A Spectacular Grazing Occultation for Aldebaran at Dawn

aldebaran moon
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.

Planetpalooza: All Bright Planets Visible in the July Dusk Sky

Moon and Venus
Moon and Venus
Venus and the waxing crescent Moon above the Grand Palais in Paris, France from May 17th. Image credit: Gwenael Blanck

Missed the planets in the dusk sky in early 2018? This summer’s astronomical blockbuster sees the return of all the classical naked eye planets in the dusk sky, in a big way.

The Sky Scene in July

This coming July 2018 features a rare look at the solar system in profile: you can see Mercury and Venus low in the dusk looking westward immediately after sunset, with Jupiter high to the south, Saturn rising in the east, and Mars rising just behind. This isn’t a true grouping or grand conjunction, as the planets span a 170 degree swath of the ecliptic from Mercury to Mars (too bad they’re not in orbital order!) but a product of our Earthly vantage point looking out over the swath of inner solar system in the evening sky.

Can you manage a “planetary marathon” and collect all five this coming Fourth of July weekend? Here’s a quick rundown of all the planetary action from west to east:

Mercury
An amazing view – Mercury through the telescope from May 5th. Image credit and copyright: Roger Hutchinson.

Mercury’s July apparition – fleeting Mercury is always the toughest of the planets to catch, low to the west. -0.3 magnitude Mercury actually forms a straight line with the bright +1st/2nd magnitude stars Castor and Pollux in Gemini the Twins later this week on the evening of June 27th. Mercury reaches greatest elongation 26 degrees east of the Sun on July 12th, presenting a half illuminated, 8” disk. The angle of the evening ecliptic is canted southward in July, meaning that the position of the planets in the evening sky also favors southern viewers. July also presents another interesting mercurial challenge, as Mercury passes in front of the Beehive Open cluster (Messier 44) in the heart of the constellation Cancer on the night of July 3rd/4th.

planets
The span of the planets through late-July at dusk. Credit: Stellarium.

Venus this summer – higher up at dusk, brilliant Venus rules the evening sky, shining at magnitude -4. Venus is so bright that you can easily pick it up this month before sunset… if you know exactly where to look for it. Venus reaches greatest elongation 46 degrees east of the Sun on August 17th, presenting a featureless half-illuminated disk 25” in diameter near a point known as dichotomy. Venus also flirts with the bright star Regulus (Alpha Leonis) in July, passing a degrees from the star on July 10th. Fun fact: Venus can actually occult (pass in front of) Regulus and last did so on July 7th, 1959 and will do so next on October 1st, 2044.

Jupiter
Jupiter, with the shadow of Europa in transit from June 6th. Image credit and copyright: Ralph Smyth.

Jupiter Rules – The King of the Planets, Jupiter rules the sky after darkness falls, crossing the astronomical constellation Libra the Scales. Fresh off of its May 9th opposition, Jupiter still shines at a respectable magnitude -2 in July, with a disk 36” across. Jupiter heads towards quadrature 90 degrees east of the Sun on August 6th, meaning the planet and its retinue of four Galilean moons cast their respective shadows off to one side. In fact, we also see a series of fine double shadow transits across the Jovian cloud tops involving Io and Europa starting on July 29th.

saturn
The glorious planet Saturn. Image credit and copyright: Paul Stewart

…and Saturn makes five: Stately Saturn never fails to impress. Also just past its June 27th opposition, the rings are still tipped open narrowing down only slightly from last year’s widest angle of 27 degrees, assuring an amazing view. Shining at magnitude 0 and subtending 42” (including rings) in July, Saturn traverses the star-rich fields of the astronomical constellation Sagittarius the Archer this summer. Look at Saturn, and you’re glimpsing the edge of the known solar system right up until William Herschel discovered Uranus on the night of March 13th, 1781.

The origins of a dust storm: Mars from late May. Image credit and copyright: Efrain Morales.

Enter Mars: We saved the best for last. The Red Planet races towards a fine opposition on July 27th. This is the best approach of Mars since the historic 2003 opposition, and very nearly as favorable: Mars shines at magnitude -2.8 at the end of July, and presents a 24.3” disk. More to come as Mars approaches!

And as with many an opposition, dust storm season has engulfed Mars. Be vigilant, as the ‘Red’ Planet often takes on a sickly yellowish tint during a large dust storm, and this cast will often be apparent even to the naked eye. NASA’s aging Opportunity rover has fallen silent due to the lack of sunlight and solar power, and it’s to be seen if the rover can ride out the storm.

The path of the Moon – The Moon makes a good guidepost as it visits the planets in July. The first eclipse season of 2018 also begins in July, with a partial solar eclipse for Tasmania, SE Australia and the extreme southernmost tip of New Zealand on July 13th and wrapping up with a fine total lunar eclipse favoring Africa, Europe, Asia and Australia on July 27th. Note that this eclipse is only 14 hours after Mars passes opposition… we expect to see plenty of pictures of a ruddy Mars near a Blood Moon eclipse.

The Moon also makes a handy guide to catch each of the planets in the daytime sky… though you’ll need binoculars or a telescope to nab Mercury or Saturn (also, be sure the Sun is physically blocked out of view while hunting for Mercury in the daytime sky!) Here are the respective passes of the Moon near each planet in July:

Planet Date Time Moon Phase/illumination Distance
Mercury July 14th 23UT/7PM EDT Waxing crescent/5% 2.1 degrees
Venus July 16th 4UT/00AM EDT Waxing crescent/14% 1.5 degrees
Jupiter July 21st 2UT/10PM EDT Waxing gibbous/63% 4.2 degrees
Saturn July 25th 5UT/1AM EDT Waxing gibbous/94% 2 degrees
Mars July 27th 16UT/12 EDT Full Moon/100% 8 degrees

Unfortunately, the telescopic planets Uranus and Neptune are left out of the July evening view; Uranus is currently crossing the constellation Aries and Neptune resides in Aquarius, respectively. Pluto is, however, currently in the direction of Sagittarius, and you can also wave to NASA’s New Horizons spacecraft en route to its New Year’s Day 2019 KBO destination Ultima Thule (nee 2014 MU69) near the waxing gibbous Moon on the night of July 26th.

The Moon, Pluto and New Horizons on the evening of July 26th. Credit: Starry Night

And finally, another solar system destination in Ophiuchus the Serpent Bearer beckons telescope owners in July: asteroid 4 Vesta.

All of this is more than enough planetary action to keep planetary observers and imagers up late on forthcoming July evenings.