StarDate

Anchorage, Alaska, isn’t quite the “land of the midnight Sun.” Tonight, there are about five hours between sunset and sunrise. But it is a land of midnight sunlight, because twilight never completely fades.

Twilight is the transition between day and night. Earth’s atmosphere scatters sunlight from the dayside to the fringes of the nightside.

But when, exactly, does twilight end? When is the sky really dark? As you might expect, astronomers have their own definition.

Astronomical twilight begins or ends when the Sun is 18 degrees below the horizon – about twice the width of your fist held at arm’s length. That’s when the sky’s as dark as it’s going to get.

Because of the Sun’s motion, astronomical twilight lasts a minimum of about an hour and 10 minutes. But because the Sun usually rises and sets at an angle, twilight can last a good deal longer.

During much of June and July, when the days are longest, twilight for much of the northern hemisphere lasts all night. The Sun never drops far below the horizon, so even though it’s out of sight, its light never disappears. So the people of Anchorage need some good blackout curtains to get a dark night’s sleep.

If you want a few hours of darkness, head south – someplace like Miami Beach. It gets a full seven hours between evening and morning twilight – hours that might be illuminated by the neon lights of South Beach, but not by the Sun.

Script by Damond Benningfield

Cold hands, warm heart. Still waters run deep. Feed a cold, starve a fever. The list of pithy old sayings goes on and on. But here’s a new one for you: long Sun, short Moon. It means that the full Moon does the opposite of what the Sun does. So when the Sun is in the sky for a long time, the full Moon makes itself scarce.

And that’s the case tonight. The full Moon of June has many names, including Flower, Strawberry, and Honey Moon. But it’s also known as the Short-Night Moon. That’s because it’s in view for less time than any other full Moon of the year.

From the northern hemisphere, the Sun passes highest across the sky at this time of year, and remains in view longest.

But because of the trail they follow, the Sun and the full Moon are like opposite ends of a seesaw. When one is up, the other is down. So right now, the full Moon passes low across the sky. And it rises around sunset and sets around sunrise.

The difference is greater as you go farther north. From Miami, the Sun will be above the horizon for almost 14 hours today, with the Moon popping into view for less than 10 and a half hours. From Duluth, Minnesota, it’s almost 16 hours versus less than eight hours. And from Anchorage, the Sun graces the sky for 19 hours, with the Moon showing up for a stingy hour and a half – an especially short appearance for the Short-Night Moon.

More about night and day tomorrow.

Script by Damond Benningfield

On average, Antares is the fifteenth-brightest star in the night sky. It looks like an orange-red gem at the heart of the scorpion. Tonight, though, it looks a little feeble. It hasn’t gotten any fainter. Instead, it stands especially close to the almost-full Moon. It looks a little washed out in the powerful moonlight.

Antares really is one of the more impressive stars in the galaxy. It’s probably 12 to 15 times the Sun’s mass, hundreds of times the Sun’s diameter, and tens of thousands of times its brightness.

Ere long – at least on the astronomical timescale – Antares will get even more impressive – but only for a while. Sometime in the next million years or so, it’s expected to blast itself apart in a titanic explosion – a supernova. For a few months, it’ll shine brighter than the combined glow of billions of normal stars.

As it fades, its demolished outer layers will form a nebula – a colorful cloud of gas and dust, energized by the blast and by the decay of radioactive elements. Over thousands of years, the nebula will expand and fade. That will leave only the star’s dead core – a tiny, super-dense corpse known as a neutron star – the almost-invisible remnant of the mighty heart of the scorpion.

Antares stands close to the Moon at nightfall, and the Moon will move closer to it during the night – washing out this brilliant star.

We’ll have more about the Moon tomorrow.

Script by Damond Benningfield

The rings of Saturn are among the most beautiful features in the solar system. They’re wide enough to span the distance between Earth and the Moon. And they’re made of bits of ice and dust – like tiny “moonlets” orbiting the giant planet.

The first person to propose that idea was Giovanni Cassini, who was born 400 years ago today. He worked in several fields, from astrology to engineering.

