Jupiter’s moon Europa continues to be a source of wonder and scientific intrigue. As one of the four Galilean Moons (so-named because of their founder, Galileo Galilee), Europa is one of Jupiter’s largest satellites and is considered one of the best bets for finding extraterrestrial life in the Solar System. And recently, it joined its cousins (Io and Callisto) in passing in front of a star.
This type of rare event (a stellar occultation) allows astronomers to conduct unique observations of a celestial body. In Europa’s case, the occultation took place in 2017 and allowed astronomers to make more precise measurements of Europa’s size, its position relative to Jupiter, and its true shape. All this was made possible by the ESA’s Gaia Observatory, which let astronomers know exactly when and where to look for the moon.
An unusual celestial spectacle unfolds for observers around the Great Lakes region next Tuesday at dawn. The Moon has been faithfully occulting (passing in front of) the bright star Aldebaran for every lunation now since January 29th, 2015. These split-second events have touched on nearly every farflung corner of the Earth. Now the United States and Canada get to see the penultimate event, as the waning crescent Moon occults Aldebaran one last time for North America.
Many news outlets are advertising this as the “last occultation of Aldebaran until 2033” which isn’t entirely true: the Moon will occult Aldebaran twice more worldwide, once on August 6th and September 3rd. Both of these events, however, involve a thin crescent Moon and occur over high Arctic climes, so I wouldn’t be surprised if they go unwitnessed by human eyes. The next cycle of Aldebaran occultations then resumes on August 18th, 2033.
Four stars brighter than +1st magnitude lie along the Moon’s celestial path in our current epoch: Antares in Scorpius, Regulus in Leo, Spica in Virgo, and Aldebaran in the eye of Taurus the Bull. Fun fact: this celestial situation is also slowly changing, partly because of the slow 26,000 year-plus long top-like wobble of the Earth’s axis known as the Precession of the Equinoxes, but also because of stellar proper motion, which is slowly bringing stars into and out of the Moon’s path over millennia. For example, until 117 BC, the Moon could also occult Pollux in the constellation of Gemini the Twins.
The circumstances for the July 10 event: The morning of July 10th sees the 11% illuminated, waning crescent Moon meet the +0.9 magnitude star Aldebaran under pre-dawn skies. When the Moon is waning, the bright limb leads the way, covering up the star during ingress and revealing once again during egress. The Moon moves its own half a degree (30 arcminute) diameter once every hour, and how long you’ll see Aldebaran covered up depends on your location. The geographic “sweet spot” for the occultation is eastern Minnesota, northeastern Iowa, northern Wisconsin, Lake Superior, the Upper Peninsula of Michigan, Ontario and northern Quebec… though the farther east you are, the brighter the skies will be, until the occultation begins under dark to twilight dawn skies and ends after sunrise.
Tales from the Graze Line
Folks based along a narrow path running for Iowa, across Wisconsin and Michigan into Ontario and Quebec are in for a very special treat, as Aldebaran just grazes in southern limb of the Moon. Instead of one single wink out, Aldebaran will flash multiple times, as it shines down through the jagged valleys along the limb of the Moon, an amazing sight to witness and catch on video.
Here are some times and circumstances for selected cities in the path of the occultation:
Fort Dodge, Iowa
Notes: all locations listed are in the Central (CDT) time zone (UT-5 for summer time). All times listed are in Universal Time (UT), with the Moon and Sun altitude listed for the beginning and end of the event, rounded to the nearest minute.
Not on the graze line? Well, the rest of us will see a very photogenic near miss on the morning of July 10th… and you might just be able to track Aldebaran up into the daytime sky (make sure you physically block the Sun out of view) if you’ve got clear blue, high contrast skies.
The Moon also occults several fainter stars across the V-shaped Hyades open star cluster around the same time worldwide, as well. One such notable event is the occultation of the +3.7 magnitude star Gamma Tauri for the United Kingdom:
You can follow the July 10th occultation using nothing more than a Mk-1 eyeball, as you can see both the star and the Moon… though binoculars or a telescope will definitely help, as Aldebaran will be tough to pick out against the bright limb of the Moon. Occultations—especially grazing events—really lend themselves to video astrophotography and are simple to capture through a telescope. Just be sure to balance the exposure setting so you can follow the star all the way up to the bright limb of the Moon.
