StarDate

Venus has the most volcanic surface of any world in the solar system. Two-thirds of it is paved by volcanic plains, which hold more than 1600 major volcanoes -- and more than 85 thousand in all. But it’s not clear whether any of them is active today. There’s evidence of recent eruptions, but there hasn’t been a smoking gun -- uh, volcano -- until this year.

It’s hard to see eruptions on Venus -- or anything else, for that matter -- because clouds blanket the entire planet. The only way to see the surface is with radar, which peeks through the clouds. The best radar look to date came from the Magellan spacecraft, which orbited Venus in the 1990s.

Two researchers recently pored over thousands of pairs of Magellan images snapped months or years apart. They were looking for changes in the volcanoes -- signs of recent activity. And they found a change on the flank of Maat Mons, the planet’s biggest volcano. It’s three miles high, and the opening at its peak is 20 miles across.

An image from early 1991 showed a circular, mile-wide pit on the side of Maat Mons. Eight months later, the feature was twice as wide, had a jagged shape, and appeared to be filled with lava. Computer models found that the only reasonable explanation was a volcanic eruption -- the most direct evidence of an active volcano on Venus.

Venus is the “evening star” now. It’s shining at its brightest -- a beautiful and possibly active planet.
 

Script by Damond Benningfield

Support McDonald Observatory

First, an important notice: Today’s program has a lot of numbers. So grab your phone or pencil and get ready -- there might be a quiz.

The reason for the numbers is Earth’s location in its orbit. Today, we’re farthest from the Sun for the entire year -- 94 and a half million miles. That’s one and a half million miles farther than average.

The distance changes because Earth’s orbit is an ellipse -- like a circle that’s been stretched out. So during the year, the distance to the Sun varies. We’re closest in January, and farthest in July.

The average distance from the center of Earth to the center of the Sun is known as an A-U -- an astronomical unit. It’s a basic yardstick for measuring distances within the solar system and in other star systems.

But using the Earth-Sun distance to define the astronomical unit has some problems. For one thing, the distance changes, and not just because of Earth’s lopsided orbit. So in 2012, astronomers adopted a precise, constant value for the A-U. Are those phones or pencils ready? The A-U is now defined as 149 billion, 597 million, 870 thousand, 700 meters. That’s 92 million, 955 thousand, 807.273 miles.

Now, about that quiz: Lined up end to end, how many large pepperoni pizzas could fit between Earth and the Sun? A-plus to everyone who figured it out, because we haven’t -- but we are hungry for pizza.
 

Script by Damond Benningfield

Support McDonald Observatory

Finding the right parking spot is always a challenge -- especially on the Moon. For the first Apollo landing, a half century ago, NASA picked a smooth, flat, easy-to-reach site near the equator -- someplace not likely to dent the fenders.

When the astronauts of the new Artemis program reach the Moon, the parking will be a lot more complicated. NASA’s looking at sites near the south pole. Such sites are trickier to reach, they’re more rugged, and many of them aren’t always in view of tracking stations.

But they’re a lot more interesting. Deposits of water ice may lurk at the bottoms of craters that never see the Sun. And the sites are inside the Moon’s largest impact basin, so astronauts can explore many layers of rock, which record much of the Moon’s history.

Mission planners have to take many factors into account as they ponder the landing sites. A site has to offer a big landing zone, plenty of sunlight, safe slopes, and interesting targets for study.

Science teams have identified more than a dozen sites that meet those criteria. The list includes the rims of several craters that might have ice in their dark recesses. The teams have even mapped routes for astronauts to explore -- at challenging parking spots near the south pole.

The Moon climbs into view by midnight tonight. The planet Saturn looks like a bright star well to its left. They’ll be closer tomorrow night, with Saturn rising above the Moon.
 

Script by Damond Benningfield

Support McDonald Observatory

Most of what makes up the universe is dark -- dark matter and dark energy. No one knows what they are, which is why they’re described as “dark.” But we observe their effects on the stuff we can see. That shows that they account for more than 95 percent of all the matter and energy in the universe. Everything that makes up stars, planets, and people is just crumbs.

