StarDate

The shortest day of the year — the day with the least amount of daylight — is coming up on the winter solstice, which is less than three weeks away. One of the odd things about the solstice, though, is that it’s not the date of the latest sunrise or earliest sunset. In fact, for the latitudes of most of the United States, the Sun is setting earliest for the year right about now.

That’s because Earth’s rotation on its axis and its orbit around the Sun aren’t quite synchronized.

Our planet spins at a fairly constant rate. But its orbit isn’t a perfect circle, so Earth covers different distances on different dates. The combination means that the Sun reaches local noon — its highest point in the sky — at different times as measured on the 24-hour clock.

Right now, solar noon comes a few minutes earlier than noon on the clock. And if noon comes earlier, so does sunset. The amount of that offset varies with latitude. From 30 degrees north — the latitude of Houston or Jacksonville — earliest sunset is today. From 40 degrees — Philadelphia or Columbus — it’s December 7th. And from Seattle or Duluth, it’s around the 10th.

Over the next few weeks, solar noon will slip a little later each day. So the latest sunrises come about the same gap after the solstice — in early January. That sequence works from north to south. And right in the middle is the December solstice — the first day of winter and the shortest day of the year.

Tomorrow: a first.

Script by Damond Benningfield

The ice fields of Antarctica might seem like an odd place to hunt for rocks. But the Antarctic is the best place on Earth to hunt for rocks from beyond Earth. Meteorite hunters have picked up thousands of space rocks there. And they’re getting ready to hunt for more. Several expeditions are scheduled to hit the ice this month.

Meteorites fall all across Earth. But most of them splash into the oceans, or they hit regions of land where they’re hard to pick out — you can’t tell them from all the Earth rocks.

In Antarctica, they fall all across the continent and are frozen in the ice. As the ice flows toward the ocean, it sometimes runs into barriers, such as mountain ranges. That forces some of the meteorites toward the surface, where the ice can be blown away by the wind. So any rocks sitting atop the ice probably are meteorites.

The leading meteorite-hunting team is from the United States. Known as ANSMET, it’s recovered more than 22,000 meteorites during its 40 years of operation — more than a third of all the known meteorites on Earth.

This year, the group will explore an area known as the Miller Range. Scientists have already found thousands of meteorites there. But some of the ice hasn’t been explored yet, so it could offer up many more.

A British group is heading south as well. Last year, it tested some gear that’s used to find landmines as a way to find iron meteorites below the surface — rocks from beyond Earth hidden by the ice.

Script by Damond Benningfield

It’s balloon season in Antarctica. NASA’s getting ready to launch three giant balloons from McMurdo Station, on the Antarctic coast. At least it will try to launch them. Last year, high winds kept its balloons grounded. So some of this year’s launches are re-dos.

Antarctica provides a few advantages for scientific balloons, which carry telescopes and other instruments. At this time of year it offers around-the-clock daylight, so the balloon stays at a constant altitude — it doesn’t bob up and down as the Sun rises and sets. And air currents loop around the south pole, so a balloon circles the continent. That allows it to stay aloft for weeks without drifting far from its launch spot.

One of this year’s missions is called SuperTiger. It’s designed to look for the heaviest cosmic rays. These are the nuclei of heavy atoms — elements like iron and cobalt. They form in massive stars, or in collisions between neutron stars. They’re then boosted to near lightspeed. Studying these particles from altitudes of 25 miles or so will help scientists figure out where they came from and how they were given such a powerful “kick.”

Another mission is BLAST. It was scheduled to fly last year, but was grounded by the weather. It’ll measure grains of dust between the stars. The grains are caught in magnetic fields, which can influence the formation of new stars. Tracing the dust grains reveals the magnetic fields, helping astronomers learn more about how stars are born.

Script by Damond Benningfield

The Milky Way Galaxy is a big mishmash of stars with different origins. Some, such as our own Sun, were born in the Milky Way itself. But many others were born in other galaxies. These galaxies merged with the Milky Way, and our galaxy made their stars its own.

An immigrant star cluster is low in the south-southwest at nightfall. It’s about 27,000 light-years away, near the triangular outline of Capricornus, the sea-goat, which is close to the upper left of the Moon this evening. Through binoculars, the cluster looks like a small, hazy smudge of light.