Occultations have inspired those who witnessed them back through pre-telescopic times. A Greek coin from 120 BC may depict an occultation of Jupiter by the Moon. Sultan Alp Arslan was said to have been inspired by a close pairing of Venus and the crescent Moon after the Battle of Manzikert in 1071 AD, adopting the celestial spectacle of the star and crescent which adorns several national flags today.
Also, keep an eye out for an optical illusion described in The Rime of the Ancient Mariner (the poem, not the song by Iron Maiden inspired by the epic tale of the same name), where the protagonist witnesses:
“While clome above the Eastern Bar,
The horned Moon, with one bright Star,
Almost atween the tips.”
This illusion is often referred to as the Coleridge Effect.
Don’t miss this fine occultation of Aldebaran… it’ll be awhile before we see the Moon meet the star again.
-Extra credit: if anyone is planning a live stream of the occultation next Tuesday, let us know.
The 86% illuminated waning gibbous Moon occults the +0.9 magnitude star Aldebaran across North America, the Northern Atlantic and Europe. The Moon is three days past Full during the event. Both are located 136 degrees west of the Sun at the time of the event. The central time of conjunction is ~6:40 Universal Time (UT). The event occurs during the daylight hours over western Europe and northwestern Africa and under darkness for southeastern North America, including the eastern United States and Mexico. The Moon will next occult Aldebaran on November 15th, 2016. This is occultation 24 in the current series of 49 running from January 29th, 2015 to September 3rd 2018.
The graze line is of particular interest during this event. We’re talking about the very edge of the footprint of the Moon’s ‘shadow’ cast by Aldebaran, running through Canada and bisecting the United States. Observers based along this line could see a spectacular ‘grazing occultation’ of Aldebaran by the Moon. We usually think of the limb of the Moon as a smooth curve, but it’s actually jagged. What you may see is Aldebaran wink in and out as light shines down those lunar valleys and is alternately blocked out behind peaks and crater rims. This is an unforgettable sight, and makes for great video. A record of a grazing occultation by multiple observers can also be used to create a profile of the lunar limb. That light from Aldebaran took 65 years to get here, only to be blocked by our Moon at the very last second.
And observers (myself one of them) based in Europe shouldn’t count themselves out. Like brighter planets, you can spy a +1 magnitude star such as Aldebaran near the daytime Moon using binoculars or a telescope… if, of course, you have a high contrast deep blue sky and know exactly where to look for it. The International Occultation Timing Association has a page for the event with a complete list of ingress and egress times for key cities on three continents in the path. We’ll be watching the Wednesday event – clear skies willing — from our present basecamp in the Andalusian foothills just outside of Jimena de la Frontera, Spain.
During our current epoch, the Moon can only occult four +1st magnitude stars: Regulus, Spica, Antares and Aldebaran. The slow motion movement of the Moon, the Earth and the background stars make this prestigious A-list change over time: until about two millennia ago, you could also count the bright star Pollux in Gemini among them.
In the current century, (2001-2100 AD) the Moon occults Aldebaran 247 times, topped only by Antares (386 times) and barely beating out Spica (220 times).
Timing an occultation is fun and as easy as shooting video of the Moon through a telescope at the appointed time of ingress or egress. Practice on framing the dazzling Moon first well in advance — probably the toughest part is getting the exposure of the bright limb stopped down enough to still see and image the star. We find that shooting anywhere from 1/100th to 1/500th frame rate for a gibbous Moon is about right. Don’t be afraid to crank up the magnification a bit, so you can place the bulk of the Moon out of view. Also, catching occultations of stars and planets during waning Moon phases are more challenging than waxing, as the star will ingress behind the bright leading limb and later reappear behind the dark trailing limb (waxing is vice versa).
Observing: Running an audible time hack in the background such as WWV radio out of Fort Collins, Colorado can provide a precise record of the occultation.
But wait, there’s more. When the Moon occults Aldebaran, its also crossing the background V-shaped open star cluster known as the Hyades. Worldwide the waning gibbous Moon also occults Gamma, 51, and Theta^1 and Theta^2, SAO 93975, and 119 Tauri. Chances are, there’s an occultation for YOU to catch this week, regardless of your location.
