StarDate

If you miss this morning’s eclipse, there’s always another one. It might not be a solar eclipse, and it might not be visible from your location. But every year, four or five eclipses are visible from somewhere on Earth.

The next one comes up in two weeks — a partial lunar eclipse. It’ll be visible from a good bit of the world, but not much of the United States. The next great eclipse will roll around on April 8th — a total solar eclipse that will track across the U.S.

Eclipses occur thanks to the relative motions of Earth, Sun, and Moon. When the geometry is right, the Moon passes directly between Earth and the Sun, creating a solar eclipse. And when the Moon is on the opposite side of our planet, it can pass through the shadow of Earth, creating a lunar eclipse.

But total solar eclipses are doomed. Right now, the Moon and Sun appear almost exactly the same size in our sky. But when the Moon is a little farther from Earth than average, it’s not big enough to black out the Sun. That leaves a ring of sunlight around the Moon — an annular eclipse like today’s.

But the Moon is moving away from Earth at about an inch and a half per year. Over the next half a billion years or so, that will add up. The Moon will move so far that it will never be big enough to cover the Sun. Total solar eclipses will disappear — replaced by ever-brighter annular eclipses as the Moon recedes into the distance.
 

Script by Damond Benningfield

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An annular solar eclipse isn’t quite the same spectacle as a total eclipse. The Moon doesn’t completely cover the Sun, so we don’t see the corona — the Sun’s hot but faint outer atmosphere. But it still offers a beautiful experience. The sky looks dusky, the air cools, and leafy trees produce shadows that look like rings. And viewed with the right safety gear, the eclipse itself is impressive, with a “ring of fire” encircling the Moon.

And that’s what a narrow band of the United States will see tomorrow. The path of “annularity” — when the Sun completely encircles the Moon — will sweep ashore on the coast of Oregon at about 9:13 local time. It’ll then slide southeastward, passing across six other states and several major cities before heading into the Gulf of Mexico, around noon.

Solar eclipses take place when the new Moon glides between Earth and the Sun. If the Moon is at its average distance from Earth or closer, it produces a total eclipse. For this eclipse, though, the Moon will be about 8,000 miles farther than average. So, at most, it’ll block 91 percent of the Sun’s disk.

Other than Hawaii, the rest of the U.S. will see a partial eclipse, with the Moon blocking a smaller fraction of the Sun. The closer to the path of annularity, the greater the extent of the eclipse.

Please look at the eclipse only with proper eye protection, or follow it online — as a ring of fire encircles the Moon.
 

Script by Damond Benningfield

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An annular solar eclipse will grace American skies on Saturday. Skywatchers along a narrow path from Oregon to Texas will see the Sun encircle the Moon with a bright “ring of fire.” The rest of the country will see a partial eclipse, with the Moon covering a smaller portion of the Sun.

Earth isn’t the only world to see solar eclipses.

From the Moon, the Sun is eclipsed when Earth crosses in front of it. From Earth, we see that as a lunar eclipse — the Moon turns orange or red as it passes through Earth’s shadow. From the Moon, you’d see a beautiful ring of color around Earth.

Phobos and Deimos, the moons of Mars, are so small that they can’t completely cover the Sun. But they periodically cross in front of it, blocking part of its light.

Jupiter’s big moons all create total eclipses. Jupiter is farther from the Sun than Earth is, so the Sun appears just one-fifth as wide. so it’s easier to cover up. And Jupiter has only a slight tilt on its axis, so its moons come into alignment with the Sun more often.

For most of the moons, however, the geometry is such that they cover both the body of the Sun and the corona — the Sun’s faint outer atmosphere.

Callisto, the most distant of the big moons, is five times farther from Jupiter than the Moon is from Earth. But it’s also about 40 percent wider than the Moon. That makes it the closest match for an Earth-like solar eclipse — providing the best view of the corona.
 

Script by Damond Benningfield

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There’s nothing like a big public failure in your first days on the job to inspire confidence. But that’s what NASA faced 65 years ago today, when it was just 10 days old. It launched its first spacecraft, Pioneer 1. The probe was aiming for the Moon. It never got there.

