No matter where you live on Earth’s globe – no matter what time it happens for you – the solstice is your signal to celebrate seasonal change.

What is it? The December solstice marks the sun’s southernmost point in the sky, for the entire globe, for this entire year. On this solstice, the sun will be overhead at noon as viewed from the Tropic of Capricorn.
When is it? In 2023, the December solstice falls at 3:27 UTC on December 22 (9:27 p.m. central on December 21).
Note: For us in the Northern Hemisphere, the December solstice will mark the longest nights and shortest days of the year. For the Southern Hemisphere, it will mark the shortest nights and longest days. After this solstice, the sun will move north again.

What is a solstice?

The earliest people on Earth knew that the sun’s path across the sky, the length of daylight, and the location of the sunrise and sunset all shifted in a regular way throughout the year. They built monuments such as Stonehenge in England and at Machu Picchu in Peru to follow the sun’s yearly progress.

But today, we see the solstice differently. We can picture it from the vantage point of space, and we know that the solstice is an astronomical event. It’s caused by the tilt of Earth’s axis and by its orbital motion around the sun.

Earth doesn’t orbit upright. Instead, it’s tilted on its axis by 23 1/2 degrees. Through the year, this tilt causes Earth’s Northern and Southern Hemispheres to trade places in receiving the sun’s light and warmth most directly. It’s this tilt, not our distance from the sun, that causes winter and summer. In fact, we’re closest to – not farthest from – the sun at the turn of every new year. But we in the Northern Hemisphere are moving into winter. That’s because the Northern Hemisphere leans farthest away from the sun for the year around this time.

The December solstice

At the December solstice, Earth is positioned so the sun stays below the North Pole’s horizon. As seen from the latitude 23 1/2 degrees south of the equator, at the imaginary line encircling the globe known as the Tropic of Capricorn, the sun shines directly overhead at noon. This is as far south as the sun ever gets, and all locations south of the equator have day lengths greater than 12 hours.

Meanwhile, all locations north of the equator have day lengths shorter than 12 hours.

For us on the northern part of Earth, the shortest day comes at the solstice. After the December solstice, the days will get longer, and the nights shorter.

It’s a seasonal shift that nearly everyone notices.

Where should I look to see signs of the December solstice in nature?

Everywhere.

For all of Earth’s creatures, nothing is so fundamental as the length of daylight. After all, the sun is the ultimate source of all light and warmth on Earth.

In the Northern Hemisphere, you’ll notice late dawns and early sunsets, the low arc of the sun across the sky each day, and how low the sun appears in the sky at local noon. Look at your noontime shadow, too. Around the time of the December solstice, it’s your longest noontime shadow of the year.

In the Southern Hemisphere, it’s opposite. Dawn comes early, dusk comes late, the sun is high, and it’s your shortest noontime shadow of the year.

Why doesn’t the earliest sunset come on the shortest day?

The December solstice marks the shortest day of the year in the Northern Hemisphere and longest day in the Southern Hemisphere. But the earliest sunset – or earliest sunrise if you’re south of the equator – happens before the December solstice.

Instead of focusing on the time of sunset or sunrise, the key is in what is called true solar noon, which is the time of day that the sun reaches its highest point in its journey across your sky.

In early December, true solar noon comes nearly 10 minutes earlier by the clock than it does at the solstice around December 21. With true noon coming later on the solstice, so will the sunrise and sunset times.

It’s this discrepancy between clock time and sun time that causes the Northern Hemisphere’s earliest sunset and the Southern Hemisphere’s earliest sunrise to precede the December solstice.

This happens primarily because of the tilt of the Earth’s axis. A secondary but another contributing factor to this discrepancy between clock noon and sun noon comes from the Earth’s elliptical – oblong – orbit around the sun. Earth’s orbit is not a perfect circle, and the closer we are to the sun, the faster we move in our orbit.

Our closest point to the sun – or perihelion – comes in early January. So, we are moving fastest in orbit around now, slightly faster than our average speed of about 19 miles per second (30 km per second). The discrepancy between sun time and clock time is greater around the December solstice than the June solstice because we’re nearer the sun at this time of year.

Does latitude affect the earliest sunset?

Yes! The precise date of the earliest sunset depends on your latitude. At mid-northern latitudes, it comes in early December each year. At northern temperate latitudes farther north – such as in Canada and Alaska – the year’s earliest sunset comes around mid-December. Close to the Arctic Circle, the earliest sunset and the December solstice occur on or near the same day.

By the way, the latest sunrise doesn’t come on the solstice either. From mid-northern latitudes, the latest sunrise comes in early January.

The exact dates vary, but the sequence is always the same: earliest sunset in early December, shortest day on the solstice around December 22, latest sunrise in early January.

And so the cycle continues.

Bottom line: The 2023 December solstice takes place on December 22, at 3:27 UTC. It marks the Northern Hemisphere’s shortest day (first day of winter) and Southern Hemisphere’s longest day (first day of summer). Happy solstice to all!

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The post 2023 December solstice: All you need to know first appeared on EarthSky.

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Iceland volcano erupts, after weeks of earthquakes

For weeks, volcanologists had been predicting an eruption of a volcano in southwestern Iceland, on the Reykjanes Peninsula, in the most populated part of the country. There’d been earthquakes near the town of Grindavik, about 2.4 miles (4 km) away. And there’d already been evacuations. Then, yesterday, boom! The volcano suddenly blew, blasting lava fountains high into the sky. Ultimately, volcanologists said, the eruption was more powerful than they’d expected, lighting up the sky miles away in the center of Iceland’s capital, Reykjavik, only about 30 miles (50 km) away.