Want more? Well, the Moon continues to occult Aldebaran every lunation through 2017, and will also start a cycle of passes in front of Regulus on December 18th. In fact, the next occultation of Aldebaran on November 15th favors central Asia, and the event two lunations from now on December 13th brings the path back around the North America.
A great close out for 2016, for sure. Don’t miss this week’s occultation!
This week, we thought we’d try an experiment for tonight’s occultation of Aldebaran by the Moon. As mentioned, we’re expanding the yearly guide for astronomical events for the year in 2017. We’ve done this guide in various iterations since 2009, starting on Astroguyz and then over to Universe Today, and it has grown from a simple Top 10 list, to a full scale preview of what’s on tap for the following year.
You, the reader, have made this guide grow over the years, as we incorporate feedback we’ve received.
Anyhow, we thought we’d lay out this week’s main astro-event in a fashion similar to what we have planned for the guide: each of the top 101 events will have a one page entry (two pages for the top 10 events) with a related graphic, fun facts, etc.
So in guide format, tonight’s occultation of Aldebaran would break down like this:
Wednesday, September 21st: The Moon Occults Aldebaran
Image credit Occult 4.2
The 67% illuminated waning gibbous Moon occults the +0.9 magnitude star Aldebaran. The Moon is two days prior to Last Quarter phase during the event. Both are located 109 degrees west of the Sun at the time of the event. The central time of conjunction is 22:37 Universal Time (UT). The event occurs during the daylight hours over southeast Asia, China, Japan and the northern Philippines and under darkness for India, Pakistan and the Arabian peninsula and the Horn of Africa. The Moon will next occult Aldebaran on October 19th. This is occultation 23 in the current series of 49 running from January 29th 2015 to September 3rd, 2018. This is one of the more central occultations of Aldebaran by the Moon for 2016.
The view from India tonight, just before the occultation begins. Image credit: Stellarium
Fun Fact-In the current century, (2001-2100 AD) the Moon occults Aldebaran 247 times, topped only by Antares (386 times) and barely beating out Spica (220 times).
Or maybe, another fun fact could be: A frequent setting for science fiction sagas, Aldebaran is now also often confused in popular culture with Alderaan, Princess Leia’s late homeworld from the Star Wars saga.
Like it? Thoughts, suggestions, complaints?
Now for the Wow! Factor for tonight’s occultation. Aldebaran is 65 light years distant, meaning the light we’re seeing left the star in 1951 before getting photobombed by the Moon just over one second before reaching the Earth.
There are also lots of other occultations of fainter stars worldwide over the next 24 hours, as the Moon crosses the Hyades.
And follow that Moon, as a series of 20 occultations of the bright star Regulus during every lunation begins later this year on December 18th.
Gadi Eidelheit managed to catch the March 14th, 2016 daytime occultation of Aldebaran from Israel:
And also in the ‘Moon passing in front of things’ department, here’s a noble attempt at capturing a difficult occultation of Neptune by the Moon last week on September 15th, courtesy of Veijo Timonen based in Hämeenlinna Finland:
Lets see, that’s a +8th magnitude planet next to a brilliant -13th magnitude Moon, one million (15 magnitudes) times brighter… it’s amazing you can see Neptune at all!
Last item: tomorrow marks the September (southward) equinox, ushering in the start of astronomical fall in the northern hemisphere, and the beginning of Spring in the southern. The precise minute of equinoctial crossing is 14:21 UT. In the 21st century, the September equinox can fall anywhere from September 21st to September 23rd. Bob King has a great recent write-up on the equinox and the Moon.
Don’t miss tonight’s passage of Aldebaran through the Hyades, and there’s lots more where that came from headed into 2017!
We’re in for a celestial show from two of the sky’s glitterati this week. On Friday morning July 29 around 10:00 UT (5 a.m. CDT), the crescent Moon will occult the star Aldebaran from the eastern and southern U.S. south of a line from Toledo, Ohio through St. Louis, Tulsa and El Paso, Texas. North of that line, the Moon will slide just south of the star in a spectacular conjunction. But the real action lies within a half-mile of either side of the line, where lucky observers will see a grazing occultation.