Messier 30 is a globular cluster. It contains more than 150,000 stars, all packed into a ball that’s only about a hundred light-years in diameter.

Many of its stars, though, are jammed into the cluster’s core — a region that’s just one light-year across. By comparison, the closest star system to the Sun is more than four light-years away. So if there are any planets in the core of M30, their night skies would be ablaze with thousands of brilliant stars — so many that the night would never really get dark.

M30 orbits the center of the Milky Way in the opposite direction from most stars and star clusters. That suggests it was born in another galaxy. That galaxy merged with the Milky Way. Over time, the other galaxy’s stars and clusters spread through Milky Way — adopted members of our vast galactic home.

Script by Damond Benningfield

The Gaia Sausage isn’t made with oregano or garlic or pepper flakes. Yet it’s plenty spicy. It appears to be a streamer of stars captured by the Milky Way billions of years ago. And it includes some of the biggest and oldest star clusters in the entire galaxy.

One of those clusters is Messier 2. It’s in Aquarius, and is in the southwest at nightfall. Through binoculars, the cluster looks like a small, fuzzy patch of light.

There’s nothing small about the cluster, though. It contains 150,000 stars, packed into a ball about 175 light-years in diameter. That makes it a globular cluster. And its stars are ancient: about 13 billion years old. That means they formed when the universe was less than a billion years old.

M2 has a lot in common with at least seven other globulars in the Milky Way. The clusters are spread through a region of space that’s shaped like a sausage. It’s named for the Gaia space telescope, which has mapped the region.

The eight clusters — and possibly two others — follow similar orbits through the galaxy. They’re the same age, and their stars have about the same composition — indications that the clusters had a common origin.

They may have formed in another galaxy. The galaxy rammed into the Milky Way at least eight billion years ago. Because the Milky Way was bigger, it absorbed the other galaxy — including its globular clusters. The clusters are now long-time residents of the Milky Way — and the Gaia Sausage.

Script by Damond Benningfield

Bright, brighter, and brightest congregate in the southwest this evening. They set not long after the Sun, though, so there’s not much time to enjoy them.

The three objects are the brightest in the night sky: the Moon and the planets Venus and Jupiter. Venus is the brighter planet, quite close to the Moon. Jupiter is a little lower, so it’s tougher to spot.

The Moon looks bright only because it’s close — just a quarter of a million miles away.

In reality, though, the Moon is quite dark. On average, its surface reflects only about 10 percent of the sunlight that strikes it. That’s because the surface is made of volcanic rock. Some of it is in the form of smooth plains — the Moon’s dark features. The rest forms jumbled mountains and crater fields. These areas are lighter than the plains, but still quite dark.

Venus looks bright because it’s covered by clouds, which reflect about two-thirds of the sunlight that strikes them. It’s also close to the Sun, so it receives a lot of sunlight. And it’s close to Earth, so it reflects a lot of that light our way.

Jupiter is much farther than Venus, so it doesn’t look as bright. But it, too, is covered by clouds. And it’s the Sun’s largest planet, so it’s a big target.

The Moon and planets all set by about 90 minutes after sunset, so you need a clear horizon to see them. The view is better from more southerly latitudes. As a bonus, the planet Saturn is not far to the upper left of the bright trio.

Script by Damond Benningfield

In the afternoon of August 14th, detectors felt the universe “jiggle” in a way they hadn’t seen before — the possible result of a black hole swallowing a neutron star in a single gulp.

The “jiggles” were gravitational waves — ripples in space and time produced by the motions of extremely dense bodies. They were detected by observatories in the United States and Europe known as LIGO and VIRGO.

These observatories had already seen mergers between pairs of black holes, and pairs of neutron stars. They’d even seen one possible merger involving one of each, but the signal was too weak to confirm.

The signal in August was much stronger. Analysis showed that it was the result of a merger of an object more than five times the mass of the Sun with another less than three times the Sun’s mass. The heavier object has to be a black hole, while the lighter one is most likely a neutron star — the ultra-dense corpse of a once-mighty star. The merger took place almost 900 million light-years from Earth.