To be fair, the Pioneer project was created by the Air Force, which conducted the launch. It had already launched the first craft in the series, known at the time as Able-1, in August of 1958. It blew up during launch.

The project was turned over to NASA when it came into existence, on October 1st. The name was changed to Pioneer. And on October 11th, Pioneer 1 headed for the Moon. The plan called for it to enter lunar orbit. It would beam pictures and other observations of the Moon back to Earth. And on the way, it would provide details on Earth’s recently discovered radiation belts.

But the second stage of the booster rocket shut down early, so Pioneer wasn’t moving fast enough to escape Earth’s gravity. It did get about a third of the way to the Moon — much farther from Earth than any spacecraft had gone before. And it made good measurements of the radiation belts, as well as the magnetic field between Earth and the Moon. The craft was destroyed when it plunged back into Earth’s atmosphere.

Pioneer 2 was launched a month later. It failed, too. In fact, NASA didn’t score a successful lunar mission until 1964 — six years after the agency’s birth.
 

Script by Damond Benningfield

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A year ago yesterday, lightning bolts across parts of Europe and Asia changed direction. The switch was caused by an exploding star two billion light-years away — the possible birth of a black hole.

Gamma-ray burst GRB 221009A was first seen by space telescopes. They recorded a blast of gamma-rays, the most energetic form of light. The outburst lasted for more than five minutes — far longer than most outbursts. And it was the most powerful ever seen. Astronomers nicknamed it the BOAT — brightest of all time.

Other telescopes in space and on the ground recorded the event in every form of light. It was even bright enough to see through small telescopes.

Charged particles from the outburst flooded Earth’s upper atmosphere. They disrupted part of the ionosphere — a layer with an electric charge. That caused the ionosphere to expand, and knocked out some radio communications. Particles that flew deeper into the atmosphere caused changes in lightning activity, and even set up mild currents in the ground.

The outburst probably erupted when the core of a huge star collapsed to form a black hole. Magnetic fields created a swirling disk around the black hole. The disk powered enormous beams of energy that sliced into space from the poles of the dying star. Earth lined up along one of those beams, so we got a powerful dose of energy — announcing the birth of a black hole.
 

Script by Damond Benningfield

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The Moon lines up with a bright star and planet early tomorrow. Regulus, the heart of Leo, is close to the lower right of the Moon. And the planet Venus — the “morning star” — is to the lower right of Regulus. They climb into good view by about 4:30, and are high in the sky at dawn.

It’s an impressive alignment. But the Moon is heading toward an even more impressive alignment on Saturday: a solar eclipse. The Moon will pass across the face of the Sun, blocking most of the Sun’s light.

But it won’t be a total eclipse. The Moon will be thousands of miles farther from Earth than average. So it won’t appear quite big enough to completely cover the Sun. Instead, the Sun will encircle the Moon with a “ring of fire” — an annular eclipse.

The United States will be in the middle of the action. The lunar shadow will sweep ashore in Oregon at 9:13 local time. It then will slide to the southeast, passing across six other states and several major cities. It will head into the Gulf of Mexico around noon local time, near Corpus Christi, Texas.

The period when sunlight fully encircles the Moon will last up to 5 minutes, 17 seconds. And the path of annularity will be up to 116 miles wide. At most, the Moon will cover about 90 percent of the Sun’s disk.

The rest of the U.S. — except Hawaii — will see a partial eclipse, with the Moon blocking a smaller fraction of the Sun.

More about the eclipse later in the week.
 

Script by Damond Benningfield

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One of the first things every astronomy student learns is the H-R diagram — a plot of stars based on their temperature and true brightness. From that plot, astronomers can learn a lot about both individual stars and all stars.

One of the diagram’s creators was Ejnar Hertzsprung — the “H” in H-R. He was born 150 years ago today, in Denmark.

Hertzsprung studied chemical engineering, and worked in that field for several years. But he was more interested in stars. So even though he didn’t have any training, he became an astronomer.

Hertzsprung quickly found a relationship between a star’s brightness and its color or temperature. When he plotted this relationship for many stars, he discovered some interesting patterns.

Most of the stars fell along a gentle curve, known as the main sequence. Such stars are in the prime of life, as the Sun is, steadily “burning” the hydrogen in their cores to make helium. Other stars fell to the upper right of this line — giant stars beyond the main sequence. And still others fell to the lower left — the faint corpses of stars.