As the Moon’s orbital motion carries it eastward at the rate of one lunar diameter per hour, Aldebaran will appear to approach the sunlit northern cusp and then scrape along the Moon’s northern limb. You’ll need binoculars or a small telescope to see the initial approach, but once star reaches the semi-dark, earthlit portion of the Moon, the graze will be visible with the naked eye.
The edge or limb of the Moon appears smooth to the eye, but it’s rife with polar mountain peaks. As Aldebaran creeps along the craggy limb, it will repeatedly flash in and out of view as peaks and cliffs momentarily block it from sight. And here’s the truly amazing thing. Observers along the western section of the graze line, where the event takes place in fairly dark sky, can watch the star blink in and out of view without optical aid when it reaches the dark part of the lunar disk. Wow!
Aldebaran is no small star. An orange giant 67 light years from Earth, it’s 44 times the diameter of the Sun. That means that sometimes only a part of the star at a time will covered at a time in some cases, so the length of the flashes will vary. According to David Dunham, president of the International Occultation Timing Association (IOTA), Aldebaran will disappear for one-tenth of a second up to a second as the Moon rolls east, allowing some observers to sense the size of the star. Wow x 100!
Skywatchers further east along the graze line and in other areas where the occultation / conjunction takes place after sunrise shouldn’t pass up the chance to see the event. During last October’s occultation of Aldebaran, I was able to see and photograph the star in my 10-inch scope in daylight no problem. The moon will be closer to the Sun this time around, but give it try anyway. This is the best grazing occultation of Aldebaran visible from North America in the current 4-year series.
For the many who live either north or south of the graze line, the views will still be fantastic. You’ll either see an occultation and subsequent reappearance of the star at the dark limb … or a fine conjunction. The forecast looks good for my city, so I plan on heading out to watch an orange giant meet the skinny Moon. I wish you clear skies and happy shooting!
For more information about the event including detailed weather forecasts and grazing maps, check out the IOTA’s public announcement page. Click here for Universal Times for the disappearance and reappearance of the star for over 1,000 cities.
Have you caught sight of Mercury yet? This coming week is a good time to try, looking low to the west at dusk. We just managed to to nab it with binoculars for the first time during the current apparition this past Sunday from the rooftop of our Air BnB in Casablanca, Morocco.
Mercury is a tough grab under any circumstance, that’s for sure. Brilliant Venus and Jupiter make great guides to finding the elusive planet in late July, as it ping-pongs between the two. The waxing crescent Moon joins the scene in the first week of August, and for a very lucky few, actually occults (passes in front of ) the diminutive innermost world shortly after passing New.
Here’s the low down on everything Mercurial, and circumstances for the coming weeks.
Mercury passes 18′ from the star Regulus on Saturday, July 30th at 19:00 Universal Time (UT), representing the closest passage of a planet near a first magnitude star for 2016.
The Moon then reaches New phase, marking the start of lunation 1158 on August 2nd at 20:45 UT. The Moon then moves on to occult Mercury on Thursday, August 4th at 22:00 UT, just over 48 hours later. The occultation is visible at dusk for observers based in southern Chile and southern Argentina. The rest of us see a close pass. Note that although it is a miss for North America, viewers based on the continent share the same colongitude and will see Mercury only a degree off of the northern limb of the Moon on the night of August 4th. Mercury shines at magnitude +0.01, and presents a 67% illuminated disk 6.3” in size, while the Moon is a slender 5% illuminated.
How early can you see the waxing crescent Moon? Catching the Moon with the naked eye under transparent clear skies isn’t usually difficult when it passes 20 hours old. This cycle, first sightings favor South Africa westward on the night of August 3rd.
Mercury reaches greatest elongation 27.4 degrees east of the Sun 12 days after this occultation on August 16th.
How rare is it? Well occultations of Mercury by the Moon are the toughest to catch of all the naked eye planets, owing to the fact that the planet never strays far from the Sun. Nearly all of these events go unwitnessed, as they occur mainly under daytime skies. And while you can observe Mercury in the daytime near greatest elongation with a telescope, safety precautions need to be taken to assure the Sun is physically blocked from view. Astronomers of yore did exactly that, hoping to glimpse fleeting detail on Mercury while it was perched higher in the sky above the murk of the atmosphere low to the horizon.
