StarDate

The Moon has a brilliant companion tonight. Jupiter stands to the lower right of the Moon as night falls. Although it looks like a bright star, it’s actually the largest planet in the solar system — about 11 times Earth’s diameter and more than 300 times Earth’s mass.

Earth and Jupiter were at their closest just six weeks ago — about 410 million miles apart. Now, Earth is pulling away from the giant planet, so the gap between them is getting wider. Jupiter is about 24 million miles farther than it was in early May. And over the coming months, the gap will get wider still.

As that happens, Jupiter will grow a bit fainter. Right now, it’s the third-brightest object in the night sky. It’s outshined only by the Moon and by the planet Venus, the dazzling “evening star.” But Jupiter is already a few percent fainter than it was in early May. And by this time next month, it will have lost the “third-brightest-object” title to Mars — both because Jupiter is fading and because Mars is getting brighter.

A planet is at its best when it’s closest to Earth, which comes as it lines up opposite the Sun in our sky. Jupiter was at opposition six weeks ago. Saturn will be at opposition next week; we’ll have more about that tomorrow. Saturn is smaller and farther than Jupiter, though, so it won’t shine as brightly. And Mars will reach its opposition in about a month — allowing it to briefly outshine the giant planet Jupiter.

Script by Damond Benningfield

 

The Moon forms a wide, flat triangle with two bright lights tonight. The brilliant planet Jupiter is to the lower left of the Moon as night falls. At the same time, the star Spica, the leading light of Virgo, is a bit farther to the lower right of the Moon.

Over the past six decades, dozens of spacecraft have orbited the Moon. One of the first things they discovered is that the Moon’s gravity is “lumpy” — some parts of the Moon exert a stronger gravitational pull than others. That caused early probes to move up and down like horses on a merry-go-round as they orbited the Moon. They also crashed into the Moon much earlier than planned.

The “lumps” are known as mascons — mass concentrations. The material in these lumps is denser than the surrounding crust, so its gravitational pull is a bit stronger.

Most of the mascons are associated with “basins” — craters that are filled with volcanic rock. The impacts that carved the craters melted some of the lunar material. When the rock cooled, it was denser than the surrounding material, giving it a stronger gravitational pull.

Probes that measured lunar gravity found two sets of rings around many of the mascons. The inner rings are made of lighter material, so their gravitational pull is weak. And the outer rings contain piles of rock that was blasted out by the impacts that gouged the craters, so they have a stronger pull. Those rings help contribute to the Moon’s lumpy gravity.

Script by Damond Benningfield

For most of the year, the Sun’s path across the sky changes from day to day. It moves northward for about half of the year, and southward for the other half. But for a few days in June and December, the Sun’s path doesn’t move at all — it stands still.

Those times are the solstices, when the Sun appears farthest north or south for the year. And today marks the summer solstice — the start of summer in the northern hemisphere. It’s also the longest day of the year in the north — the longest gap between sunrise and sunset.

The Sun’s north-south motion across the sky is a bit of an illusion. The Sun doesn’t move at all. Instead, it’s our viewing angle that changes — the result of Earth’s tilt on its axis.

Our planet leans a bit with respect to its orbital path. So as Earth moves around the Sun, the poles take turns dipping toward and away from the Sun. Right now, the north pole is tipped sunward, so it’s summer in the northern hemisphere. And the pole itself is bathed in constant daylight.

At the winter solstice, in December, the north pole will tilt away from the Sun. It’ll be blanketed by darkness, while the south pole enjoys ’round-the-clock daylight.

Between those dates, the Sun rises and sets at different points along the horizon — far north in June, and south in December. For a few days around the solstices, though, its rising and setting points hardly move — the Sun comes to a standstill for the change of seasons.

Script by Damond Benningfield

For the people of southern Egypt, the summer solstice once was a time to gather. The monsoon season began about then, bringing enough rain to create shallow lakes. People gathered to water their cattle, and to offer sacrifices to the gods. So it was important to know when the solstice was coming.

At one site, people built what may have been a calendar — a ring of small stones. Some of the stones pointed north, while others aligned with the sunrise on the solstice. And some may have pointed to the rising points of bright stars.

