StarDate

When it comes to Earth’s orbit around the Sun, the more things change – well, the more things change. Over the course of a year, our distance from the Sun varies by about three million miles. But that’s changing. And we’re farthest from the Sun in early July – but that’s changing, too.

The farthest point from the Sun is known as aphelion – from Greek words that mean “far from the Sun.” And we’ll reach that point on Thursday. Earth will receive about seven percent less sunlight than it does when we’re closest to the Sun, in early January.

Earth’s orbit around the Sun is elliptical – like a slightly flattened circle. But the exact shape changes over a period of about a hundred thousand years. Right now, the orbit is getting a little more circular, so there’ll be a smaller change in the distance to the Sun. After that, it’ll get a lot more stretched out. That will cause much bigger changes in Earth’s climate between the closest and farthest points.

The timing of those points also changes. About 800 years ago, aphelion happened around the time of the summer solstice in the northern hemisphere.

Now, it’s shifted a couple of weeks later. And the shift is continuing. So, about 4400 years from now, aphelion will happen at the fall equinox, in September. It’ll return to its current spot on the calendar more than 20,000 years from now.

Tomorrow: A bright star that looks like an egg.

Script by Damond Benningfield

Lots of star clusters adorn the evening skies of summer. That’s because the glowing path of the Milky Way climbs high into the sky. It’s the combined glow of millions of stars that outline the disk of the Milky Way Galaxy. So not only does it contain lots of individual stars, it hosts many star clusters as well.

But a few clusters are far from the path of the Milky Way. One example is Messier 5. It’s in Serpens Caput – the head of the serpent – a region with not much around it.

M5 is a globular cluster – a big ball packed with several hundred thousand stars. Such clusters are scattered all across the sky. Some appear in the disk, but they’re not part of the disk – they loop high above and below it.

Globular clusters are the oldest members of the galaxy. And M5 is one of the oldest – 12 billion years or older. That means its original stars were born when the universe was only about one-tenth of its present age.

Any stars that were more massive than the Sun have burned out. So the remaining original stars are smaller and fainter than the Sun. There’s evidence that a second wave of starbirth rippled through M5 well after the cluster was formed. Some of these stars can still rival the Sun – the “youngsters” of an ancient star cluster.

Messier 5 is high in the south at nightfall. Through binoculars, it looks like a fuzzy star. A small telescope reveals some of the cluster’s individual stars.

Script by Damond Benningfield

Mars and the Moon stage a spectacular encounter this evening. The Moon will slide just a fraction of a degree from the planet, which looks like a bright star.

Something we’ve learned about both of these worlds is that they have a lot of frozen water. On the Moon, it’s mixed in with the dirt and pebbles, or buried in craters that never see the Sun. On Mars, it’s also mixed in at the surface, but it’s also found in the polar ice caps, in layers of frost, and elsewhere.

On Mars, there’s even evidence of liquid water far below the surface. A study last year said there could be a lot of water buried in spaces in the rocks about 10 miles down – enough water to cover the surface of Mars in an ocean about a mile deep.

And earlier this year, scientists in Japan reported more evidence of that water. They analyzed the observations of the InSight lander, which operated for four years.

The craft listened for “marsquakes.” Sound waves from the quakes traveled through the planet. The new study looked at how different types of waves rippled through the interior. Each type of wave travels differently as it passes through different materials – rock versus water, for example. So comparing the waves revealed the likely presence of water miles below the surface. On Earth, where there’s water, there’s life. So an ocean’s-worth of water could provide a home for life on the Red Planet.

Script by Damond Benningfield

The crescent Moon and two bright pinpoints stairstep up the western sky this evening. Regulus, the star that represents the heart of the lion, is to the upper left of the Moon. And the planet Mars is about the same distance to the upper left of Regulus. The trio sets in late evening.

The largest feature on the Moon has never been seen directly by human eyes – only by robots. That’s because it’s on the Moon’s far side – the hemisphere that always faces away from us. Only a sliver of its edge can be seen from Earth. And Apollo astronauts saw only a sliver of the opposite edge.

