The North Star is one of the most important beacons in the sky. It serves as a compass, pointing the way due north. And it also serves as the hub of the sky – all the other stars appear to circle around it as Earth turns on its axis.

More than 4,000 years ago, the architects of Egypt used the North Star to align the pyramids of Giza. And more than four decades ago, Apollo astronauts used it to help guide them to the Moon. But the two groups were aided by different north stars. The Egyptians used a star called Thuban, in Draco, the dragon. The astronauts used Polaris, at the end of the handle of the Little Dipper.

Thuban didn’t explode or fade away – it’s still in plain sight. Instead, Earth’s axis turned away from the star – an effect called precession.

It’s caused by the gravitational tug of the Sun and Moon, which cause our planet to wobble like a spinning top. As it wobbles, the axis points toward different stars. Four thousand years ago, it aimed at Thuban. Today, it aims at Polaris.

Over the next century, the pole will take slightly better aim at Polaris. Right now, the star is about two-thirds of a degree from where the axis is pointing. Around the year 2100, it’ll be less than half a degree from that spot.

After that, though, the pole will move away from Polaris. By around the year 4100, it’ll take aim at a star that’s one constellation over, in Cepheus – a North Star to guide future generations.

Script by Damond Benningfield

Follow the water is NASA’s mantra for Mars exploration. Water is a key ingredient for life. So missions to the Red Planet have sought out regions that show evidence of water, either past or present. Such regions should be the most likely to hold evidence of life.

But some scientists think the search ought to be expanded. Don’t just follow the water – follow the salt.

The Viking landers of the 1970s carried several experiments to look for microscopic life. They fed a bit of Martian dirt into small chemical laboratories. The experiments added water, nutrients, or other ingredients. They then looked for gases or radioactive elements that might be produced by such organisms.

A couple of the experiments found just what they were looking for. But most scientists later decided that the evidence was produced by chemical reactions. Others maintained that they really were produced by life.

A recent paper argued that the composition of the soil at the landing sites was very salty. On Earth, some organisms live in such conditions. They draw their water from the salts. But if you add water, they die. So the Viking landers might have found life – then killed it off. So NASA might need to expand the search – by following the salt.

Mars is especially easy to see tonight. It’s right next to the Moon at nightfall, and looks like a bright orange star. They’ll be a little farther apart as they set, in the wee hours of the morning.

Script by Damond Benningfield

For centuries, the far side of the Moon was called the dark side – not because it doesn’t get any sunlight, but because it was always hidden from view. Astronomers couldn’t shed any light on what it looked like – making it dark.

Today, scientists are trying to shed a little light on another darkness – dark energy. It appears to be making the universe expand faster as it ages. And it accounts for about 70 percent of all the energy and matter in the universe.

But so far, there’s no widely accepted explanation for what constitutes dark energy. Scientists have a lot of ideas, but none of them has been confirmed.

One of the leading ideas is that dark energy is constant, because it comes from the vacuum of space itself. As the universe expands, the amount of matter stays the same, but it’s spread across a greater and greater volume. That means each bit of matter feels a weaker gravitational pull from all the other matter. At the same time, as more space is created, so is more dark energy. It overpowers gravity, causing the universe to expand faster.

Some recent research has found support for that idea by looking at how the universe has expanded over the eons. But the same result could support the idea that dark energy isn’t constant – that it’s evolved over time. That has important implications for the fate of the universe – whether it will continue to expand forever, or someday collapse – ending in a “Big Crunch.”

Script by Damond Benningfield

If astronomers ever see a “wiggle” in the orbit of Mars, it might be the fault of a passing black hole. A study last year suggested that tiny black holes might pass through the inner solar system once every decade or so. They’re not likely to run into Earth or anything else. But their gravity might give the inner planets a minuscule nudge. And if Mars is the one getting shoved, we might be able to measure it.

The universe might be filled with “primordial” black holes created in the Big Bang. They’d be as tiny as atoms, but as massive as asteroids hundreds of miles wide.