AP reported:

The town near Iceland’s main airport was evacuated in November after strong seismic activity damaged homes and raised fears of an imminent eruption.

Iceland, which sits above a volcanic hotspot in the North Atlantic, averages an eruption every four to five years. The most disruptive in recent times was the 2010 eruption of the Eyjafjallajokull volcano, which spewed huge clouds of ash into the atmosphere and led to widespread airspace closures over Europe.

WATCH: Volcano erupts in Iceland, lava shooting into the air from fissure extending 2.5 miles (4 km) pic.twitter.com/M93QePO4wK

— BNO News (@BNONews) December 19, 2023

Will it close airports in Europe?

But this eruption is not expected to produce as much ash, AP said. In fact, according to AP:

Iceland’s Foreign Minister Bjarne Benediktsson said on X, formerly Twitter, that there were no disruptions of flights to and from Iceland and international flight corridors remain open.

Big fissure, getting bigger

Meanwhile, the New York Times reported that the volcano’s fissure is:

… some 2.5 miles [4 km] long and growing quickly … [It’s] not far from the Svartsengi Power Plant and the town of Grindavík, which was evacuated last month because of heightened seismic activity, leading to concerns than an eruption was likely.

The Times said that the direction of the lava flow is still “unpredictable.”

More reports from volcano-watchers

The moment the eruption in Iceland started. This just blows my mind!!! pic.twitter.com/xOegj91xMQ

— Volcaholic ? (@volcaholic1) December 18, 2023

Reykjanes volcano in Iceland is erupting! 154 small earthquakes in under 2 hours. 3.5km (just over 2 miles) long fissure eruption. Things can change rapidly on the ground, but this pic by the Iceland Coast Guard is stunning. pic.twitter.com/UKy93Qfyx1

— Jess Phoenix, Lava Connoisseur ? (@jessphoenix2018) December 19, 2023

Bottom line: After weeks of earthquakes, an Iceland volcano – on the Reykjanes Peninsula – began erupting powerfully on December 18, 2023. It was a bigger eruption than volcanologists had predicted!

Via AP

Via the New York Times

The post Iceland volcano erupts! See a livestream here first appeared on EarthSky.

The bright waxing gibbous moon will pass the faint but distinct Circlet asterism in Pisces the Fish on the evenings of December 19 and 20, 2023. The moon and the Circlet will be visible as darkness falls and will set after midnight.

December 19: 1st quarter moon

The instant of 1st quarter moon will fall at 18:39 UTC (12:39 p.m. CST), on December 19, 2023. The 1st quarter moon rises around noon your local time and sets around midnight.

EarthSky Minute, December 18-21, 2023

See what’s in store for in the sky for the coming days.

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EarthSky Minute, moon phases December 2023

December 21 and 22 evenings: Moon near Jupiter

On the evenings of December 21 and 22, 2023, the waxing gibbous moon will glow near the bright planet Jupiter. The moon and Jupiter will set several hours after midnight.

December solstice: December 21-22

The December solstice is at 3:27 UTC on December 22, 2023 (9:27 p.m. December 21 CST). Winter arrives in the Northern Hemisphere and summer arrives in the Southern Hemisphere.

December 22: Mercury will pass between Earth and the sun

Mercury will reach the point in its orbit known as inferior conjunction – when it passes between Earth and the sun – at 19 UTC on December 22.

Our charts are mostly set for the northern half of Earth. To see a precise view – and time – from your location, try Stellarium Online.

December 22-23 mornings: Ursid meteor shower

The predicted peak of the Ursid meteor shower is on the early mornings of December 22 and 23, 2023. The best time to watch is before dawn and after the waxing gibbous moon has slipped below the horizon.

December 23 and 24 evenings: Moon near the Pleiades

The bright waxing gibbous moon will pass the Pleiades star cluster on the evenings of December 23 and 24, 2023. The Pleiades is also known as the Seven Sisters or Messier 45 and appears as a glittering, bluish cluster of stars in the constellation Taurus the Bull. The moon and Pleiades will cross the sky together until about an hour before sunrise.

December 25 all night: Moon near Capella, Aldebaran and Betelgeuse

December 25, 2023, the bright waxing gibbous moon will lie near a trio of bright stars. It’ll be near the fiery orange star Aldebaran of Taurus the Bull and Orion’s mighty red supergiant star Betelgeuse. The bright golden star is Capella of the constellation Auriga the Charioteer. If you catch Capella low on the horizon, it may be flashing like a small disco ball. You can follow them all night until sunrise.

December 26-27, all night: Full Moon

The instant of full moon will fall at 0:33 UTC on December 27, 2023 (6:33 p.m. CST December 26).

Our charts are mostly set for the northern half of Earth. To see a precise view – and time – from your location, try Stellarium Online.

December 26 and 27 evenings: Moon near Castor and Pollux

On the evening of December 26, 2023, the full moon will pass Castor and Pollux, the twin stars of Gemini. Then on the evening of December 27, the waning gibbous moon will be closer to the Twin Stars of Gemini. They’ll rise a few hours after sunset and be visible traveling across the sky all night.