South Pole-Aitken Basin is about 1600 miles wide – one of the largest impact features anywhere in the solar system. It probably formed when a giant asteroid slammed into the Moon soon after the Moon was born.

A Chinese lander, Chang’e 6, touched down in the basin last June. A few weeks later, it brought about four pounds of rocks and dust to Earth. Analysis of some of the samples confirmed that the basin was gouged four and a quarter billion years ago. But the dark volcanic rock that coats much of the basin formed just 2.8 billion years ago, when an ocean of magma cooled and crystallized.

Samples from the near side of the Moon indicate that it was coated with magma at the same time. So the entire lunar surface was covered by an ocean of molten rock – the side we can see, and the side we can’t.

Script by Damond Benningfield

Two decades before astronauts walked on the Moon, American movie-goers got a good preview of what the trip might look like. “Destination Moon” was the first big space movie. And it was the first to accurately explain the science and engineering of a trip to the Moon.

Co-written by science-fiction author Robert Heinlein, the movie premiered 75 years ago today. In the story, millionaire businessmen decide to finance a lunar voyage. They build a nuclear-powered rocket in the desert, then send it on its way. Problems ensue, but the crew lands on the Moon and makes it back home safely.

The backers are convinced in part by a short cartoon that accurately explained how rockets work. In fact, the cartoon was so good that NASA later used a revised version to explain rockets to the public.

Destination Moon also showed the effects of acceleration and zero-gravity. During a spacewalk, one of the crew maneuvered using a tank of oxygen; the first real American spacewalker used a similar technique. And on the Moon, the crew loped along just like the Apollo astronauts.

The film didn’t get everything right. The landscape – painted by space artist Chesley Bonestell – was too sharp and craggy. And the art director added cracks to the surface like those in a dry riverbed to add a feeling of depth.

Still, Destination Moon remains one of the most accurate movies about spaceflight – a fantastic trip to the Moon long before the real thing.

Script by Damond Benningfield

Early in its history, the inner solar system was chaotic. Violent collisions might have destroyed many small worlds, while perhaps creating others – including the Moon. It probably formed when a planet as big as Mars rammed into Earth, blasting out debris that came together to make the Moon.

A recent study says that a meteorite discovered a few years ago might be a remnant of one of the demolished worlds.

NWA 15915 was discovered in Algeria. Scientists analyzed the composition, structure, and magnetic properties of the six-pound meteorite. They concluded that it’s a rare type of meteorite – it doesn’t come from any known asteroid, planet, or moon.

But it does have some similarities to Mercury, the smallest planet and the one closest to the Sun. The study suggests that NWA 15915 might have come from a Mercury-like planet born in the same region of the solar system. The planet was demolished long ago by a giant impact. But a few fragments remain.

The findings are preliminary. So it’ll take more work to confirm that a piece of a dead planet fell atop the desert sands of northwestern Africa.

Mercury itself is near the Moon this evening. It looks like a fairly bright star to the left of the Moon. They’re quite low in the sky as twilight fades, so you need a clear horizon to spot them.

Script by Damond Benningfield

It would be fascinating to get close to Cygnus X-3. Unfortunately, it also would be deadly. The system is bathed in X-rays and ultraviolet radiation. It features powerful “jets” that blast into space like energy cannons. And it probably has a black hole – a one-way trip to oblivion.

Cygnus X-3 is in the swan, which swoops across the eastern sky on these early summer evenings. The system itself is too faint to see. In fact, we can’t see its visible light even with the largest telescopes because it’s hidden behind thick clouds of dust.

But we can see it in other wavelengths, including X-rays, gamma rays, and radio waves. Those forms of energy have allowed astronomers to piece together the system’s likely story.

Cygnus X-3 probably consists of a black hole plus a brilliant companion. The companion probably is a dozen or more times the Sun’s mass, and a couple of hundred thousand times its brightness.