But Cassini’s greatest love was astronomy. He became director of the Paris Observatory, and studied the Moon and planets – especially Saturn. He discovered four of its moons, plus a dark “gap” between its two most prominent rings. That gap was named the Cassini Division in his honor.

Although it looks empty, the gap contains a smattering of dark particles. It probably was mostly cleared out by a small moon that orbits inside the gap. Its gravity pushes ring particles away.

A study a few years ago said the gap could have started much smaller than it is today. Over the past few million years, the moon moved closer to Saturn, clearing a wider region. But now, it’s moving away from Saturn. So the Cassini Division could close up – in about 40 million years.

Saturn is in the southeast at dawn, and looks like a bright star. We’re viewing the rings almost edge-on, so there’s not much to see even through large telescopes. But the view will improve over the coming months – revealing both the rings and the dark divide between them.

Script by Damond Benningfield

The world has known many great astronomers, but only a few great astronomy dynasties. One of those celebrates an anniversary tomorrow – the birth of its patriarch 400 years ago.

Giovanni Cassini was born in a small Italian village just across the border from Nice, France. Cassini the first, as he’s often called, was the first of four generations of Cassini astronomers. And he definitely was the most productive.

He’s best known for his discovery of a gap in the rings of Saturn. It’s named the Cassini Division in his honor. Later, Cassini and his son Jacques theorized that the rings were made of “swarms of tiny satellites” moving at different speeds. They understood the truth many years before it was actually confirmed.

More about Cassini and Saturn’s rings tomorrow.

Cassini made many other contributions to astronomy. He was among the first to realize that light travels at a limited speed, although he didn’t believe the speed could be calculated. And he developed a law to explain why we always see the same side of the Moon: The Moon takes the same amount of time to orbit Earth as it does to complete one turn on its axis.

For most of his career, Cassini was director of Paris Observatory. His descendants kept the job in the family for more than 120 years. Cassini the fourth ended the streak in 1793 – when he left the observatory to write the “stellar” history of this astronomical dynasty.

Script by Laura Tuma

There was a lot of talk earlier this year about an asteroid with the highest odds of hitting Earth ever calculated. The chances of an impact by asteroid 2024 YR4 in December of 2032 peaked at about three percent. The asteroid is big enough to cause major damage if it hit. As astronomers tracked it a little longer, though, they realized that’s not going to happen – the asteroid will miss by at least 60,000 miles.

Such close calls aren’t rare. Hundreds of asteroids pass within a few million miles of Earth every year. This weekend, in fact, asteroid 2014 LL26 will miss us by just two million miles. Its orbit overlaps Earth’s orbit, so it passes close fairly often. And it could hit our planet at some point in the future – though not anytime soon.

Astronomers have discovered more than 27,000 potentially hazardous asteroids. And they discover more all the time. The one that caused the kerfuffle earlier this year, in fact, was just discovered in December.

Several automated searches scan the sky every night. Those efforts yield thousands of asteroids every month. But it takes observations over a period of days or weeks to give us a good measurement of an asteroid’s orbit.

Most of the new discoveries are in the asteroid belt, between the orbits of Mars and Jupiter. But some are close enough to keep an eye on – potential hazards to life on Earth.

Script by Damond Benningfield

The Pilbara region of Western Australia is big, dry, and wide open. And it may contain the oldest cosmic “scar” on Earth: an impact crater gouged three and a half billion years ago.

Scientists discovered evidence of the crater during a brief expedition in 2021. They found some rock formations called shatter cones. Some of the cones are as tall as a house. The only known way to make them is in giant collisions with space rocks.

Follow-up work last year revealed many more of these formations. The cones were found in a rock layer that’s miles wide, but only a few dozen feet thick.

The layer also contains tiny “beads” that formed when molten rock was blasted high into the sky. The flight through the air sculpted droplets of the molten rock into balls.

Geologists found that the layer formed three and a half billion years ago, so that’s when the impact must have taken place – more than a billion years earlier than the previous record holder.

