StarDate

Like the surface of Earth, the surface of the Moon sometimes shakes, rattles, and rolls. On the Moon, though, much of the rattling appears to be caused by the gravitational influence of Earth itself.

Apollo astronauts left seismometers on the Moon to measure any motions at the surface. They listened until late 1977, when they were shut down. During that time, they recorded about 1300 moonquakes. Most of them were pretty mild, although they could rattle on for hours.

Some of the quakes took place at the surface — the result of impacts by space rocks. But most of them were centered deep inside the Moon — hundreds of miles below the surface. And they occurred in cycles of about 27 days — about the same time it takes the Moon to orbit Earth.

A couple of studies in the last few years have blamed the quakes on the tides — the same effect that raises ocean tides here on Earth. Earth’s gravitational pull on the Moon is stronger than the Moon’s pull on Earth, creating tides in the solid lunar crust. That forms cracks deep inside the Moon. The sides of the cracks rub against each other, creating stress. Eventually, the stress releases with a jolt — creating a moonquake.

Look for the Moon just above the western horizon at sunset. It’s a beautiful thin crescent. And it has a beautiful companion: the planet Venus. It’s the brilliant “evening star,” to the left of the Moon.

More about the Moon in the evening sky tomorrow.

Script by Damond Benningfield

Jules Janssen spent a lot of time in high places. The French astronomer viewed a solar eclipse from the Himalayas. He escaped from Paris in a hot air balloon. And he almost died while in the mountains of South America.

But 125 years ago this week, he topped even those feats. He took the first observations from a small observatory at the summit of Mont Blanc, the tallest mountain in Europe — almost 16,000 feet high.

Janssen specialized in studying the Sun. And one problem intrigued him. He wasn’t sure whether oxygen in the Sun’s spectrum came from the Sun itself, or from Earth’s atmosphere. He’d taken spectra from different altitudes, but wanted to go higher. So he picked Mont Blanc.

Building an observatory there took a major effort. The summit was covered with ice, so there was no rock to anchor the observatory. And everything had to be carried to the top by hand.

The observatory was finished in September of 1893. Janssen, who was nearing 70, was carried up the mountain in a sled on the 11th. A storm delayed his research, but he made his first observations a few days later.

Over the next few years, about 25 scientists visited the facility. They photographed the Sun, listened for solar radio waves, and studied the planets. They also studied the weather.

By 1906, though, the shifting ice had tilted the observatory. And in 1909, a crevasse opened beneath it, so it was torn down — ending the short life of the Mont Blanc Observatory.

Script by Damond Benningfield

Lacerta, the lizard, isn’t much to look at. Even the constellation’s brightest stars are quite faint. From light-polluted cities, in fact, they’re all but invisible.

The brightest is Alpha Lacertae. It’s about twice as big and heavy as the Sun, and a couple of dozen times brighter. It’s about a hundred light-years away — a distance that’s pretty well locked in.

The distance to the second-brightest star has been a little more uncertain. It had been thought to be about 625 light-years away, give or take a few dozen. But observations by Gaia, a European spacecraft, say the distance is only 560 light-years, with about the same amount of wiggle room.

And there was also a difference in the distance to the third-brightest star in the constellation, 5 Lacertae — about 1600 light-years for the old measurements, versus about 1750 for Gaia.

Knowing a star’s distance is pretty important. Combining the distance with how bright the star looks reveals its true brightness. Plugging all of those numbers into models of how stars work reveals the star’s size, its age, and other details. So a difference of just 10 percent in the distance can have a big impact in what we know about the star.

If you have clear, dark skies, away from most of the glow of city lights, look for Lacerta about half way up the northeastern sky at nightfall. It’s between W-shaped Cassiopeia, to the lower left, and beautiful Cygnus, the swan, to the upper right.

Script by Damond Benningfield

Most of the universe consists of empty space — enormous volumes of darkness that harbor few stars or galaxies at all. We live next to a void. It’s called the Local Void, and it starts about four million light-years away, on the other side of our home galaxy, the Milky Way. This region of darkness is gargantuan: It spans an incredible quarter of a billion light-years.