Astronomers quickly scanned the region of the sky where the gravitational waves came from at many wavelengths. They were looking for additional evidence of the merger — a flash of light as the neutron star was being pulled apart, for example. But they didn’t see a thing. That suggests the black hole may have swallowed the neutron star in a single gulp — leaving no evidence of this heavyweight merger.

Tomorrow: a bright trio in the evening sky.

Script by Damond Benningfield

Strange things have been known to happen on quiet country roads. Travelers on New Mexico highway 6563, for example, blaze past the planets of the solar system at more than 10 times the speed of light.

Or, to be more precise, they pass road signs with the names of the planets. The signs correspond to the scale of the solar system, radiating from a model of the Sun at the Sunspot Observatory. At that scale, traveling at highway speeds is like warping through space with the U.S.S. Enterprise.

Big models of the solar system have been built at many sites in the United States and other countries. They serve as tourist attractions, but they also help people understand the vast distances between planets.

There’s one in Peoria, Illinois, for example, that’s centered on a 46-foot disk of the Sun at a science museum. And one in Midland County, Michigan, is built along a historic trail.

The biggest one in the U.S. stretches 40 miles across Maine. In that model, one mile equals the distance from Earth to the Sun — 93 million miles. The Sun forms a 50-foot ring that’s inside a building at a University of Maine campus. Earth is a Styrofoam ball less than six inches in diameter.

The largest solar-system model in the world is in Sweden. The Sun is represented by a dome-shaped arena that’s 360 feet in diameter. Earth is almost five miles away, with Neptune, the most distant planet, at more than 140 miles. At lightspeed, that’s just four hours away.

Script by Damond Benningfield

Perpetual darkness and frigid temperatures don’t usually pack in the tourists. But when you throw in some shimmering curtains of light in the sky, they show up in bunches.

In the last decade or so, the aurora borealis — the northern lights — has become big business. Tourists flock to Scandinavia, Iceland, Alaska, and similar sites to see it.

The aurora is formed by the interplay between Earth’s magnetic field and the solar wind — a flow of charged particles from the Sun. The magnetic field funnels the particles into the upper atmosphere, where they zap atoms and molecules, causing them to glow.

The aurora forms in a narrow ring around the magnetic north pole. Most of the time, the ring stays far north. So the aurora is best viewed from far-northern latitudes — places where the sky stays dark for weeks at a time.

Tourist bureaus, lodges, and others are playing up the aurora. They offer package deals that include sleigh rides, dog sledding, snowmobiling, and other activities. Resorts have built glass-domed igloos so visitors can stay cozy while they watch. Train operators offer glass-topped coaches, and ship lines are flogging aurora cruises.

You don’t need a package deal to see the aurora, though. You can visit a national park or other dark-sky site within the auroral zone. And several web sites post predictions of auroral activity — letting you know when and where to view these amazing light shows.

More about astronomy tourism tomorrow.

Script by Damond Benningfield

One of nature’s most unforgettable light shows is the aurora — ribbons and curtains of color in the night sky. But the auroras on Jupiter, the Sun’s largest planet, are a thousand times more powerful — and much more complicated.

On Earth, an aurora forms when charged particles from the Sun bombard our planet’s magnetic field. The field funnels particles toward the surface. They zap atoms and molecules in the upper atmosphere, causing them to glow.

Jupiter’s auroras are fed by particles from the Sun as well. But they’re juiced up by particles from inside Jupiter’s own magnetic field.

Many of those particles come from Io, the closest of Jupiter’s four big moons. Volcanoes on Io blast gas and dust into space. Jupiter’s magnetic field gives the particles an electric charge, then guides them into the atmosphere near the poles. That keeps Jupiter’s auroras going all the time.

The auroras are also affected by the other moons. As they sweep through the magnetic particles around Jupiter, the moons push some of the particles toward the planet.

The biggest moon, Ganymede, produces its own magnetic field. That interacts with Jupiter’s magnetic field to funnel even more particles toward the surface — adding to the Jovian light show.

And Jupiter is teaming up with the planet Venus in the evening sky. Venus is the “evening star.” Fainter Jupiter is quite close to it for the next few nights.

We’ll have more about auroras tomorrow.

Script by Damond Benningfield