StarDate

The Moon is lining up for a pass through Earth’s long shadow tomorrow afternoon and evening. Unfortunately, though, it’s a passage through the bright outer part of the shadow, so few moonwatchers are likely to notice much difference.

That part of the shadow is known as the penumbra, so the Moon’s passage through it is a penumbral lunar eclipse.

Lunar eclipses occur only at full Moon because that’s when the Moon lines up opposite the Sun, which is where Earth’s shadow is. Most full Moons don’t produce eclipses, though. The Moon’s orbit around Earth is tilted slightly with respect to Earth’s orbit around the Sun. So most months, the Moon passes a little above or below the shadow, leaving the Moon uneclipsed.

When the geometry is just right, though, the Moon passes through some portion of the shadow. If the Moon is totally immersed in the dark inner portion of the shadow, then the eclipse is total, so the entire Moon looks dark orange or red. If only part of the Moon passes through the shadow, it’s a partial eclipse, so it looks like something took a “bite” out of the Moon.

And if the Moon stays in the penumbra, there’s hardly anything to notice — only the slightest darkening of the lunar disk. And that’s what happens tomorrow, with the moment of greatest eclipse at about 6:45 p.m. Central Time.

If you don’t notice the eclipse, at least you can enjoy the bright moonlight — and a bright star near the Moon, which we’ll talk about tomorrow.

Script by Damond Benningfield

Osiris-Rex is still a couple of years away from its destination. But the spacecraft will give one of its instruments a check-out over the next couple of weeks by hunting for asteroids that share Earth’s orbit around the Sun.

Such asteroids are known as Trojans. They’re found in clumps that are 60 degrees either side of a planet. They’re “herded” by the planet’s gravity, like a sheepdog keeping its flock in place.

Thousands of Trojans share the orbit of Jupiter, the solar system’s largest planet. Others have been discovered in the orbits of all the other planets except Mercury and Saturn, including one that’s known to accompany Earth. It was discovered in 2010, by another spacecraft. There could be more, though, so Osiris-Rex will take a look.

The craft, which was launched in September, is headed for a rendezvous with an asteroid named Bennu in late 2018. It’ll fly along with the asteroid for many months, then touch down and grab a small sample for return to Earth in 2023.

Beginning tomorrow, though, it’ll use one of its cameras to scan for Trojans ahead of Earth. The camera’s designed to find small rocks in orbit around Bennu, so looking for larger rocks that are farther away should be a good test.

Over the next couple of weeks, the camera will scan the entire region where Trojans might exist. That could tell us whether any more space rocks share our planet’s orbit around the Sun.

Tomorrow: a faint lunar eclipse.

Script by Damond Benningfield

If you keep intending to learn a few stars and their constellations but never seem to have the time, tonight offers an easy way to check a few off the list. Several prominent stars will surround the Moon, making it easy to find the stars and their home constellations.

As twilight fades, look for two bright stars to the left of the Moon: Pollux and Castor, the twins of Gemini. Pollux is a stellar giant that’s nearing the end of its life, while Castor is a system of at least six stars.

Next, look about the same distance to the right of the Moon for bright orange Betelgeuse, which marks the shoulder of Orion. Betelgeuse is heavier than the Sun, hundreds of times wider, and tens of thousands of times brighter. Before long the star will explode, briefly shining brighter than all the other stars in the galaxy combined.

The same fate awaits most of the other bright stars of Orion, which stretch to the lower right of Betelgeuse. Three stars form a compact line known as Orion’s Belt. Rigel, Orion’s brightest star, is on the opposite side of the belt from Betelgeuse.

Canis Minor, the little dog, stands below the Moon, marked by its single bright star, Procyon. And the brightest star in all the night sky stands to the lower right of Procyon: Sirius, the leading light of Canis Major, the big dog.

That gives you four constellations in a single glance, perhaps jump-starting those plans to learn more of the geography of the night sky.

Script by Damond Benningfield

Stars are big and heavy, but they’re not solid. Because they’re made of gas, they’re just about as easy to stretch and squish as a blob of Silly Putty. Stars that spin very fast, for example, bulge out at the equator, so they look like flattened beachballs. And stars in certain stages of life puff in and out like a set of breathing lungs.