The Local Void isn’t completely empty, though. It houses a few lonely galaxies. And despite their peculiar location, they seem perfectly ordinary.

The nearest known inhabitant of the Local Void is a dwarf galaxy known as KK 246. The galaxy is 23 million light-years from Earth — about 10 times farther than the Andromeda galaxy, which is the nearest giant galaxy to our own.

Like most galaxies, KK 246 is much smaller and fainter than the Milky Way. And as far as we can tell, it has no neighbors within 10 million light-years. In contrast, more than a hundred galaxies lie within that same distance of the Milky Way.

But like our galaxy, KK 246 contains gas that’s giving birth to new stars. Dwarf galaxies that lie near big ones often lose their gas to the giants. But with no neighbors, KK 246 doesn’t have that problem. It’s simply an average-looking small galaxy in the middle of nowhere — in the vast emptiness of the Local Void.

The galaxy is in Sagittarius, which is low in the south at nightfall. Its stars form the outline of a teapot.

Script by Ken Croswell

Neptune is the bluest planet in the solar system. Most of that color comes from methane atop the planet’s atmosphere. It absorbs red light, allowing only blue light to reflect back into space. But Neptune is too blue for methane to account for all of the color. So far, though, astronomers don’t know what else might be enhancing the blue.

It’s just one of the mysteries about the Sun’s most remote major planet. And solutions to many of them probably require a direct look at Neptune’s atmosphere.

So far, only one spacecraft has visited Neptune. Voyager 2 flew past it in 1989. Just about everything else we know about Neptune has come from telescopes on or near Earth. But even though Neptune is four times the diameter of Earth, it’s so far away that it’s a tiny target.

Knowing more about Neptune, though, will tell us more about how all the planets of the solar system took shape. It’ll also tell us more about planets in other star systems. About a third of the planets discovered to date are similar to Neptune.

In a recent study, a team of scientists suggested parachuting a probe into Neptune’s atmosphere. It would measure the composition, temperature, and structure of the air and the clouds. The measurements would answer many questions — while leaving plenty of mystery about this blue giant.

Neptune is at its brightest now, although you need a telescope to see it. It’s low in the southeast at nightfall, near the bottom of the outline of Aquarius.

Script by Damond Benningfield

Astronomers have been studying Neptune for only a bit more than one year — one Neptunian year. The planet takes 165 Earth years to orbit the Sun. But it was discovered just 172 years ago. That means that, since its discovery, it’s made only a bit more than one full orbit, so we’ve seen only one year on the planet.

That’s not a lot of time to learn what’s happening there. Astronomers have seen one cycle of seasons. But seasons can be quite different from one year to the next. And a planet’s overall climate can change, too. So it can take many years to figure out how a planet’s atmosphere works.

And in the case of Neptune, we’ve had really good “eyes” on the planet for only the last few decades. The Voyager 2 spacecraft flew past Neptune in 1989, giving us our only close-up look. Hubble Space Telescope has watched it since the 1990s. And new technologies have greatly improved the view from the ground over that period as well.

So to really to get to know the planet, astronomers will have to keep watching it for years — Earth years and Neptunian years, too.

And this is an especially good time for them to be watching the planet. It lines up opposite the Sun in our sky, so it rises at sunset and is visible all night. Neptune is closest for the year, too — about 2.7 billion miles away — so it’s also at its brightest. Even so, you need a telescope to find this remote giant.

We’ll have more about Neptune tomorrow.

Script by Damond Benningfield

A blue giant is putting on its best face this week. It’s in view all night, and it shines brightest for the year, too. Unfortunately, though, you still need a telescope to see it.

Neptune is the fourth-largest planet in the solar system. But it’s also the most remote of the Sun’s major planets, so it’s not easy to find.

In fact, Neptune was discovered not because of its brightness, but through its gravitational influence. Astronomers noticed some oddities in the orbit of Uranus, the next planet in toward the Sun. They determined that it was being pulled by another planet. They even calculated the other planet’s orbit. One astronomer then asked some colleagues to look for it. They did — and there it was.

At least three astronomers may actually have seen Neptune through telescopes well before then. But it looked like an ordinary star, so they didn’t pay much attention to it. If they’d looked again a few days after they first saw it, they would have noticed that it moved against the background of stars, revealing its true nature: a giant planet far from the Sun.