December 29 morning: Moon near the Beehive

For the second time this month, the moon will slide by the famous – but faint – Beehive star cluster. This time it will be a waning gibbous moon on the morning of December 29, 2023. You will need binoculars to spot the dozens of stars in the busy Beehive star cluster. Also nearby will be the twin stars, Castor and Pollux, in the constellation Gemini. They’ll rise late the night before and be high in the morning sky.

December 30 and 31 mornings: Moon near the Sickle

On the mornings of December 30 and 31, 2023, the waning gibbous moon will float near Regulus, marking the bottom of the backward question mark asterism called the Sickle. Regulus is the brightest star in Leo the Lion. And the moon will lie close to Regulus on the final morning of 2023. They’ll rise late in the evening the night before and be high in the morning sky.

Our charts are mostly set for the northern half of Earth. To see a precise view – and time – from your location, try Stellarium Online.

Planets in December 2023

December mornings: Venus

The blazing light of Venus will continue to dominate the morning sky through the end of the year. However, it will be sinking lower each morning after it reached its greatest elongation from the sun in October. Luckily, it will rise early enough to see it in dark skies. Venus will begin the month at -4.2 magnitude and will dim slightly as it recedes from Earth, ending the month at -4.1 magnitude. A lovely waning crescent moon will join Venus on the morning of December 9, 2023, when they appear about 4 degrees apart. Also, Venus will appear close to the star Spica at the beginning of the month, and then they drift apart more each morning. Venus begins the month in the constellation Virgo the Maiden and will move into Libra the Scales by mid-month. Then by month’s end, it will be in the constellation Scorpius the Scorpion.

December most of the night: Jupiter

Bright Jupiter will draw your attention most of the night. It will be very obvious in the east at sunset and will be visible until a few hours before dawn. It will shine near the pretty Pleiades star cluster in the constellation Taurus the Bull. Jupiter reached its closest point to the Earth in early November. And it reached opposition overnight on November 2-3, 2023, when we flew between it and the sun. So, as Jupiter recedes from Earth, it’ll fade a bit in our sky. It will lie in the dim constellation Aries the Ram. It will shine at -2.7 magnitude by month’s end. The waxing gibbous moon will float by Jupiter on December 22, 2023.

December evenings: Saturn

Golden Saturn will shine in the southwest after sunset during December. It will be in the dim constellation Aquarius the Water Bearer. Our solar system’s beautiful ringed planet will be fading a bit this month as it recedes from Earth and will shine at +0.9 magnitude for most of the month. Saturn will be a little brighter than the nearby lonely star, Fomalhaut, which shines around 1st magnitude. The waxing gibbous moon will visit Saturn on the evenings of December 17, 2023. Saturn will set by mid-evening your local time this month.

December evenings: Mercury

Mercury will be emerging low in the morning sky the last few days of the year on its way to its first greatest morning elongation of 2024.

Where’s Mars?

It’s too close to the sun in the morning sky to be visible for the rest of 2023. It’ll be back next year.

Thank you to all who submit images to EarthSky Community Photos! View community photos here. We love you all. Submit your photo here.

Looking for a dark sky? Check out EarthSky’s Best Places to Stargaze.

Sky dome maps for visible planets and night sky

The sky dome maps come from master astronomy chart-maker Guy Ottewell. You’ll find charts like these for every month of 2023 in his Astronomical Calendar.

Guy Ottewell explains sky dome maps

Heliocentric solar system planets

The sun-centered charts come from Guy Ottewell. You’ll find charts like these for every month of 2023 in his Astronomical Calendar.

Guy Ottewell explains heliocentric charts.

Some resources to enjoy

For more videos of great night sky events, visit EarthSky’s YouTube page.

Watch EarthSky’s video about Two Great Solar Eclipses Coming Up

Don’t miss anything. Subscribe to daily emails from EarthSky. It’s free!

Visit EarthSky’s Best Places to Stargaze to find a dark-sky location near you.

Post your own night sky photos at EarthSky Community Photos.

Translate Universal Time (UTC) to your time.

See the indispensable Observer’s Handbook, from the Royal Astronomical Society of Canada.

Visit Stellarium-Web.org for precise views from your location.

Almanac: Bright Planets (rise and set times for your location).

Visit TheSkyLive for precise views from your location.

Bottom line: Visible planets: Venus dominates the morning sky. Jupiter is visible most of the night, Saturn sets late evenings and Mercury is low on the western horizon. Visible planets and night sky, here!

The post Visible planets and night sky for December first appeared on EarthSky.

Rain that doesn’t reach the ground

Have you seen clouds that are pouring rain … but the rain never reaches the ground? Meteorologists call this rain by the name virga. You see virga in places where the air is dry, and often warm. The rain evaporates as it falls, before hitting Earth. So you might see virga in a desert, or at high altitudes, for example, in the western U.S. and Canadian prairies, the Middle East, Australia and North Africa. Virga isn’t rare. But it’s delicate and very beautiful. Maybe you’ve seen it lots of times, but never knew it had a name?

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Virga on radar

Sometimes, when you’re looking at your weather app, you might see what looks like rain or snow on the radar, but nothing is falling outside. Instead, look up at the clouds and see if you can spot virga. The radar is picking up precipitation in the air which is just not reaching the ground. As weather.gov says:

The radar isn’t lying, rather, the the rain or snow is not hitting the ground. If you have a dry air mass in place in the low levels, sometimes rain cannot completely penetrate that dry layer before it evaporates.