And astronomers using a space telescope have recently discovered binary stars that get stretched out, then wiggle like a bowl of Jell-O when they spring back to a rounder shape.

The Kepler satellite was built to look for planets in other star systems. But its observations also reveal details about the stars.

An example is “heartbeat” stars. They got the name because, when you plot how bright they are, the line looks like a chart of a beating heart, with regular peaks and valleys.

A recent study found that these systems consist of two stars in an elongated orbit around each other. At their closest, the stars may be separated by only a few times the size of the stars themselves. The gravity of each star tugs on the other, stretching the star out. The increased surface area of the two stars makes the system brighter. When the stars move away from each other, they snap back to a rounder shape, so they get fainter.

As they change to a more spherical shape, though, their surfaces jiggle. Astronomers may someday use the jiggles to probe conditions inside the stars — stars that are constantly changing shape.

Script by Damond Benningfield

When you look at pictures of the Sun, you see a fairly sharp edge, as though the Sun has a solid surface. But that’s not the case at all. Hot gas outside that surface extends many thousands of miles into space. It’s so thinly spread, though, that we can see right through it. What we see as the surface is the point where the gas is so dense that we can’t see through it to the layers below.

For some other stars, though, it’s harder to make out any surface at all. The star is surrounded by so much gas that it’s as though it was enveloped by a glowing fog.

An example is Aldebaran, the brightest star of Taurus the bull. The bright orange star is quite close to the Moon as darkness falls tonight.

Aldebaran is a red giant — a star that’s nearing the end of its life. It’s converted the original hydrogen in its core to helium. The helium can’t “fuse” together to produce energy, so the core is shrinking and getting hotter. That’s triggered nuclear reactions in a shell of hydrogen around the core.

The heat from the “burning” hydrogen, and from the helium core, pushes outward on the layers of gas above them. That’s caused Aldebaran to puff up to dozens of times its original size. The gas at the surface of the star is quite thin. And a good bit of it is blowing out into space in the form of a strong wind. That surrounds the star with a spray of hot gas that makes it hard to see any “surface” at all — giving Aldebaran a “fuzzy” appearance.

Script by Damond Benningfield

Our sedate, middle-aged Sun spins on its axis about once a month. This leisurely rate keeps the Sun nice and round, like a beachball. But some stars rotate much faster. In fact, some spin so fast that they flatten themselves out, like a squashed beachball.

An example is Gomeisa or Beta Canis Minoris, the second-brightest star of Canis Minor, the little dog. It’s in the east at nightfall, above much brighter Procyon, the constellation’s leading light.

Gomeisa is several times more massive than the Sun, and several times wider as well. But it spins much faster than the Sun does, so it probably makes one turn in about a day.

Since a star is made of gas, that high-speed rotation forces material outward at Gomeisa’s equator. That gives the star its compressed appearance.

The rotation has another important effect. Gas at the equator is moving at more than half a million miles per hour. And since the equator is bulging outward, it “feels” less of a gravitational grip than material at the poles. So some of the hydrogen gas at the equator gets flung out into space, like water whirling away from a spinning lawn sprinkler. That surrounds Gomeisa with a disk of gas that’s several times wider than the star itself.

If you could get close to Gomeisa, you’d see one other interesting effect. Because the gas at the poles is closer to the center of the star, it’s much hotter. So the star’s poles are brighter than its equator — a two-toned appearance for a flattened star.

Script by Damond Benningfield

If anyone is watching a binary star system in Ophiuchus in a couple of million years, they might see an assortment of planets unlike any other. Some of the planets might orbit the individual stars, while some might circle around both stars.

IRS 43 consists of two proto-stars — balls of gas that aren’t yet hot enough to ignite nuclear fusion in their cores. Each is roughly as massive as the Sun, and is still growing — gas and dust is still falling onto their surfaces.

But some of the gas and dust may survive to make a planetary system unlike anything astronomers have yet seen. They’ve found binaries where planets orbit one or both of the individual stars, or where planets orbit both of the stars — but never both in the same system.