In fact, a quick search of ye ole web reveals very few convincing captures of an occultation of Mercury (see the video above). The closest grab thus far comes from astrophotographer Cory Schmitz on June 3rd 2016 based in South Africa:
Can’t wait til next week? The Moon crosses the Hyades open star cluster this week, occulting several stars along the way. The action occurs on the morning of Friday, July 29th culminating with an occultation of +1 magnitude Aldebaran by the 23% illuminated Moon. Texas and Mexico are well-placed to see this event under dark skies. A small confession: we actually prefer occultations of planets and stars by the waxing Moon, as the dark edge of the Moon is leading during ingress, making it much easier to witness and the exact moment the Moon blots out the object.
Still want more? The Moon actually goes on to occult Jupiter on August 6th for the South Pacific. Viewers farther west in southeast Asia might just spy this one in the daytime. This is the second occultation of Jupiter by the Moon in a series of four in 2016.
Keep and eye on those planets in August, as they’re now all currently visible in the dusk sky. The Moon, Regulus and Venus also form a tight five degree triangle on the evening of August 4th, followed by a slightly wider grouping of Venus, Jupiter and the Moon around August 25th.
More to come on that soon. Be sure to check the planet Mercury off of your life list this coming week, using the nearby waxing crescent Moon as a guide.
Wow! Check out this video of the Moon passing in front of Saturn from a viewpoint in Brisbane, Australia. This type of phenomenon, called an occultation, happens when one celestial body passes in front of the other from an observer’s standpoint. You can see some information about a June 10 occultation of Saturn, for example, at this link.
“There has been a fair amount of post-processing done on the images to get to this result. The first stage was to adjust the source images so that detail was visible both on Saturn and on the Moon. This is because the two objects are quite different in brightness, and so each individual exposure results in a slightly over-exposed Moon and a slightly under-exposed Saturn,” wrote Teale Britstra, who created the video, on Vimeo.
“After initial processing, the series of images were imported into video editing software, and the resulting footage stabilized to eliminate some small tracking errors between shots,” Britstra continued.
“There was also one LARGE tracking error, where I had to physically move the telescope. This was because the Moon was sinking towards the western horizon and some nearby, large trees which would have obscured the shot had the scope not been moved. This can be seen in the resulting footage as the period where the Moon appears to slow down and slightly change direction.”
Observers in Australia and New Zealand had a special treat this week: watching Saturn disappear behind the Moon during an event called an occultation. (You can read all the details of how and why this happens here in our preview article.) Catching an event like this with a camera is tricky… the bright Moon can wash out the comparatively tiny (from our vantage point) planet Saturn. But here, several astrophotographers had success. Above is a nice view from Silveryway on Flickr.
Ian Musgrave from Australia used a 4″ Newtonian telescope, with a “Point and Shoot” Canon IXUS attached with inifinty to infinity focussing, 3xZoom, and a 25mm eye piece. You can see his entire set of images on his website here.
Peter Lake, also from Australia not only took images of the event, but also did a live Hangout on G+.
“Live hangouts and driving a telescope live is a tricky business,” Peter wrote on his website. “I lost focus playing around trying to improve the image due to the thin cloud.” He added that the night sky wasn’t ideal that evening. “The full moon was shining through thin clouds, washing out a bit of the detail.”
You can watch a replay of Peter’s Hangout below:
Sarah and Simon Fisher from the UK captured this “diffused” view of Saturn close the Moon on the evening of May 13, 2014.
Want to get your astrophoto featured on Universe Today? Join our Flickr group or send us your images by email (this means you’re giving us permission to post them). Please explain what’s in the picture, when you took it, the equipment you used, etc.
Funny thing. Skywatchers are often just as excited to watch a celestial object disappear as we are to see it make an appearance. Early Wednesday morning (May 14) the Full Flower Moon will slip in front of Saturn, covering it from view for about an hour for observers in Australia and New Zealand. If you don’t live where the dingoes roam, no worries. You can watch it online.And no matter where you are on the planet, the big moon will accompany the ringed planet across the sky this Tues. night-Weds. morning.