Do you want to learn to identify virga when you see it? Check out the photos on this page from our global EarthSky community. Once you acquaint yourself with the variations of virga, you’ll be able to spot it in your own sky. If you capture a photo of virga, submit it to us!

Photos of virga from EarthSky’s community

More photos

Bottom line: Learn what virga is and how it forms, and see great photos to help you learn how to identify it yourself!

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The post Virga is rain that doesn’t reach the ground first appeared on EarthSky.

10 tips for watching the Geminids

2023’s Geminid meteor shower is expected to peak on December 14. This year, a thin waxing crescent moon won’t interfere viewing the meteors. And, the radiant point rises in mid-evening. So there will be darkness – on both the evenings of December 13 and 14 – from mid-evening until dawn. Read more about this year’s Geminid meteor shower.

Moon or no moon, the Geminid meteor shower is always worth a look. You never know when you’ll be surprised by a bright fireball. The radiant, which is near the star Castor in Gemini the Twins, rises in mid-evening for all of us around the globe. And then, it reaches nearly overhead around 2 a.m. (that’s why the shower is best then).

While it’s possible to see up to 120 meteors per hour on a dark night, when the radiant is overhead, it’s still exciting to see even just a handful of Geminid meteors. For your best chance to see the most Geminids, make sure you’re in a dark-sky location. Here are 10 tips to get the most out of the Geminids in 2023.

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1. The peak viewing time is around 2 a.m.

Geminid meteor numbers tend to intensify as evening deepens into late night, with the greatest number of Geminids likely falling an hour or two after midnight, when the meteor shower’s radiant point appears highest in the sky as seen from around the globe. That time holds true no matter your time zone. The waxing crescent moon will set soon after sunset on both December 13 and 14. So, you’ll be able to watch the Geminids in a moonless sky. Visit Sunrise Sunset Calendars to find the moonrise and moonset time for your specific location. Be sure to check the moonrise and moonset box.

2. Get away from city lights.

For optimum viewing, find a dark place to observe in the country.

3. Give yourself a wide-open view of the sky.

A farmer’s field? A stretch of country road? A campsite with a clear view in one or more directions? An open sky will increase your chances of seeing some meteors.

4. Watch for Geminids for an hour or more.

The 2023 Geminid meteor shower will be better if you let your eyes adapt to the dark. That can take as long as 20 minutes. Plus, the meteors tend to come in spurts, followed by lulls. Be patient! You’ll see some.

5. You don’t need to find the radiant point.

You don’t need to look in a single direction – or locate the Geminid’s radiant point – to have fun watching the shower. The meteors will appear all over the sky. The radiant point is interesting, though. If you track Geminid meteors backward on the sky’s dome, you’ll find them streaming from this point, within the constellation Gemini the Twins. Hence this shower’s name.

6. Pay attention to the moon.

The moon will be in a waxing crescent phase, the moon will not interfere with the Geminid meteors. However, if the moon was out – because you can look in any direction to spot Geminids – you can look at areas of the sky away from the moon. That’s because anything in the moon’s vicinity will likely be washed out by its bright light. Another tip for watching in moonlight: place yourself in a moon shadow. Observing from the shadow of a barn, or mountain, even a tree, can help you see more meteors.

7. Think about the Geminids’ parent object.

Most meteors in annual showers originate in comets. But the parent of the Geminid meteor shower is a mysterious body named 3200 Phaethon. This solar system object is termed an Apollo (near-Earth) asteroid, and it might be a dormant comet. This tidbit may not help you watch the shower, but it’s fun to discuss as you wait for the next meteor. Click here for more on weirdly comet-like 3200 Phaethon.

8. Bring along a buddy.

Both of you watch different parts of the sky. If you see one, shout “Meteor!” Let your eyes rove casually in all parts of the sky.

9. No special equipment needed.

You may be more comfortable with a reclining lawn chair, blankets, snacks and a hot drink. And leave your electronic devices in your pocket or the car. Even night mode is bright enough to ruin your night vision.

10. Enjoy nature.

As a wise man once said, meteor watching is a lot like fishing. You go outside. You hope you catch some!

Bottom line: The Geminids make up a rich and much-loved annual meteor shower that peaks in mid-December. Maximize your viewing time with these tips.

The post 10 tips for watching the Geminids in 2023 first appeared on EarthSky.

The Geminid meteor shower is peaking on the nights of December 13 and 14. Read more about 2023’s Geminid meteor shower.

Weirdly comet-like 3200 Phaethon

Most meteors in annual showers have comets as their sources. But not December’s Geminid meteors, whose source, known as 3200 Phaethon, is a strange hybrid of an asteroid and a comet. This “rock-comet” isn’t icy, like a comet is. But it brightens as it nears the sun, as comets do. And it’s been observed to have a tail when nearest the sun. Plus, it spawns the Geminid meteor shower. And so scientists have long puzzled over 3200 Phaethon. How can a rocky asteroid leave behind debris that sparks a meteor shower? Where does its tail come from?

In 2023, a couple of new studies provide insights on 3200 Phaethon, parent object of one of the year’s best meteor showers.

Available now! 2024 EarthSky lunar calendar. A unique and beautiful poster-sized calendar showing phases of the moon every night of the year. Makes a great gift!