Nabta Playa is near the southern border of Egypt, about 65 miles west of the Nile River. Excavations have shown that the site was inhabited as early as 7,000 years ago. People built structures and dug wells. They also dug burial pits and quarried sandstone. Some of the rocks were used in the calendar circle and the possible sightlines to the stars.

These structures weren’t on the scale of Stonehenge. Most of the rock markers were only a few inches to a few feet tall. And the calendar circle spanned only about 12 feet. Yet that would have been enough to sight the rising of the Sun and stars at key times of the year.

The rocks may have been aligned with Arcturus, Sirius, and Alpha Centauri — three of the brightest stars in the night sky. These alignments could have helped the people of Nabta keep time with the seasons — and with the life-giving rains of summer.

More about the summer solstice tomorrow.

Script by Damond Benningfield

Most of us spend a fair amount of time trying to get rid of the dust in our homes. But some scientists at NASA spend time trying to catch dust. They use high-altitude aircraft to catch particles of dust that come from outside Earth’s atmosphere.

Thousands of tons of cosmic dust enter the atmosphere every year. Most of the particles are grains from comets and asteroids. Collecting them reveals more about these big chunks of ice, rock, and metal. And since comets and asteroids are leftovers from the birth of the planets, the dust grains also reveal a lot about conditions in the early solar system.

So NASA has been flying dust collectors aboard two types of aircraft for more than 30 years — modified versions of a spy plane and a Korean War-era bomber. The craft reach the stratosphere — altitudes of 50,000 feet or higher. Even there, most of the particles they pick up come from Earth — bits of sand, pollen, volcanic ash, and others. But there’s enough space dust to make it worth the effort.

At high altitude, the aircraft deploy flat, round collectors that are coated with a sticky oil. They come in two sizes — as big as a saucer or a dinner plate. The big one catches 20 or so grains of cosmic dust per hour.

NASA scientists sort through thousands of particles to pick out the ones that came from beyond Earth. Other scientists from around the world can then study the dust grains — learning more about the solar system from its tiniest members.

Script by Damond Benningfield

The 50 states have all kinds of symbols. South Carolina has an official heritage horse, for example — the marsh tacky. Kansas has a flying fossil, the pteranadon. And Vermont has a fly-fishing fly, the Governor Aiken bucktail streamer. But only Utah has an official star cluster: the Beehive Cluster, for the beehive state.

And for the next few evenings, a bright marker points to the beehive: Venus, the “evening star.” It’s low in the west as night falls. The Beehive Cluster is close to the upper left of Venus tonight, and even closer to the planet tomorrow night. Binoculars reveal a swarm of stars, so it’s a beautiful sight.

The cluster is one of the closest to Earth, at a distance of about 600 light-years. It contains more than a thousand stars. They’re packed into a region that’s about three dozen light-years across. The brighter, heavier stars congregate at the center of the cluster, with the lighter, fainter stars on the outskirts.

The stars of the Beehive were all born together, from the same cloud of gas and dust, about 600 million years ago. That means the cluster has made about three turns around the Milky Way. As clusters move through the galaxy, they usually fall apart quickly. Stars are stripped away by the gravity of the Milky Way’s other stars, gas, and dust. The fact that the Beehive has survived this long could mean that it’ll be around for much, much longer — billions of years into the future.

Script by Damond Benningfield

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When you look at a star, you’re looking back in time. The star HD 140283, for example, is 190 light-years away, so we see the star as it looked 190 years ago.

But the star itself allows us to see much farther back in time — to the very early universe. That’s because the star is one of the oldest ever seen.

Astronomers used several lines of evidence to determine that age. Hubble Space Telescope measured the star’s distance, which revealed its true brightness. That was combined with measurements of its temperature and composition. Plugging those numbers into models of how stars work yields an age of about 14.5 billion years.

That’s older than the universe. But uncertainties in the numbers provide some wiggle room. So the star probably formed just a few hundred million years after the Big Bang.