Such black holes might account for much of the “dark matter” in the universe. It makes up about 85 percent of all matter, but it produces no energy, so we can’t see it. We know it’s there only because its gravity pulls on the visible matter around it. Scientists have suggested it might consist of subatomic particles, but efforts to find them have come up empty.

If primordial black holes make up the difference, they would zip through the solar system fairly often.

The study found that it would be hard to measure the effect of such a black hole on Earth or the Moon. But if one passed within a few hundred million miles of Mars, its gravity could change the planet’s orbit by a tiny amount. Over a few years, the change would add up, allowing astronomers to measure it. So if the orbit of Mars gives a little wiggle, it just might be caused by a passing black hole.

Script by Damond Benningfield

The poles of the giant planet Jupiter look like works of art: They resemble “The Starry Night” by Vincent Van Gogh. Light-colored whirlpools spin through a dark background. There’s one whirlpool at the center of each pole, with a ring of whirlpools around it.

These beautiful spirals are cyclones – giant storms much like tropical storms on Earth. And they’re powered by the same process as those on Earth: Warm, humid air rises higher into the atmosphere, where it cools and condenses to form clouds. Jupiter’s rotation deflects the rising air, making the system spin.

But the Jovian storms are much bigger than those on Earth – the largest are the size of continents. Their winds max out at more than 200 miles per hour.

Jupiter’s north pole has nine storms – one in the middle, with a ring of eight around it. They’re all about the same size. And the configuration appears to be stable. Other cyclones try to push their way in, but they’re blocked by the existing storms.

The south pole has only six of the cyclones – five storms encircling the one at the pole. They’re much bigger than the storms at the north pole. An extra storm pushed into the ring a few years ago, but it was ejected – leaving Jupiter’s south pole with a sextet of cyclones.

Jupiter is close to the upper left of the Moon at nightfall, and looks like a brilliant star. The true star Aldebaran, the eye of the bull, is farther to the Moon’s lower left.

Script by Damond Benningfield

The body and tail of the Great Bear are well known. They form the Big Dipper. But you have to go a long way from the dipper to find the bear’s nose. It is marked by Muscida – a name that means “muzzle.”

Muscida is much bigger and brighter than the Sun. It’s also much more massive than the Sun, so it’s aged out in a hurry. It’s used up the hydrogen in its core, and is getting ready to move to the next major phase of life – even though it’s four billion years younger than the Sun.

The star has at least one companion, and maybe two. The confirmed companion is a planet. It’s bigger and heavier than Jupiter, the giant of our own solar system. It orbits Muscida at about four times the distance from Earth to the Sun. At that range, the planet is likely to get roasted as Muscida enters the next phase, which will make the star much puffier and brighter.

The “maybe” companion is a small, faint star that’s billions of miles away. Astronomers haven’t watched the stars long enough to confirm that they’re bound to each other. But the stars share a common motion through space, which is much faster than the stars around them. That suggests the system is a runaway – zipping through space in a big hurry – perhaps destined to leave the galaxy behind.

The Big Dipper is standing upright as night falls, with the bowl above the handle. Muscida is well above the dipper. Unless you’re in a light-polluted city, it’s easy to see.

Script by Damond Benningfield

Finding your way to the other bodies of the solar system is a tough chore. They’re a long way away, they’re fairly small targets, and they’re in constant motion. So it helps to have some navigational beacons to guide you.

One of those beacons is the Sun – the brightest object in the sky. And for many spacecraft, another is Canopus, the second-brightest star in the night sky.

It’s a long way from the Sun in the sky, so tracking both objects allows a craft to triangulate its position. That provides a precise fix on its location in space.

Canopus would make a good navigational beacon for any civilization within a thousand light-years of it. That’s because it’s extremely bright – more than 10 thousand times brighter than the Sun. So even though it’s more than 300 light-years away, it outshines all the other stars except Sirius, which is less than nine light-years away.

Canopus shines so brightly because of its size and its temperature. It’s about 70 times the Sun’s diameter, and its surface is thousands of degrees hotter. The combination makes Canopus one of the most impressive stars in our part of the galaxy.