The asteroid could have been miles wide, and blasted a crater more than 60 miles across. The effects of the collision would have been felt around the world.

In fact, researchers say the impact could have helped shape the world. Major asteroid impacts could have traveled deep, churning things up far below the surface. That could have created the “seeds” that gave birth to the continents when Earth was young.

We’ll talk about potential future impacts tomorrow.

Script by Damond Benningfield

You can’t tell just by looking, but the universe undergoes constant change. Stars explode. Quasars flare up. Asteroids zip past Earth. And soon, astronomers will be able to generate super-high-definition movies of those changes almost every night of the year. That’s because a new telescope dedicated to “time-domain” astronomy is about ready to take its first looks at the heavens.

The telescope is the centerpiece of the Vera Rubin Observatory. It’s named for an astronomer who provided strong evidence for the existence of dark matter. It’s atop an 8700-foot mountain in Chile.

The telescope’s main mirror, which gathers and focuses starlight, is 8.4 meters across – almost 28 feet. It has a wide field of view, allowing it to photograph the entire southern sky every few nights. It’ll record its observations on the largest digital camera ever built – 3200 megapixels.

Astronomers will use those observations to learn more about dark energy and dark matter, and to map the Milky Way Galaxy. And they’ll watch for things that change. They’ll discover asteroids and comets – both close to Earth and deep in the outer solar system. They’ll see novas, supernovas, and other brilliant flare-ups. And the observatory will send out immediate notices of each new outburst, allowing other astronomers to make detailed follow-up observations – learning much more about our constantly changing universe.

Script by Damond Benningfield

Before astronauts could land and walk on the Moon, NASA had to be sure they could do three things: live in space for days at a time, catch and link up with other spacecraft, and work outside their ship. And it took stabs at two of those goals 60 years ago today, with a mission called Gemini 4.

  [AUDIO: 3, 2, 1, Ignition … Liftoff …]

Astronauts James McDivitt and Ed White were scheduled to spend four days in space. That was longer than the first seven American missions combined.

And White would make the first American spacewalk. He’d float outside the cabin for a few minutes, using a small “gun” of compressed air to move around.

And three and a half hours after launch, it was time to get started:

CAPCOM: Gemini 4, Hawaii capcom. We just had word from Houston, we’re ready to have you get out   whenever you’re ready. Okay, my feet are out. … Okay, I’m out.

The spacewalk went well – very well. [WHITE: I feel like a million dollars!] In fact, it went so well that White didn’t want it to end.

HOUSTON: The flight director says get back in!

     McDIVITT: This is Jim, you got any message for us?

     CAPCOM: Gemini 4, get back in!

     McDIVITT: Okay…

Actually working during a spacewalk turned out to be a lot harder than White had made it look. It took several more missions to work out the kinks. But the success of Gemini 4 helped make it possible for astronauts to walk on the Moon just four years later.

Script by Damond Benningfield

If you step outside at dawn on June third of 2033, you’ll see the planet Venus standing due east – the brilliant “morning star.” But if you don’t want to wait that long to experience that beautiful view, then take a look at dawn tomorrow – Venus will be standing in the same spot in the sky. Not only that, it stands at that spot in the sky every eight years.

In fact, anytime you see Venus – whether as the morning star or evening star – you can find it at that same spot eight years later. That’s because there’s a near-“resonance” in the orbital cycles of Earth and Venus. Venus completes 13 orbits around the Sun for every eight orbits that Earth makes. The ratio isn’t exact, but it’s quite close.

Thanks to that, Venus follows five repeating cycles across our sky. It’s like the planet is a toy train on a looping, winding track. It hits every point along the track with every cycle. If you plot that motion from the perspective of Earth, it traces out a flowing pattern like five rose petals.

The clockwork precision makes it easy to predict the entire sequence of Venus’s appearances. We know that it always remains in view in the morning sky for about 263 days, disappears for about 50 days, then moves into the evening sky.

So if you miss the view of Venus tomorrow, or the next day, or the day after that, just mark it in your calendar – and look for it in the same location eight years later.

Script by Damond Benningfield