Moon-Saturn occultation from Perth, Australia Feb. 22, 2014 captured by Colin Legg
Occultations of stars happen swiftly. The moon’s limb meets the pinpoint star and bam! it’s gone in a flash. But Saturn is an extended object and the moon needs time to cover one end of the rings to the other. Planetary occultations afford the opportunity to remove yourself from planet Earth and watch a planet ‘set’ and ‘rise’ over the alien lunar landscape. Like seeing a Chesley Bonestell painting in the flesh.
As the moon approaches Saturn, the planet first touches the lunar limb and then appears to ‘set’ as it’s covered by degrees. About an hour later, the planet ‘rises’ from the opposite limb. Planetary occultations are infrequent and always worth the effort to see.
Seen from the northern hemisphere and equatorial regions, the nearly full moon will appear several degrees to the right or west of Saturn tomorrow night (May 13). As the night deepens and the moon rolls westward, the two grow closer and closer. They’ll be only a degree apart (two full moon diameters) during Wednesday morning twilight seen from the West Coast. Northern hemisphere viewers will notice that the moon slides to the south of the planet overnight.
Skywatchers in Australia will see the moon cover Saturn during convenient early evening viewing hours May 14:
* 8:09 p.m. local time from Adelaide
* 9:05 p.m. Brisbane
* 8:50 p.m. Melbourne
* 8:53 p.m. Canberra
* 8:56 p.m. from Sydney (More times and a map – click HERE)
Before the occultation, Saturn will shine close to the moon’s upper right and might be tricky to see with the naked eye because of glare.
Binoculars will easily reveal the planet, but a telescope is the instrument of choice. Even a small scope magnifying at least 30x will show Saturn and its rings hovering above the bright edge of the moon. Stick around. About an hour later, Saturn will re-emerge along the moon’s lower left limb.
Meanwhile, back in the western hemisphere, we’ll watch the nearly full Flower Moon make a close pass of the planet. If you’ve had difficulty finding the celestial ring bearer, you’ll have no problem Tuesday night. Take a look at Saturn’s wonderful system of rings in your telescope – they’re tipped nearly wide open this year. For even more fun, see how many moons you can spot. And don’t forget, you can watch it online courtesy of astrophysicist Gianluca Masi. His Virtual Telescope website will broadcast the occultation live starting at 10:15 Universal Time May 14 (6:15 a.m. EDT, 5:15 CDT, 4:15 MDT and 3:15 PDT).
Rings are a tough phenomenon to spot. As late as 1977, astronomers thought that the only thing in the solar system with rings was the planet Saturn. Now, we can add the first asteroid to the list of ringed bodies nearby us. The asteroid 10199 Chariklo hosts two rings, perhaps due to a collision that caused a chain of debris circling its tiny surface.
Besides the 250-kilometer (155-mile) Chariklo, the only other ringed bodies known to us so far are (in order of discovery) Saturn, Uranus, Jupiter and Neptune.
“We weren’t looking for a ring and didn’t think small bodies like Chariklo had them at all, so the discovery — and the amazing amount of detail we saw in the system — came as a complete surprise,” stated Felipe Braga-Ribas of the National Observatory (Observatório Nacional) in Brazil, who led the paper about the discovery.
The rings came to light, so to speak, when astronomers watched Chariklo passing in front of the star UCAC4 248-108672 on June 3, 2013 from seven locations in South America. While watching, they saw two dips in the star’s apparent brightness just before and after the occultation. Better yet, with seven sites watching, researchers could compare the timing to figure out more about the orientation, shape, width and more about the rings.
The observations revealed what is likely a 12.4-mile (20-kilometer)-wide ring system that is about 1,000 times closer to the asteroid than Earth is to the moon. What’s more, astronomers suspect there could be a moon lying amidst the asteroid’s ring debris.
If these rings are the leftovers of a collision as astronomers suspect, this would give fodder to the idea that moons (such as our own moon) come to be from collisions of smaller bits of material. This is also a theory for how planets came to be around stars.
The rings haven’t been named officially yet, but the astronomers are nicknaming them Oiapoque and Chuí after two rivers near the northern and southern ends of Brazil.
Because these occultation events are so rare and can show us more about asteroids, astronomers pay attention when they occur. Part of the Eastern Seabord enjoyed a more recent asteroid-star occultation on March 20.
The original paper, “A ring system detected around the Centaur (10199) Chariklo”, will soon be available on the Nature website.