New research from Lowell Observatory

The Planetary Science Journal published a new study about 3200 Phaethon in April 2023.

On December 13, 2023, Spaceweather.com quoted astronomer Karl Battams of the U.S. Naval Observatory – a co-author of the April study – as saying:

Our work has upended years of belief about 3200 Phaethon, the source of the Geminids. It’s not what we thought it was.

In other words, since its discovery in 1983, 3200 Phaethon has appeared to be a rocky asteroid. NASA’s STEREO spacecraft first observed its tail – which appears when 3200 Phaethon passes near the sun in its 524-day orbit – in 2009 and 2012.

But, according to the story in Spaceweather.com today (December 13), Qicheng Zhang at Lowell Observatory in Flagstaff was “never convinced.” Spaceweather.com explained:

For one thing, the Geminid debris stream is massive (1,013 kg or 2,233 pounds), while the tail of 3200 Phaethon is puny, providing less than 1% of the mass required to explain the Geminids.

‘The tail we see today could never supply enough dust to supply the Geminid meteor shower,’ says Zhang.

Zhang, Battams, and colleagues decided to take a closer look. Using coronagraphs on the Solar and Heliospheric Observatory (SOHO), they monitored Phaethon as it passed by the sun in 2022. Color filters on the spacecraft revealed no dust or rock. Instead, Phaethon’s tail is made of sodium gas.

And therein lies the twist. Meteor showers are made of meteoroids, not gas. Suddenly, the Geminids are a mystery again.

‘We’re back to square one,’ says Zhang. ‘Where do the Geminids come from?’

Source: Sodium Brightening of (3200) Phaethon near Perihelion

More new research on 3200 Phaethon

Could the answer come from more new research, published on November 2, 2023, by a team at the University of Helsinki? This new study appears in the journal Nature Astronomy.

This team compared an infrared spectrum of 3200 Phaethon – from NASA’s Spitzer space telescope – to infrared spectra of known meteorites. They found Phaethon’s spectrum showed olivine, carbonates, iron sulfides and oxide minerals. Those substances are also found in the composition of CY carbonaceous chondrite meteorites (a rare type of meteorites with only six samples available for study).

When CY meteorites are exposed to high temperatures, the carbonates in the meteorites produce carbon dioxide. That releases water vapor, and the sulfides release sulfur gas. Could that be what’s happening in 3200 Phaethon also? Is that why this object has a tail when nearest the sun?

Source: Thermal decomposition as the activity driver of near-Earth asteroid (3200) Phaethon

NASA also found sodium coming off 3200 Phaethon’s surface

Scientists with NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, first announced sodium fizzing from the asteroid’s surface in 2021. Their statement explained that this asteroid:

… brightens as it gets close to the sun. Comets typically behave like this: When they heat up, their icy surfaces vaporize, causing them to become more active and brighten as the venting gases and dust scatter more sunlight. But what is causing Phaethon to brighten if not vaporizing ices?

So it’s been thought for a few years that sodium could play a role in the formation of 3200 Phaethon’s tail.

Read more: Fizzing Sodium Could Explain Asteroid Phaethon’s Cometlike Activity

All that, and blue, too

By the way, the comet-like behavior of this asteroid isn’t the only unusual thing about it. For one thing, 3200 Phaethon has an odd color for an asteroid. Most asteroids are dull grey to red, depending on the type of material on their surface. 3200 Phaethon is blue. It’s not the only blue asteroid, but blue asteroids make up only a fraction of all known asteroids. And Phaethon isn’t just blue. It’s one of the bluest of similarly colored asteroids (or comets) in the solar system.

Here’s another odd feature of 3200 Phaethon. While comets tend to have more elliptical orbits, asteroid orbits are more circular. 3200 Phaethon’s orbit – which is now exceedingly well known – is highly elongated, reminiscent of some comets. Its orbit crosses the orbits of Mars, Earth, Venus and Mercury.

Plus, its orbit brings 3200 Phaethon closer to the sun than any other named asteroid (though some smaller, unnamed asteroids come even closer). At its closest point, Phaethon is only 13 million miles (20.9 million km) from the sun. That’s less than half of Mercury’s closest distance.

The name of this object – 3200 Phaethon – honors its relationship to the sun. In Greek mythology, Phaethon was the son of the sun god Helios.

A potentially hazardous asteroid

3200 Phaethon is classified as a potentially hazardous asteroid. But that doesn’t mean it’s a threat to Earth. It just means two things. First, 3200 Phaethon is big. The latest estimates (2021) suggest it’s 3.6 miles (5.8 km) wide. It’s big enough to cause significant regional damage if it were to strike Earth. Second, it makes periodic close approaches to Earth. But astronomers know of no upcoming strike by this object in the foreseeable future.

In 2017, 3200 Phaethon came closer to Earth than it will again until 2093. At its closest in 2017, it was still about 26 times the moon’s distance away.

Videos of 3200 Phaethon

Both amateur and professional astronomers watched 3200 Phaethon as carefully as they could in 2017. For example, Northolt Branch Observatories in London, England, created the animation below from images it captured in 2017.

Steven Bellavia also produced a video (below) of 3200 Phaethon in 2017. He commented then that he’d endured cloudy weather and subfreezing temperatures in order to capture the images.

