A Spectacular Grazing Occultation for Aldebaran at Dawn

aldebaran moon
Aldebaran versus the crescent Moon from April 2017. Image credit and copyright: frankastro
aldebaran moon
Aldebaran versus the crescent Moon from April 2017, post grazing occultation. Image credit and copyright: frankastro.

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.

July 10th occultation
The footprint for the July 10th occultation of Aldebaran by the Moon. Note that this is a daytime event across the Arctic, except for the tiny lower left corner of the footprint falling over the Great Lakes region at dawn. Credit: Occult 4.2.

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.

Graze line
A close study of the southern graze line for the July 10th event. Credit: IOTA/Google Maps

Here are some times and circumstances for selected cities in the path of the occultation:

Location Ingress Egress Moon altitude Sun altitude Duration
Minneapolis 8:30 8:47 1deg/3deg -16deg/-14deg 17 minutes
Green Bay 8:39 8:40 5deg/5 deg -13deg <1 minute
Thunder Bay 8:32 8:54 5deg/8 deg -12deg/-9 deg 22 minutes
Fort Dodge, Iowa N/A 8:37 0.1 deg -18 deg <1 minute

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:

Gamma tauri
The footprint for the July 10th Gamma Tauri event. Credit: Occult 4.2

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.

moon graze
The grazing occultation of Aldebaran on July 10th. The direction of motion for the Moon spans one hour. Credit: Stellarium.

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 International Occultation Timing Association (IOTA) welcomes observations of any occultations worldwide… in the case of a lunar graze, observations can be used to map out the profile of mountains and valleys along the edge of the Moon.

A Spectacular Set of Conjuctions on Tap for the Moon, Mars and Saturn this Weekend

Saturn passing behind the lunar limb on May 15th.

Got clear skies this July 4th weekend? The Moon passes some interesting cosmic environs in the coming days, offering up some photogenic pairings worldwide and a spectacular trio of occultations for those well placed observers who find themselves along the footprint of these events.

The path of the Moon on July 5th, 6th and 7th. Credit: Stellarium

First, let’s look at our closest natural neighbor in space. The Moon reaches first quarter phase on Saturday, July 5th at 11:59 Universal Time (UT)/7:59 AM EDT. First Quarter is a great time to observe the Moon, as the craters along the jagged terminator where the Sun is just starting to rise stand out in stark profile. Watch for the Lunar Straight Wall and the alphabet soup of elusive features known as the Lunar X or Purbach Cross and Lunar V on evenings right around First Quarter phase.

Starry Night
Mars off of the limb of the Moon as seen from North America on the evening of July 5th. Credit: Starry Night.

Our first conjunction stop on this weekend’s lunar journey is the planet Mars. Although the Moon occults — that is, passes in front of a given planet from our Earthly perspective — exactly 16 naked eye planets in 2014 (24 if you add in Uranus events and 1 Ceres and 4 Vesta on September 28th), the Moon will only occult Mars once in 2014, on the night of July 5th/6th. Northern South America and southern Central America will have a front row seat, while the rest of North America will see a close pass less than one degree from the lunar limb. This will still present a fine photographic opportunity, as it’ll be possible to snag Mars and the limb of the Moon in the same field of view. The Moon will be 56% illuminated during the conjunction, and Mars will present an 88% illuminated disk 9.2” across shining at magnitude +0.3.

Occult 4.0
The occultation path for Mars. Graphics created using Occult 4.0.

Both will be 96 degrees east of the Sun during geocentric (Earth-centered) conjunction, which occurs around 1:00 UT on July 6th or 9:00 PM EDT on the evening of the 5th. For those positioned to catch the occultation, it’ll take about a minute for “Mars set” to occur on the lunar limb. The last occultation of Mars occurred on May 9th, 2013 and the next won’t happen ‘til March 21st, 2015.

The footprint of Lambda Virginis…

Next up, the Moon occults the +4.5th magnitude star Lambda Virginis on July 7th centered on 8:26 UT. This event is well placed for observers in Hawaii on the evening of July 6th. Located 187 light years distant, the light that you’re seeing departed the far-flung star on 1827, only to be interrupted by the pesky limb of our Moon a second prior to arrival on Earth. This star is also of note as it’s a spectroscopic binary, and while you won’t be able to resolve the pair at a tiny separation of just 0.0002” apart, you just might be able to see the pair “wink out” in a step wise fashion that betrays its binary nature. The Moon misses the brightest star in Virgo (Spica) this month, as it’s wrapped up a series of occultations of the star in early 2014 and won’t resume until 2024. Aldebaran, Antares and Regulus also lie along the Moon’s path on occasion, and the next cycle of bright star occultations resume with Aldebaran in January 2015. You can check out a list of fainter naked eye stars occulted by the Moon this year here courtesy of the International Occultation Timing Association.

… and the occultation footprint  for Saturn.

And finally, the Moon visits Saturn, now residing just over the border in the astronomical constellation of Libra. This occultation occurs just 49 hours after the Mars event at 2:00 UT on July 8th (10:00 PM EDT on the evening of July 7th) and favors observers in the southernmost tip of South America. As with Mars, North America will see a close miss, although it will also be possible to squeeze Saturn in the same field of view as the Moon at low power, though it’ll sit about a degree of off its limb. We’re in a cycle of occultations of Saturn this year, with 11 occurring in 2014 and the next on August 4th. The reason for this is that Saturn moves much more slowly across the sky than Mars from our perspective, making for a relatively sluggish moving target for the Moon. Saturn shines at +0.6 magnitude as the 75% illuminated Moon passes by and subtends 42” with rings and will take about five minutes to pass fully behind the Moon.

These events will make for some great pics and animation sequences for sure… can you spot Saturn or Mars near the lunar limb with binoculars or a telescope before sunset? Or catch ‘em in the frame during a local fireworks show? Let us know, if enough pics surface on Universe Today’s Flickr page, we may do a post weekend roundup!