Among other things, the diagram tells astronomers how stars evolve. And plotting an individual star reveals its mass, its stage of life, and more.

Hertzsprung published his results in 1911. American astronomer Henry Russell reported similar findings two years later. Today, their work forms the Hertzsprung-Russell diagram — one of the most important tools in astronomy.
 

Script by Damond Benningfield

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The Moon is closing in on the brilliant “morning star.” They’re far apart tomorrow, but will stand almost side by side on Tuesday. As a bonus, the heart of the lion joins them.

The morning star is Venus, the second planet from the Sun. Since Earth is the third planet, Venus has a limited range of motion across our sky. It’ll reach its farthest point from the Sun in a couple of weeks. That’ll give us several good hours to enjoy it before it’s overpowered by twilight.

Venus spends only half of its time in the morning sky. It spends the other half in the evening sky — an average of about nine and a half months in each phase. But it’s not in view for all of that time. It’s visible for about 263 days in the morning sky, then vanishes for about 50 days as it passes behind the Sun. It spends another 263 days in the evening sky, then disappears for about a week as it passes between Earth and the Sun.

Venus will remain in view in the morning sky through the rest of this year and well into 2024. It should disappear by early May, then return to view a couple of months later — as the evening star.

For now, look for Venus in the east at dawn. Regulus, the heart of the lion, is close by. The Moon will be high above them tomorrow, closer on Monday, and quite close on Tuesday. The Moon will slide away after that — heading toward another spectacular encounter, on Thursday. We’ll tell you more about that next week.
 

Script by Damond Benningfield

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Pollux, the brighter “twin” of Gemini, stages a beautiful encounter with the Moon tonight. They’re quite close as they climb into good view, by 1:30 or 2. Castor, the other twin, stands above them.

Pollux looks bright for a couple of reasons. For one thing, it’s a close neighbor — just 34 light-years away. And for another, it really is bright — more than 30 times the Sun’s brightness. That’s because it’s a giant — a bloated star that’s nearing the end of its life. In fact, Pollux is the closest giant to the Sun.

A star becomes a giant when it’s converted the original hydrogen in its core to helium. It briefly burns through a shell of hydrogen around the core, then starts burning the helium. Pollux appears to be in the helium-burning phase.

Those changes make the core hotter. In turn, that makes the star’s outer layers puffier. Now, Pollux is nine times wider than the Sun. As it continues to evolve, it’ll get even bigger and brighter.

Pollux is only about 725 million years old, compared to four and a half billion years for the Sun. It became a giant at a much younger age because it’s twice as massive as the Sun, and heavier stars consume their nuclear fuel more quickly.

Pollux will remain a giant for a few hundred million years. Then it will expel its outer layers, leaving only its hot, dead core. That whole process will play out in the Sun as well — beginning in about six billion years.
 

Script by Damond Benningfield

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It’s not often that a single star transforms our view of the universe. But it happened with a star that was photographed 100 years ago tonight. The star proved that there’s more to the universe than just the Milky Way Galaxy — much more.

The star is known today as M31-V1. It’s in M31, the Andromeda Galaxy. A century ago, M31 was called the Andromeda Nebula. Many astronomers thought it was a mote of matter inside the Milky Way. Others thought it was a separate galaxy — an “island universe” of billions of stars.

On the night of October 5th of 1923, Edwin Hubble snapped a picture of a small segment of M31. He identified three stars that had grown much brighter since the last time he looked at them. Follow-up observations showed that one star’s brightness changed from night to night.

That star was a Cepheid — a supergiant star that’s unstable. It pulses in and out like a beating heart. The duration of the pulses revealed the star’s true brightness. And from that, Hubble calculated its distance — a million light-years. Studies of other Cepheids confirmed the distance. So M31 had to be well beyond the Milky Way — vastly expanding our understanding of the universe.

Modern observations put the distance to M31 at two-and-a-half million light-years. And the galaxy is visible to the unaided eye. Under dark skies, it looks like a faint smudge of light, in the north-northeast at nightfall.
 

Script by Damond Benningfield

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