The key measurement is composition. The star contains only tiny amounts of elements heavier than hydrogen and helium, which were forged in the Big Bang. Almost everything else was created inside stars. As the stars died, the heavier elements were expelled into space, where they could be incorporated into new stars. Since HD 140283 has some of these elements, it’s not from the first generation of stars. But it has so little of them that it’s probably a member of the second generation — born more than 13 billion years ago.

This ancient star is in Libra, which is in the south at nightfall. The star is visible through binoculars.

Script by Damond Benningfield

There’s a lot to look at in the early evening sky tonight, including the Moon and three planets. To see everything at once, though, you need to look at just the right time — about an hour and a half to two hours after sunset.

The crescent Moon is low in the west then, with the night sky’s second-brightest object close to the lower right: Venus, the brilliant “evening star.”

If you let your gaze arc up across the south, you’ll come to Jupiter, the solar system’s largest planet. It outshines everything except the Moon and Venus, so it’s an easy target.

If you look closely, you’ll see that it has a bright companion right now: Zubenelgenubi. The star is one of the leading lights of Libra, the balance scales. But its name means “the southern claw.” That indicates that the star once belonged to the adjoining constellation Scorpius.

Then allow your gaze to drop to the southeastern horizon for the third planet, Saturn. It’s quite bright, but it’s also quite low in the sky, so any buildings or trees along the horizon can block it from view. It will climb all the way across the southern sky during the night.

By the wee hours of the morning, Venus and the Moon will be gone from view. But Jupiter and Saturn will still be around. And yet another planet will have joined them by then — Mars. It shines bright orange as it heads toward its best showing of the year in just a few weeks, completing a beautiful line-up of planets in the June sky.

Script by Damond Benningfield

There’s a beautiful encounter in the western sky the next couple of evenings — a conjunction between the Moon and the planet Venus. Venus is the “evening star,” so you just can’t miss it. It stands above the Moon tonight, and about the same distance to the right of the Moon tomorrow night.

The Moon is the barest of crescents tonight. Sunlight will illuminate only a sliver of the lunar hemisphere that faces our way. As the sky darkens, though, you’ll be able to see the dark portion of the Moon, lit up by earthshine — sunlight reflected off of Earth. In fact, if you were on the Moon, you’d see an almost-full Earth hanging in the sky, shining dozens of times brighter than a full Moon.

Venus goes through a cycle of phases, too. Right now, it’s about three-quarters illuminated, which helps make it nice and bright. Venus is too far away to see the phases with the eye alone, or even with most binoculars. But that cycle is an easy target for small telescopes.

Venus emerged from behind the Sun a few months ago, so it was almost full. Now, it’s circling back toward Earth, so we’re starting to see some of its nightside. Over the coming months, the illuminated fraction of the planet will grow smaller as Venus prepares to cross between Earth and the Sun. But Venus will move closer to Earth as that happens. So even though we see less and less of its dayside, Venus will actually shine brighter — enhancing the beauty of the evening star.

Script by Damond Benningfield

Hundreds of big space rocks follow paths that bring them close to Earth. And it’s possible that some of these rocks could someday hit our planet. Such an impact could cause destruction on a regional or even global scale.

One of the best ways to study and track these potential killers is with radar. Scientists use big radio telescopes to beam pulses of energy at the target. Some of the radio waves reflect back to the telescope. In some cases, scientists use two telescopes — one to send out radio waves, and another to catch their reflection.

Those waves can reveal the asteroid’s size, shape, and something about its composition. Most important, they also help measure the asteroid’s position with a high degree of accuracy. That provides a good plot of its orbit, letting us know if the asteroid might threaten us in the future.

The first radar observations of an asteroid took place 50 years ago this week, with radio telescopes in Massachusetts and California. They looked at 1566 Icarus, which was passing just a few million miles from Earth.

The initial observations were crude, and some of them turned out to be wrong. But radar scans during another close pass, in June of 2015, revealed a lot more. Icarus is an oddly shaped chunk of rock that’s about a mile long. It spins once every two and a quarter hours. And it orbits the Sun once every 1.1 years — a potentially troublesome asteroid that we keep an eye on with radar.

Script by Damond Benningfield