The history of 3200 Phaethon

3200 Phaethon was the first asteroid discovered via spacecraft, on October 11, 1983. Astronomers Simon F. Green and John K. Davies noticed it while searching Infrared Astronomical Satellite data for moving objects. Charles T. Kowal confirmed it optically and said it was asteroid-like in appearance. The object received the provisional designation 1983 TB. Two years later, in 1985, using the convention for naming asteroids, astronomers assigned it its asteroid number and name: 3200 Phaethon.

Before 3200 Phaethon, scientists linked all known meteor showers to comets and not asteroids.

Thus, 3200 Phaethon surprised them from the beginning, because – while it looked like an asteroid – it appeared to be the source of the annual Geminid meteor shower. Astronomers began calling 3200 Phaethon a comet-asteroid hybrid, an asteroid that behaves like a comet. Later, they began using the term rock-comet.

What else will we learn about this object, as the years pass?

Bottom line: The Geminid meteor shower has a unique source – 3200 Phaethon – sometimes called a comet-asteroid hybrid, or a rock-comet. In November 2023, scientists found the composition of 3200 Phaethon matches a rare type of meteorite that releases gas when heated to temperatures like 3200 Phaethon experiences when it nears the sun. And in 2021, scientists suggested that some of this object’s comet-like behavior might stem from sodium fizzing from its surface.

Source: Thermal decomposition as the activity driver of near-Earth asteroid (3200) Phaethon

Via University of Helsinki, JPL and U.S. Naval Research Lab

The post Mysterious 3200 Phaethon – parent to the Geminids – revealed? first appeared on EarthSky.

When the sun dies

What does death mean for the sun? It means our sun will run out of fuel in its interior. It’ll cease the internal thermonuclear reactions that enable stars to shine. It’ll swell into a red giant, whose outer layers will engulf Mercury and Venus and likely reach the Earth. Life on Earth will end.

If the sun were more massive – estimates vary, but at least several times more massive – it would explode as a supernova. So … no supernova. But what? What happens next? An international team of astronomers used a stellar data-model that predicts the life cycle of stars to answer this question.

Their research was published in the peer-reviewed journal Nature Astronomy and is available to read at arXiv.org. It suggested that the sun is almost exactly the lowest mass star that – at the end of its life – produces a visible, though faint, planetary nebula.

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About planetary nebulae

The name planetary nebula has nothing to do with planets. It describes a massive sphere of luminous gas and dust, material sloughed off an aging star. In the 1780s, William Herschel called these spherical clouds planetary nebulae because, through his early telescope, planetary nebulae looked round, like the planets in our solar system.

Astronomers already knew that 90% of all stars end their active lives as planetary nebulae. They were reasonably sure our sun would meet this fate. The key word here is visible. For years, scientists thought the sun has too low mass to create a visible planetary nebula.

Albert Zijlstra of the University of Manchester in England is a co-author of the study. He said in a statement:

When a star dies it ejects a mass of gas and dust – known as its envelope – into space. The envelope can be as much as half the star’s mass. This reveals the star’s core, which by this point in the star’s life is running out of fuel, eventually turning off and before finally dying.

It is only then the hot core makes the ejected envelope shine brightly for around 10,000 years – a brief period in astronomy. This is what makes the planetary nebula visible. Some are so bright that they can be seen from extremely large distances measuring tens of millions of light-years, where the star itself would have been much too faint to see.

The fate of our sun

Will that be the fate of our sun? Will it – at the end of its life – become briefly visible to alien astronomers on planets millions of light-years away? These astronomers say no. They say their models predict that our sun – though forming a planetary nebula at the end of its life – will remain faint.

Read more about this study from the University of Manchester

By the way … what happens next? Eventually, the planetary nebula will disperse and fade. With its thermonuclear fuel gone, the sun will no longer be able to shine. The immensely high pressures and temperatures in its interior will slacken. The sun will shrink down to become a dying ember of a star, known as a white dwarf, only a little larger than Earth.

Bottom line: A study suggests our sun is about the lowest mass star that – at the end of its life – produces a visible, though faint, planetary nebula. What that is … and more on the fate of our sun, here.

Source: The mysterious cut-off of the Planetary Nebula Luminosity Function

Via University of Manchester

The post When our sun dies, what will happen to Earth? first appeared on EarthSky.

The orange star Aldebaran – the fiery eye of the Bull in the constellation Taurus – is easy to find. It’s part of a V-shaped group of stars – the Hyades – that forms the Bull’s face. You can locate Aldebaran using the famous constellation Orion as a guide. Notice the three stars of Orion’s Belt. Then draw an imaginary line through the Belt to the right. The first bright star you come to will be Aldebaran with its distinctive reddish-orange glow.

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When to spot Aldebaran

Aldebaran is the 14th brightest star, but five of those that outshine it are only barely visible or not visible at all from much of the Northern Hemisphere. Aldebaran is primarily a winter and spring star for us on the northern part of Earth. That’s when this orange star is most easily visible in the evening sky. By early December, it rises shortly after sunset and is visible all night. Three months later it is high to the south at sunset, and sets at around midnight. By early May, it hangs low about the western sunset glow – and before the end of the month, it’s lost altogether. It returns to the predawn sky around late June.

By the way, although it appears among them, Aldebaran is not actually a member of the V-shaped Hyades cluster. It is actually much closer to us in space than the other Hyades stars.

History and mythology of Aldebaran

Artists often depict Aldebaran as the fiery eye of Taurus the Bull. Because it is bright and prominent, ancient Persians honored Aldebaran one of the Four Royal Stars, the other three being Regulus, Antares and Fomalhaut.