Scientists have been working for decades to determine just what dark matter and dark energy really are. And they’re set to get a good new tool to help with that quest as early as this month -- a European space telescope that will look into the early universe.

Euclid will study the shapes and motions of galaxies out to distances of about 10 billion light-years. That means it will see the galaxies as they looked when the universe was less than a third of its current age.

The craft will use gravitational lensing to study the shapes of galaxies. The gravity of galaxy clusters distorts the view of the galaxies that appear behind them. The way those distant galaxies are distorted reveals how much dark matter is contained in the intervening clusters.

The telescope also will measure how fast the galaxies are moving away from us -- a result of the universe’s expansion. That will tell us when dark energy began to dominate the universe, causing it to expand faster.

Euclid should study 10 billion objects across a third of the sky -- shedding some light on the dark universe.
 

Script by Damond Benningfield

Support McDonald Observatory

The Moon is full tonight. It’s known as the Hay Moon or Thunder Moon. And because July is the month in which humans first walked on the Moon, it’s sometimes called the Apollo Moon as well.

Whatever you call it, it’s especially bright. That’s because this month’s full Moon closely matches up with the Moon’s closest point to Earth. Late tomorrow, it’ll pass less than 224,000 miles away -- about 15,000 miles closer than average.

Such a close approach makes the tides especially strong. The tides are caused by the gravitational pull of the Moon and Sun. When the Moon is full, the Moon and Sun are on the same line, so their gravity pulls together. That makes the high tides higher and the low tides lower. And when the Moon is closer to us, its gravitational pull is a little firmer than average, jacking up the tides even more.

The full Moon doesn’t usually coincide with its closest approach to Earth. The Moon is full every 29 and a half days. But it’s closest to Earth once every 27 and a third days. So most months, they don’t sync up very well. This month is an exception -- a full Moon that’s especially close.

One other factor that goes into creating the tides is Earth’s distance from the Sun. We’ll be at our farthest on Thursday, so the Sun’s influence is a little lower than average. But the Moon is far more important in raising the tides -- which will be especially prominent over the next couple of days.
 

Script by Damond Benningfield

Support McDonald Observatory

Sirius, the brightest star in the night sky, is out of sight now. Thanks to its rising and setting times and our viewing angle, it’s hidden in the Sun’s glare. It’ll return to view in a few weeks.

Thousands of years ago, Sirius first poked into view in the morning sky in early July, when the weather was heating up. Sirius is known as the Dog Star, so the period of the summer’s greatest heat -- when the Dog Star emerged into full view -- came to be known as the Dog Days. The period begins around July 3rd and ends in mid-August.

“Dog Days” is a phrase we still use today, even though Sirius is nowhere in sight. It’s moved because an effect known as precession causes the stars to slide through the seasons.

The Dog Star is also known as Canicula -- a Latin word that means “little dog.” In parts of Mexico, Texas, and other regions, the Dog Days are sometimes called “La Canicula.”

The people of ancient Egypt used several calendars. In one, the new year began with the first appearance of Sirius in the dawn sky. In another, the year was exactly 365 days long -- a quarter of a day shorter than the true year. So the calendar slid out of sync with the Sun by one day every four years. But it came back around once every 1,461 of those years. That span is known as the canicular period -- the time it took the canicular star to return to its rightful place in the calendar, climbing into view in early July.
 

Script by Damond Benningfield

Support McDonald Observatory

There’s a nice lineup in the western evening sky right now, like three airplanes queuing up for landing. And the line will get tighter over the coming evenings before the bright lights diverge, headed for different gates.

The brightest of the three lights is Venus. It’s the brilliant “evening star” -- the brightest light in the night sky other than the Moon. And the airplane analogy is an apt one. It really does look like an approaching airliner with its landing lights turned on. But if you keep an eye on it for a minute or two, you’ll see that it doesn’t change position -- it stays with all the other lights around it.

Mars stands to the upper left of Venus. It’s the faintest of the three lights. Even so, it looks like a modestly bright orange star. And its proximity to Venus will help it stand out.

The final member of the lineup is Regulus, to the upper left of Mars. The star marks the heart of Leo, the lion. It’s a little brighter than Mars, but no match for Venus.