The bright star is blowing huge amounts of gas into space. The black hole grabs some of the gas, which forms a spinning disk around the black hole. Some of the gas is funneled into high-speed jets that fire into space. One of those jets is aimed almost directly at Earth.

The brilliant companion star is likely to explode in the next million years or so, with its core collapsing to form another black hole. But the blast might rip the system apart – perhaps causing Cygnus X-3 to fade away.

Script by Damond Benningfield

425 years ago, a “new” star flared to life near the neck of Cygnus, the swan. The star slowly faded, then flared twice more during the 17th century. It’s remained visible ever since. And someday soon, it’ll flare up again – for the last time: It’ll explode as a supernova.

P Cygni is more than 5,000 light-years away, so it must be extremely bright for us to see it at all. And in fact, it’s one of the brightest stars in the entire galaxy – 600 thousand times brighter than the Sun.

P Cygni is so brilliant because it’s 35 to 40 times the mass of the Sun. Such a monster burns through the nuclear fuel in its core in a hurry. So even though P Cygni is only a few million years old – compared to four and a half billion years for the Sun – it’s nearing its end.

The earlier outbursts might have erupted because the star’s interior is unstable. It gets so hot that the star blasts some of the gas at its surface into space. There’s evidence that similar outbursts took place thousands of years earlier.

P Cygni is likely to explode within a couple of million years. Its core might collapse to form a super-dense neutron star – or even a black hole.

Under dark skies, P Cygni is visible to the eye alone. At nightfall, it’s in the east-northeast, close to the right of Sadr, the bright star that connects the swan’s body to its wings.

More about the swan tomorrow.

Script by Damond Benningfield

Cygnus, the swan, soars gracefully through summer nights. Its brightest star, Deneb, is in the northeast at nightfall. It marks the swan’s tail. The swan’s body stretches to the right, parallel to the horizon. The wings extend above and below, connected to the body by the star Sadr.

Cygnus contains many star clusters. The list includes several that stretch from Sadr to the south, roughly along the swan’s neck. The clusters contain a few dozen to a few hundred stars. All of them are young – no more than about 10 million years old. And many of them are especially hot, bright, and massive.

The clusters are indirectly related. They belong to much larger collections of young stars, plus the raw materials for making more stars. A “wave” passed through that region of the galaxy, squeezing gigantic clouds of gas and dust. Clumps of material within the clouds collapsed, forming stars.

Over the next few million years, the most massive stars will explode as supernovas. Shockwaves from the blasts may compress more pockets of gas and dust, creating more stars. But the clusters themselves won’t survive much longer – at least on the galactic timescale. They’ll be pulled apart by the gravity of the surrounding stars and clouds, so their stars will go their separate ways.

The clusters are easy targets for good binoculars. One is just a whisker from Sadr. Several others trail off to the right – sparkly decorations for the swan.

Script by Damond Benningfield

The Royal Observatory at Greenwich has been one of the most important skywatching sites in history – not so much for what it told us about the stars, but for its role right here on Earth. Its location marked the starting point for measuring longitude – the position east and west on the globe. It also marked the time standard for the entire world: Greenwich Mean Time.

The observatory was established on today’s date in 1675, by King Charles II. It was built on a hill near London, overlooking the Thames.

Greenwich was created to provide highly accurate maps of the stars, and to measure the motions of the Sun, Moon, and planets. The work was designed to help sailors determine their longitude. Establishing longitude at sea was extremely difficult – and dangerous; many ships crashed because their navigators didn’t know where they were.

The observations also played a key scientific role: they helped confirm that the motions of the Sun, Moon, and planets were governed by Isaac Newton’s laws of gravity.

In 1833, the observatory began a “time service.” It dropped a ball from a tall pole at precisely 1 p.m. That allowed mariners to set their clocks before they sailed. Greenwich later transmitted the time to the whole country by telegraph, then radio.

The observatory was moved in the 20th century, and closed in 1998. Today, the Greenwich site is a museum – preserving an important part of world history.

Script by Damond Benningfield