And Canopus also is a good beacon for skywatchers in the far south of the United States – south of about Little Rock. Right now, it stands just above the southern horizon about an hour and a half after sunset. It’s almost directly below Sirius, which is much higher – the brightest stars in the night sky.

Script by Damond Benningfield

Sirius, the brightest star in the night sky, arcs across the southwest on March nights. It’s low in the south at nightfall and wheels westward later on. Not only is it especially bright, it’s also especially twinkly – it shifts from blue to red to white in the blink of an eye.

Sirius is visible from most of Earth’s surface – all but the high Arctic. But the second-brightest star has a more limited range. Canopus is a stunner from the southern hemisphere, but it’s tough to see from the United States. You have to be south of about San Diego or Dallas to find it. And even then, it never climbs more than a few degrees above the horizon.

Canopus itself is much more impressive than Sirius. It’s far larger and more massive, and thousands of times brighter. But it’s also 300 light-years farther, so it can’t quite match Sirius’s luster.

Canopus is at the end of its prime phase of life, so it’s quite puffy. If it took the Sun’s place in our own solar system, it would extend all the way out to Mercury. But no one is quite sure whether it’s getting even bigger, or if it’s starting to “deflate” after going through a puffier phase. Either way, Canopus will continue to shine brightly in Earth’s night sky – even if not everyone can see it.

If you’re far enough south, look for Canopus just above the southern horizon as night falls. The star is due south about 8 p.m., almost directly below Sirius.

Script by Damond Benningfield

Our three closest celestial neighbors highlight the early evening sky tonight – the Moon and the planets Venus and Mercury. Venus is the brilliant “evening star.” Fainter Mercury stands well below the Moon. It looks like a bright star, but it’s so low in the sky that it’s easy to miss.

The Moon is the closest neighbor. It orbits Earth once every four weeks, at an average distance of less than a quarter of a million miles.

Venus and Mercury are our closest planetary neighbors. Mercury is the nearest planet to the Sun, Venus is second, and Earth is third. So Venus passes closer to Earth than any other planet – as little as 24 million miles away. Combined with its size and its unbroken cloud cover, that makes Venus the brightest object in the night sky other than the Moon.

Mercury passes as little as 50 million miles from Earth. Mars can actually come closer than that. But on average, Mercury is much closer than Mars. In fact, the average distance to Mercury and Venus is the same – about 93 million miles.

If you know the layout of the solar system, that number may sound familiar – it’s the average distance from Earth to the Sun. But it’s also the average distance to any object that stays inside Earth’s orbit.

Mercury is climbing away from the Sun as seen from Earth now, while Venus is dropping toward the Sun. They’ll pass each other in about 10 days, lining up side by side in the evening twilight.

Script by Damond Benningfield

The Grand Canyon is impressive – a multi-colored chasm that’s 275 miles long, up to 18 miles wide, and more than a mile deep. But compared to the largest canyon system on Mars, it’s like something a toddler dug in a sandbox. Valles Marineris extends a quarter of the way around the planet – almost 2500 miles from end to end. At its widest, it spans 120 miles – so far that you couldn’t see from one rim to the other. And it’s more than four miles deep.

Valles Marineris is named for the Mariner 9 orbiter, which discovered the canyon half a century ago. And geologists have been debating its origin ever since. They’ve wondered whether it was carved by water, ice, winds, volcanic activity, or some combination.

Today, the leading view says it began as a crack in the crust. Over millions of years, a giant volcano near the eastern end of the canyon grew larger and larger. It became so heavy that it fractured the crust. The walls of that crack then collapsed, making it wider. The wind scoured out more of the landscape. And parts of the canyon may have been eroded by water or ice – giving Mars one of the grandest canyons in the solar system.

And Mars is in grand view. It passed closest to Earth a few weeks ago, so it’s still pretty close. It looks like a bright orange star, high in the east at nightfall. Castor and Pollux, the twin stars of Gemini, line up to its left and lower left.

Script by Damond Benningfield