Watch over the coming nights as Venus and Mars slide toward Regulus, which is dropping lower in the sky day by day. A week from now, Mars and Regulus will stand side by side, almost touching. Venus will be close by, but it won’t quite reach either of the other lights. After July 10th, Mars will pull away from Regulus, while Venus drops away. It will disappear from view by about the end of the month -- ending a beautiful lineup in the evening sky.
 

Script by Damond Benningfield

Support McDonald Observatory

The Moon teams up with the heart of the scorpion tonight. It’s their second close approach this month — even closer than the first one. In fact, they’ll get so close that it will look like they could reach out and touch each other.

The scorpion’s heart is the star Antares. It’s much more massive than the Sun, and hundreds of times the Sun’s diameter. Because it’s so puffy, its outer layers are fairly cool, so they shine reddish orange. Unfortunately, the bright Moon will wash out most of that color.

Antares and the Moon team up once every four weeks or so, as the Moon rolls across the background of stars. They were close together on the nights of June 2nd and 3rd, though not as close as they are tonight as seen from North America.

When the geometry is just right, they can appear closer still. In fact, the Moon can pass directly in front of the star, blocking it from view. And it’ll do just that four times this year. Two of them will be visible from all or most of the United States, on the nights of August 24th and November 14th — vanishing acts for frequent companions.

Antares won’t disappear tonight, but it’ll come close. It stands to the lower left of the Moon at nightfall, separated by just two or three degrees, depending on your location. The Moon will slide toward the star during the night. As seen from most of the U.S., they’ll be at their closest about the time they set, a few hours before sunrise.
 

Script by Damond Benningfield

Support McDonald Observatory

Venus and Mars are planetary opposites. One is hot; the other, cold. One has a thick atmosphere; the other, not much atmosphere at all. Even their names are opposites. Venus is named for the Roman goddess of love and beauty, while Mars bears the name of the god of war. Right now, they do share a location — close together in the west at nightfall.

Venus and Mars flank Earth. Venus is the next planet in toward the Sun, with Mars the next planet out. Venus is about the size of Earth, with Mars a little more than half that big.

The combination of location and size explains the differences.

Because Venus is bigger and heavier, its gravity is stronger. That allowed it to hang on to its early atmosphere. With the Sun so close, that made the planet hotter. The extra heat evaporated any water Venus might’ve had, and baked gases out of its rocks. That made the atmosphere thicker, trapping more heat. Today, the atmosphere is more than 90 times as dense as Earth’s, and it’s hot enough to melt lead.

Mars, on the other hand, is small and light, so its gravity is weak. That allowed much of its atmosphere to leak into space, making the planet colder. Today, the air is less than one percent as thick as Earth’s, and Mars is almost always far below freezing.

Venus blazes as the “evening star,” with fainter Mars close to its upper left. They’ll stay close for a few more nights, then start to move apart — putting some distance between planetary opposites.
 

Script by Damond Benningfield

Support McDonald Observatory

The bright gibbous Moon is in “balance” tonight. That’s because it’s moving into the constellation Libra, the balance scales. In fact, Libra’s two brightest stars are close to the Moon this evening.

Originally, those two stars formed the claws of Scorpius, the scorpion, which spreads out to the lower left of Libra. And even today, the names of those stars represent that heritage: Zubeneschamali and Zubenelgenubi, the northern and southern claws.

But thousands of years ago, the claws were pulled away from the scorpion and assigned to a new constellation: Libra. At least in part, that may be because, at the time, the Sun passed across that part of the sky at the autumn equinox. Day and night are roughly equal then — a sort of “balance” in the heavens.

Thanks to an effect known as precession, though, the Sun is no longer in Libra at the equinox. Earth wobbles on its axis like a spinning gyroscope that’s running down a little bit. As it wobbles, our perspective on the stars changes, so the Sun appears against different constellations. Today, it passes through Virgo at the equinox, and doesn’t cross through Libra until about a month later.

Look for Zubenelgenubi not far to the left of the Moon as night falls, with Zubeneschamali farther to the Moon’s upper left — the former claws of the scorpion that are now in cosmic “balance.”

Tomorrow: un-balanced planets in the evening sky.
 

Script by Damond Benningfield

Support McDonald Observatory