The name Aldebaran is from the Arabic for the follower, presumably as a hunter following prey, which was likely the star cluster we call the Pleiades. Some viewed the latter as a flock of birds, perhaps doves. According to Richard Hinckley Allen in his classic book Star Names: Their Lore and Meaning, people once applied the name Aldebaran to the entire Hyades star cluster, a large loose collection of faint stars.

More star lore

In Hindu myth, Aldebaran was a beautiful young woman named Rohini, disguised as an antelope and pursued by her lecherous father, disguised as a deer, Mriga. Several other ancient peoples associated the star with rain. In a Dakota Sioux myth, Aldebaran was a star which had fallen to the Earth and whose killing of a serpent led to the formation of the Mississippi River. Allen notes a number of other alternate names, but precious little mythology is known for Aldebaran separately.

Also, Aldebaran is the name of one of the chariot horses in the movie and book “Ben Hur.”

On a different note, astronomer Jack Eddy has suggested a connection with the Big Horn Medicine Wheel, an ancient circle of stones atop a mountain in Wyoming. Eddy wrote that the ancient Americans may have used this site as a sort of observatory to view the rising of Aldebaran just before the sun in June to predict the June solstice.

Interestingly, in about two million years, the NASA space probe Pioneer 10, now heading out into deep space, will pass Aldebaran.

Science of Aldebaran

Aldebaran is a huge aging star. The diameter is about 44 times the size of our sun. If Aldebaran replaced our sun, its surface would extend past the orbit of Mercury.

Aldebaran glows with the orangish color of a K5 giant star. In visible light, it is about 153 times brighter than the sun, although its surface temperature is lower, roughly 4,000 K (about 3,700 degrees C or 6,700 degrees F) compared to 5,800 K (about 5,500 C or 10,000 F) for the sun.

Although Aldebaran is associated with the stars of the Hyades, it’s much closer at 65 light-years distant. The Hyades are about 150 light-years away.

Aldebaran is an erratic variable with minor variations too small to see with the eye. Also, five faint stars are visible near Aldebaran, but so far none have been confirmed to be gravitationally bound to Aldebaran.

Aldebaran’s position is RA: 4h 35m 55s, dec: 16°30’35”

Bottom line: Aldebaran is an enormous, orange-colored star that marks one of the eyes of Taurus the Bull. It also marks one point of the V-shape of the Bull’s face.

The post Aldebaran is Taurus the Bull’s bloodshot eye first appeared on EarthSky.

Graham Jones of timeanddate.com joins with Deborah Byrd of EarthSky to explain why the December 4-5 quarter moon – aka a ‘half moon’ – is more than 50% illuminated. Plus, we share some key moon dates for 2024, from the closest moon to the shortest lunar month. Original article from timeanddate.com on November 27, 2023. Reprinted with permission. Edits by EarthSky.

Quarter moon or half moon?

From our perspective on Earth, the 3rd quarter moon on the night of December 4-5, 2023, will be the most-illuminated quarter moon of this year. It’ll be 50.137% lit, as seen by us on Earth.

A quarter moon looks half-illuminated in Earth’s sky. Some people even call it a half moon. And we in astronomy often say a quarter moon – aka a half moon – appears from Earth to be 50% illuminated.

But it’s not true. Instead, every quarter moon (half moon) is always slightly more than 50% illuminated.

And the December 4-5, 2023, quarter moon is the most illuminated of all the quarter moons this year.

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A fraction more than half?

Who knew that a quarter moon is always more than half illuminated? That’s not common knowledge, even among astronomers. But it’s true.

Here’s why. In astronomy, we define the instant of quarter moon as when the sun and moon are separated by 90 degrees, as seen from Earth.

But, in order for the moon to appear exactly 50% illuminated for an observer on Earth, the sun and Earth must be separated by 90 degrees, from the perspective of the moon.

See? It’s a tiny difference, but a measurable one. From Earth, at every quarter moon, a fraction more than 50% of the Earth-facing side of the moon is illuminated.

Not to scale

The illustration above is a very rough representation of the triangle formed by the sun, Earth, and moon. In reality, the distance from Earth to the moon is about 30 times the diameter of Earth. The distance from Earth to the sun, meanwhile, is around 12,000 times Earth’s diameter.

In other words, although our not-to-scale illustration shows a small and compact triangle, the actual triangle is extremely long and narrow.

As the moon travels along its orbit, the moment when it is exactly 50% illuminated comes around 20 minutes after the moment of 3rd quarter, or before the moment of 1st quarter.

(Our illustration ignores an effect called parallax, where the moon’s position differs by a tiny amount depending on your observing location on Earth. Moon phase calculations are based on the centers of the Earth and moon.)

December 5 is also the farthest quarter moon

Although the 3rd quarter moon on December 5 won’t look different from any other quarter moon, it will also be the farthest quarter moon from Earth in 2023. This is not a complete coincidence: as the Earth-moon distance gets larger, the percentage of the moon’s face that is lit up increases.

Similarly, it is no coincidence that December 5 falls near perihelion (early January), when Earth is closest to the sun. A smaller Earth-sun distance also leads to a greater percentage of the moon’s face being lit up.

Lesser-known moon dates for 2024

The moon’s orbit around Earth is never the same from one lunar month to the next, leading to events such as supermoons and micromoons.