IRS 43, though, includes three disks of gas and dust — one around each star, and a giant one encircling the entire system. Each of the disks is aligned in a different direction, so they don’t line up in a neat and orderly way.

Some of the material in the disks could coalesce to form planets. So each star could have worlds of its own, with perhaps a few more shared between them.

The astronomers who discovered this odd configuration aren’t sure what caused it. Perhaps the original cloud that collapsed to form the two protostars was unusually turbulent. Or perhaps it was stirred up when the protostars kicked a third one out of the system — setting the stage for the birth of an unusual set of planets.

Script by Damond Benningfield

The country’s most famous rodent is scheduled to issue a major weather forecast today. If the groundhog known as Punxsutawney Phil sees his shadow, we’ll have six more weeks of winter. If not, then we’re in for an early spring.

That’s the legend, at least — one that was first celebrated 130 years ago today, in the hamlet of Punxsutawney, Pennsylvania. And we’ve been celebrating Groundhog Day ever since.

It’s one of several celebrations that have been tied to February 2nd, which is known as a cross-quarter day because it occurs roughly halfway between a solstice and an equinox. In some cultures, the date marked the end of winter, not the middle of it as it does in the modern western calendar.

The people of Germany and other parts of Europe thought that bears and other animals, such as the hedgehog, came out of hibernation at that time of year to test the weather. If it was warm enough, they’d leave their dens. If not, they’d go back in and wait out winter’s remaining chill.

German immigrants brought that idea with them when they settled in Pennsylvania. They found a lot of groundhogs there, so they made the groundhog the new arbiter of the weather.

Most early calls on the groundhog were made in private. In 1887, though, the editor of the Punxsutawney newspaper declared that his town’s groundhog was the only true judge of the weather. He organized a public event at Gobbler’s Knob — the first public celebration of Groundhog Day.

Script by Damond Benningfield

Odd little February, which is named for the Roman god of purification, is the shortest month of the year. Historians aren’t exactly sure why it’s so short. But tracing its evolution gives us a capsule history of the evolution of the modern western calendar.

Our calendar is a descendant of the earliest Roman calendar. It included only 10 months, beginning with March. The year ended with about 60 days that weren’t part of any month.

That system didn’t work very well, though, so two months were added to the end of the year — January and February. They stayed at the end of the year for several centuries, until the start of the year was moved to January.

The lengths of the 10 original months were changed to give the newcomers a total of 56 days. But the Romans feared even numbers, so they added a day to January to give it 29. Since February was the month for repentance and for honoring the dead, it stayed at an unlucky even number.

This version of the calendar contained only 355 days, so an extra month was added every other year. In those years, the last five days of February were dropped.

After that, February remained unchanged until Julius Caesar introduced the basic calendar that we use today — a calendar that includes July, which Caesar named for himself. While updating the calendar, he might have added a day to February. If so, it was taken away by Augustus Caesar, who added it to the month that bears his name: August.

Script by Damond Benningfield

Mars just keeps getting wetter. New studies turn up new sources of water on the planet all the time. Late last year, in fact, geologists reported the discovery of a slab of water ice that’s big enough to fill Lake Superior, the largest of the Great Lakes.

Modern-day Mars is a vast desert — it’s cold and extremely dry. And the air is too thin for any liquid water to exist on the planet’s surface. But Mars was much warmer and wetter in the distant past, with rivers, lakes, and perhaps even a giant ocean. Much of that water has escaped into space, but some remains — most of it in the form of ice, either in the polar caps or buried below the dusty surface.

Geologists at the University of Texas at Austin uncovered some of that ice. They used radar aboard Mars Reconnaissance Orbiter to probe deep below the surface of Utopia Planitia, a vast plain that’s halfway between the equator and the north pole. The radar detected a layer of ice that’s as big as the state of New Mexico, and up to 500 feet thick.

That layer could contain clues about how the Martian climate has changed over the eons. So digging into it might tell us more about why some regions kept their water, while others lost it all.

And Mars is in great view tonight. The Sun’s fourth planet looks like a modest orange star close to the upper right of the Moon this evening. The much brighter planet Venus stands to the lower right of the Moon, shining as the brilliant “evening star.”

Script by Damond Benningfield