar eclipse to cross the continental United States until 2012.
In Baltimore, the eclipse will be partial. At 1:28 p.m., more than 80 percent of the sun will be obscured by the moon, creating an odd twilight. Mostly clear skies were predicted today for Baltimore.
Observers are warned never to look directly at the sun, even when it is partly obscured, because the retina of the eye can be permanently damaged.
Today's eclipse is an annular eclipse because the moon will be near the most distant point in its orbit, leaving a brilliant ring, or "annulus," of sunlight.
Weather permitting, the full ring will visible along a 145-mile-wide band from West Texas to the coast of Maine.
Seeking the best view, Dr. Dunham picked Texas.
For 15 years now, he has made it his passion to chase solar eclipses around the world. But he's no thrill-seeker. Slender, graying and in constant motion, the 51-year-old scientist is in dogged pursuit of a single measurement -- the diameter of the sun -- or more precisely, changes in the diameter of the sun that may produce changes in Earth's climate.
The diameter can't be measured directly because of the blurring effects of Earth's atmosphere. So Dr. Dunham and his associates do it indirectly, by measuring the width of the moon's shadow during solar eclipses.
They believe they have already detected a decrease of more than 200 miles in the sun's diameter between measurements deduced from eclipse observations in 1925, and their own measurements in 1979. "But we still don't know whether it's a real long-term trend or something periodic," said Dr. Alan D. Fiala, 51, of the U.S. Naval Observatory, who has shared the project since 1979. He made plans to view today's eclipse from Indiana or another, clearer spot in the Midwest.
A good weather eye and a willingness to travel is a must in this business. The chase has taken them as far as India, Siberia and Tasmania. Dr. Dunham, who has now seen 12 solar eclipses, is already making plans to be in Bolivia Nov. 3.
But on Sunday, as the weather forecast worsened, the map at hand was one of the United States.
"I'm going to fly home and drive to Ohio," he announced.
To his relief, Monday dawned clear and hopeful over El Paso, and Dr. Dunham canceled his airline reservations.
As president of the U.S. section of the International Occultation Timing Association, he was calling the signals for dozens of other scientists, amateurs and reporters from around the world who had gathered here.
Since 1984, Dr. Dunham and Dr. Fiala have enlisted the help of the association -- an enthusiastic band of amateur astronomers who scramble around their countries or around the globe to time the passage of the moon and asteroids in front of stars.
The data help professional astronomers determine the shapes and orbits of asteroids, and the motions of stars around the Milky Way.
Eberhard Bredner, 55, a school administrator from Hamm, Germany, and European president of IOTA, flew to El Paso with his wife.
"Ten years ago we were in Morocco . . . and a Moroccan TV cameraman said, 'You are crazy driving from Germany to Morocco to see an eclipse.' I said, 'No, no. I will see any new one.' "
The German contingent assisting Dr. Dunham was strong yesterday when he gathered up his video gear, telescope and laptop computer and headed for Sierra Blanca, Texas, about two hours east of El Paso.
A two-stop-sign town at the side of Interstate 10, Sierra Blanca was dozing in the heat yesterday when Dr. Dunham pulled into town to scout sites for observations.
With him was IOTA volunteer Reinhold Buechner, of Oberursel, Germany. By nightfall, three more German IOTA members and their telescopes had arrived and a fifth observer was due this morning.
Except for the observers, the town's mom-and-pop motels were nearly empty, its streets deserted, and the loudest noises were the swallows feeding their young.
Meanwhile, 145 miles to the northwest, a second team was assembling near Truth or Consequences, N.M., to observe at the northern edge of the moon's shadow.
Observers can't measure the moon's shadow by simply watching it pass and marking its limits. The shadow's edges are too indistinct for that. Instead, IOTA teams spread out across both the northern and southern edges of the predicted shadow path and train their video cameras and telescopes on the eclipse itself.
They are watching for the moment when the northern and southern edges of the moon and sun line up precisely. It would be an easy job if the moon were smooth and stood still. They could simply walk across the shadow path until they found the spot where the first sunlight slipped from behind the moon.
But the moon is covered by mountains and valleys. During an eclipse, sunlight streams through the valleys on the lunar profile, while it is blocked by high mountains.
That creates a broken arc of sunlight at the rim of the moon known as "Baily's Beads" -- named for the 18th-century English astronomer Francis Baily, who first described them.
So how do IOTA observers know when they're standing at the shadow's edge?
They don't. Their job is simply to record the "bead" patterns on video tape, with a continual timing signal broadcast on shortwave radio as their soundtrack.
Later, astronomers review each tape and try to match the bead patterns to well-mapped valleys along the lunar profile.
Knowing each observer's position to within 40 feet, the time of each sunbeam to within a tenth of a second, and the identity of each lunar valley the beam illuminated, computers can work out the beam's trajectory and the precise limits of the moon's shadow.
With the shadow measured, the astronomers punch in well-defined numbers for the moon's diameter and distance, and the sun's distance, and the geometry yields a precise figure for the sun's diameter.
The sometimes ragtag operation "causes problems," Dr. Fiala admitted. "But that was the only way to get anybody to do it."
He expressed optimism about the future. "There are some people in IOTA who are quite experienced at this," he said. "The data now is quite a bit more reliable than it was 10 years ago when we first started."
Better still, new instruments designed to measure the solar diameter from space -- without the need for a solar eclipse -- are scheduled to be launched next year aboard an orbiting solar observatory.
That will eliminate the need for Dr. Dunham and Dr. Fiala's observations, but not for the data they have already collected. And that raises another problem.
The IOTA observations are so numerous, and so time-consuming to process, that many from past eclipses have yet to be analyzed by the Naval Observatory.
Dr. Fiala's application for a three-year federal grant of less than $500,000 to hire personnel to get the work done was recently rejected.
"We got very high ratings," he said. But no money.
HOW TO WATCH
The American Academy of Ophthalmology recommends against watching Tuesday's eclipse of the sun directly, even through filters endorsed by some astronomy groups. Here are some safe ways to view the eclipse indirectly:
* Punch a nail or pencil through one end of cardboard box, allowing sun's image to project inside to the other end of the box. Hold the box overhead to watch.
* Punch a 1/4 -inch hole in a piece of paper. Tape paper completely over a small, high-quality mirror. Use a lump of clay to position mirror on sill of an open, south-facing window so mirror projects sun's image onto a far wall. Keep the room dark. Don't look into the sunbeam.
* Punch a 1/8 -inch pinhole in the center of a piece of cardboard. Stand with your back to the sun and hold the cardboard so the sun shines through the hole and onto a second piece of white cardboard. Focus the image by changing the distance between pieces of cardboard.