So, again, in 2023, the most-illuminated quarter moon will be the 3rd quarter moon of December 5. And, in 2024, the most-illuminated quarter moon will be the 3rd quarter moon of December 22.

Here are seven lesser-known lunar events for 2024.

January 25: Longest moon phase of 2024. There will be 8.225 days between full moon on January 25 and 3rd quarter moon on February 2.

March 10: Closest moon of 2024. The most extreme Earth-moon distances occur at new moon or full moon. At new moon on March 10, the moon will be 221,764 miles (356,895 km) away.

May 30: Shortest lunar month of 2024. The moon will cycle through all its phases – from 3rd quarter moon on May 30 to 3rd quarter moon on June 28 – in 29.195 days.

August 19: Shortest moon phase of 2024. The moon will take 6.625 days to wane from full moon on August 19 to 3rd quarter moon on August 26.

October 2: Farthest moon of 2024. At new moon on October 2, the distance to the moon will be 252,597 miles (406,516 km).

November 23: Longest lunar month of 2024. It will take the moon 29.868 days to go from 3rd quarter on November 23 to 3rd quarter on December 22.

December 22: Most-illuminated quarter moon of 2024. And so it comes around again!

Bottom line: Some quarter moons are fuller than others! The most-illuminated quarter moon of 2023 is on December 5. This happens when the moon is in a slightly different position in its orbit … a little bit after 3rd quarter, or a little bit before 1st quarter. Learn more unique moon dates for 2024.

The post 2023’s most-illuminated quarter moon is tonight first appeared on EarthSky.

Sailing stones leave a trail in the desert

How could rocks – driven by some unseen force – slide across the ground, leaving behind a trail? That’s what happens at Racetrack Playa, a dry lake bed at Death Valley National Park in California. The definitive solution to this long-standing mystery finally came in 2014, from two cousins. Their work showed that the rocks are nudged into motion by melting panels of thin floating ice, driven by light winds, in winter.

The sailing stones, or sliding stones, of Racetrack Playa have been observed and studied since the early 1900s. It was long thought that strong winds pushed the stones. More fanciful theories involved magnetic fields. And, of course, as is the case in many good mysteries, space aliens were sometimes implicated as well.

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The mystery of the sailing stones solved

But today most sources agree that the work released in August 2014 solved the mystery once and for all. That’s when a group of (very patient) researchers aided by the Scripps Institution of Oceanography, NASA and others announced their findings.

In a statement, Richard D. Norris and his cousin James M. Norris said that the rock movements occurred during a rare combination of conditions in winter. There had to be a shallow layer of water in the dry lake bed and nighttime temperatures cold enough for the formation of a thin layer of ice. On sunny days, melting caused the ice to break into large floating panels that, driven by light winds, pushed against the rocks to move them. Of course, this will leave tracks on the desert floor. The editor- and peer-reviewed journal PLOS ONE published their study.

The video above – from the Slithering Stones Research Initiative – shows a sailing or sliding or slithering stone of Death Valley’s Racetrack Playa in motion. See it? It’s the big rock in the foreground.

How they studied the sailing stones

The two cousins launched their investigation of sailing stones in 2011. That’s when they founded what they called the Slithering Stones Research Initiative. They established a weather station near Racetrack Playa and added 15 of their own stones to the playa. The added stones had GPS tracking units attached.

Then, they watched. On December 4 and December 20, 2013, their setup – which used time-lapse photography – caught on camera rocks that were sliding across the playa at up to 15 feet (3-5 meters) per minute. They saw many other instances of sailing stones as well, becoming the first people in recorded history to see the stones in motion.

In the statement, Richard Norris said,

Science sometimes has an element of luck. We expected to wait five or 10 years without anything moving, but only two years into the project, we just happened to be there at the right time to see it happen in person.

They described what they saw in their PLOS ONE paper:

The largest observed rock movement involved >60 rocks on December 20, 2013, and some instrumented rocks moved up to 224 meters between December 2013 and January 2014 in multiple move events.

What their study revealed

They said in their paper that watching the stones move enabled them to see the cause:

In contrast with previous hypotheses of powerful winds or thick ice floating rocks off the playa surface, the process of rock movement that we have observed occurs when the thin, 3- to 6-millimeter ‘windowpane’ ice sheet covering the playa pool begins to melt in late morning sun and breaks up under light winds of ~4–5 meters/second [about 5 yards/second].

Floating ice panels tens of meters [yards] in size push multiple rocks at low speeds of 2–5 meters/minute [2-5 yards/minute] along trajectories determined by the direction and velocity of the wind as well as that of the water flowing under the ice.

How often do they move?

Racetrack Playa stones only move once every two or three years and most tracks last for three or four years. Stones with rough bottoms leave straight striated tracks while those with smooth bottoms wander.

According to Wikipedia, these sailing stones are slabs of dolomite and syenite ranging from a few hundred grams to hundreds of kilograms.

It’s nice to know what causes them to move! If you have time (six minutes), check out the video below, which features the Norris cousins telling their story.

Bottom line: The mystery of the sailing stones of Racetrack Playa in Death Valley was solved in 2014. Scientists found that thin floating panels of ice driven by light winds pushed the rocks.

Do you want to know what’s up in the night sky? Visit our EarthSky visible planets and night sky guide.

The post What moves the sailing stones of Death Valley? first appeared on EarthSky.