Stardate

Most of the United States will see a rare astronomical alignment late today: a partial solar eclipse. And across a narrow swath of the western U.S., the eclipse will be even rarer — the Moon will be completely enfolded within the Sun’s disk, leaving a thin but bright ring of sunshine around the Moon.

A similar eclipse took place on May 10th, 1994, although this one is shifted about a third of the way westward around the globe. And in fact, the two eclipses are like cousins — both are members of an eclipse “family,” known as a Saros.

We’re all familiar with the Moon’s month-long cycle of phases. But the Moon also has other cycles — its distance from Earth and its relation to the Sun’s path across the sky, among others. Three of these cycles overlap every 6,585-and-a-third days — a bit more than 18 years. When they overlap, there’s an eclipse.

Each Saros cycle lasts for centuries. It begins with partial eclipses that are visible from one of the poles, then moves across Earth’s disk with total or annular eclipses, then finishes with more partial eclipses at the opposite pole.

Today’s eclipse is part of Saros cycle 128. The first eclipse in the series took place in the year 984, and the last will be in 2282.

The annular part of today’s eclipse begins around 6:24 p.m. Pacific Time, at the California-Oregon border. It ends at 8:39 p.m. Central Time, as the Sun and Moon set over western Texas.

Script by Damond Benningfield, Copyright 2012

More about the eclipse

For more skywatching tips, astronomy news, and much more, read StarDate magazine.

The Sun and Moon will team up to produce a brilliant ring of fire across the western United States tomorrow afternoon — an annular solar eclipse.

The eclipse occurs because the Moon will pass directly between Earth and Sun, covering the Sun’s disk. But the Moon is near its farthest point from Earth, so it won’t be quite big enough to cover the entire disk. Instead, a thin ring of sunlight will encircle the Moon.

The annular eclipse will be visible across a narrow strip of Earth’s surface that begins in China, wraps across the Pacific Ocean, and ends in the western United States. From the U.S., the path of the eclipse begins at the California-Oregon border around 6:24 p.m. Pacific Time. It then sweeps to the east-southeast, ending over western Texas when the Sun and Moon set at 8:39 p.m. Central Time. The annular portion of the eclipse will last up to about five minutes.

Most of the rest of the United States will see a partial eclipse — the Moon will cover a fraction of the Sun, but it won’t be completely enfolded within the Sun’s disk.

One note of caution, though. While the Moon will hide most of the Sun’s disk, the visible ring is still bright enough to cause eye damage. To view the eclipse, look through dark welder’s glass, or build a “projector” by poking a pinhole in the side of a cardboard box and watching the sunlight projected inside the box — the progress of a solar eclipse.

We’ll have more about the eclipse tomorrow.

Script by Damond Benningfield, Copyright 2012

More about the eclipse

For more skywatching tips, astronomy news, and much more, read StarDate magazine.

A solar eclipse is a happy scientific coincidence. As seen from Earth, the Moon and Sun are the same size in the sky. So when the geometry is right, the Moon can cover up all or most of the Sun’s disk.

An eclipse of the “most” variety is coming up on Sunday afternoon for parts of the western United States. It’s called an “annular” eclipse because the Moon won’t cover up all of the Sun. Instead, it’ll leave a thin but bright ring of sunlight around the Moon.

Solar eclipses occur when the new Moon crosses directly between Sun and Earth. They don’t happen at every new Moon, though, because the Moon’s orbit around Earth is tilted with respect to Earth’s orbit around the Sun. So most months, the Moon passes a little above or below the Sun as seen from Earth.

But two or three times a year, on average, the geometry is just right, creating an eclipse. Sometimes it’s total, sometimes it’s annular, and sometimes it’s just partial — the Moon obscures only a part of the Sun’s disk.

The “coincidence” part comes in because of the relative sizes of the Moon and Sun in the sky. The Sun is 400 times wider than the Moon, but it’s also 400 times farther, so they take up the same amount of sky.

The Moon is moving farther from Earth, though — at about an inch-and-a-half a year. So in a few hundred million years the Moon will be too far away to ever completely cover the Sun — and the spectacle of a total solar eclipse will disappear.

Script by Damond Benningfield, Copyright 2012

For more skywatching tips, astronomy news, and much more, read StarDate magazine.

A solar eclipse is coming up on Sunday afternoon and evening for most of us in the United States. Most will see a partial eclipse, with the Moon covering up only a portion of the Sun’s disk. But across a 200-mile-wide path from California to Texas, the Moon will fit inside the solar disk, surrounding the dark Moon with a ring of fire — a configuration known as an annular eclipse.

The timing of the eclipse is known down to the second. In fact, the motions of Earth, Moon, and Sun are so well understood that astronomers have already prepared equally precise timings of eclipses that won’t take place for centuries.

Such precise timing hasn’t always been the case, though. In fact, an eclipse forecast made 250 years ago helped turn a French astronomer into the toast of French science and society.

Madame Nicole-Reine Lepaute was the court astronomer to King Louis XV. In 1762, she wrote a paper predicting the circumstances of an eclipse that would be visible from Paris two years later. Her predictions were right on target.

Her husband, the king’s clockmaker, built a special clock to celebrate her accomplishment. It included a diagram of the eclipse from her paper, plus a small figure of Urania, the muse of astronomy.

The clock is still around today — not in Paris, but on the campus of the University of Arizona — marking the never-ending motions of Earth around the Sun.

We’ll have more about Sunday’s eclipse tomorrow.

Script by Damond Benningfield, Copyright 2012

For more skywatching tips, astronomy news, and much more, read StarDate magazine.

WHEELWRIGHT: My name is Brian Wheelright, I am a PhD student in optical sciences at the University of Arizona, and I work at the Steward Observatory Mirror Lab. And what we’re looking at today is part of our solar lab, where were trying to get solar energy to be cheap and competitive with fossil fuels.

For three decades, the Steward mirror lab has been making some of the world’s largest telescope mirrors. Early this year, for example, it was working on four mirrors that, if turned on edge, would each be taller than a two-story building. It’s an enterprise that requires a lot of time, money, and expertise.

A few years ago, lab founder Roger Angel began turning that expertise to a new project: developing a cheaper way to generate solar power.

Technicians heat panes of ordinary window glass until they get soft, then mold them to a curved shape. They then mount the panes on a test stand in an abandoned swimming pool on the University of Arizona campus. The glass reflects sunlight into a special collector — a combination of lenses and solar cells that’s more efficient than most solar power systems today.

The rig borrows from techniques used to build telescopes — but with some differences.

WHEELWRIGHT: In telescope tracking, you have to be precise, and so it’s very expensive. Here, we just want it strong enough so that it survives the wind, but not so strong that it becomes expensive. Kind of a Sun telescope — an energy telescope is what they used to call this.

A larger test project is under construction. Like all telescopes, it’s designed to capture the light of a star — in this case our own star: the Sun.

Script by Damond Benningfield, Copyright 2012

For more skywatching tips, astronomy news, and much more, read StarDate magazine.