And this week, that giant lines up opposite the Sun in our sky. Because of that, it rises at sunset and is in view all night. It’s near the eastern edge of the stick-figure outline of Aquarius, the water bearer. Neptune is closest to Earth around opposition, too, so it shines at its brightest — a faint blue dot among the stars.

More about Neptune tomorrow.

Script by Damond Benningfield

Earth’s magnetic field is constantly changing. Right now, for example, the magnetic north pole is moving about 40 miles per year. And the strength of the magnetic field has dropped over the last couple of centuries. That’s led to speculation that the field is about to make the biggest change of all: a complete reversal, with the north magnetic pole becoming the south pole and vice versa. But a recent study says that might not be the case.

Earth’s magnetic field is generated deep in its core. Our planet has a solid iron core that’s surrounded by a layer of molten metal. The solid and liquid cores rotate at different speeds. That creates electric currents, which in turn produce a magnetic field. The field flips probably are caused by changes in the core, although scientists don’t yet know how this works.

Over the past 20 million years, the field has flipped an average of about once every 250,000 years. But the last reversal took place 780,000 years ago. And there’s a spot over the South Atlantic where the field is quite weak. That’s fed the idea that the field is about to reverse.

But the recent study looked at the state of the magnetic field at several times over the last 50,000 years or so. It found several eras when the field looked almost exactly like it does today. None of those eras led to a permanent switch. So the researchers say that despite the fluctuations, the field probably isn’t due for a reversal any time soon.

Script by Damond Benningfield

Flat Earthers aside, everyone knows that our planet Earth is round. But what many don’t realize is that we’ve known this for thousands of years — long before the voyages of Christopher Columbus.

The key clue came from the Moon. Every now and then, the Moon passes through Earth’s shadow, producing a lunar eclipse. The shadow is round — a clear indication that Earth itself is, too. Greek philosopher Aristotle made that argument more than 2300 years ago.

Our planet owes that round shape to gravity. It squeezes the planet into a near-perfect sphere. In fact, Earth’s diameter through its equator is only 27 miles greater than through the poles — a difference of just a third of a percent. That difference arises because Earth spins, producing an outward force that creates that tiny bulge around the equator.

Gravity also squeezes the other planets into round shapes. The two least-round planets are the ones that spin the fastest: Jupiter and Saturn. Even though they’re many times larger than Earth, they complete one turn in just 10 or 11 hours. That high-speed rotation puffs up their equators. As a result, both planets are thousands of miles thicker through their equators than their poles, so they look like squashed beachballs.

The largest body in the solar system — the Sun — is also round. It spins slowly, though, so there’s little to distort the shape dictated by gravity — the same force that squeezes Earth and the Moon into spheres.

Script by Ken Croswell

Some stars spin so fast that they’re shaped more like lozenges than balls. An example shines in the southernmost part of the constellation Ophiuchus, which is in the southern sky at nightfall.

51 Ophiuchi is about 400 light-years from Earth. It’s a young blue star, hotter than the Sun and about four times as massive. But what really sets it apart are its rapid spin and its odd shape. The star’s equator whirls around at 600,000 miles per hour or faster. By contrast, the Sun, which is an almost perfect sphere, rotates at just 4500 miles per hour.

As you might imagine, centrifugal force has puffed up 51 Ophiuchi’s equator, so the star isn't round like the Sun. Recent observations reveal that at the equator, it’s about eight times the Sun’s diameter. From pole to pole, though, it’s less than six times as large as the Sun.

And 51 Ophiuchi isn't alone. Other rapidly spinning stars are also shaped like squashed beachballs. Altair, in the eagle, and Regulus, the heart of the lion, both spin so fast that if you could view them close up, they would look like glowing lozenges rather than spheres.

But 51 Ophiuchi is much farther away than those stars are, so it’s barely visible to the unaided eye. To see it, you need a good starchart and a nice, dark sky, away from city lights. You’ll find this intriguing star in the south as darkness falls — a star that has spun itself out of shape.

Tomorrow: rounding off planet Earth.

Script by Ken Croswell