bright star occultations Archives

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 29^th, 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 6^th and September 3^rd. 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 18^th, 2033.

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 +1^st 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 10^th 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:

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 10^th… 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 10^th 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.

As the first eclipse season of 2013 comes to an end this weekend, an extremely subtle lunar eclipse occurs on the night of Friday, May 24^th going into the morning of Saturday, May 25^th. And we do mean subtle, as in invisible to the naked eye… this eclipse only lasts 34 minutes in duration and less than 2% of the disk of the Moon enters the bright outer penumbra of the Earth’s shadow!

So, why talk about such a non-event at all?

Great things come from such humble beginnings. And while this weekend’s eclipse is one mostly for the almanacs and astronomical tables rather than a true observational event, it also marks the start of a new lunar saros cycle.

This weekend’s eclipse is one of five for 2013, a year which contains two solars and three lunars. This eclipse marks the end of the first “eclipse season” of the year, a time when the intersection of the Moon’s orbit (known as nodes) and the ecliptic nearly coincide with the position of the Sun (for a solar eclipse at New Moon) and the Earth’s shadow (for a lunar eclipse at Full Moon).

The current season began with a very slight partial eclipse on April 25^th, followed by an annular eclipse on May 10^th. It will last only 33 minutes and 45 seconds in duration starting at 03:53:11 UTC on May 25^th. The Moon will be high over the Americas at the time, but again, shading on the southern limb of the Moon will be too slight to be seen.

Curiously, SLOOH will be providing live coverage of the eclipse, although again, it will be too slight to see.

The Full Moon just nicks the Earth’s penumbra in the early morning hours of May 25th. (Created by the author in Starry Night).

What is a saros? A saros is a period of 18 years 11 days and 8 hours after which an eclipse cycle lines up, producing a similar eclipse to the one that preceded it 18 years before. Note that due to its 8 hour offset, the Earth will have rotated 120° and the visibility region will have shifted westward.

In said period, three lunar cycles very nearly line up;

The Anomalistic month (the period the Moon takes to go from one perigee to another) = 27.555 days.

The Draconic month (the period the Moon takes to return to the same node) = 27.212 days.

The Synodic month (the most familiar one, the period between similar phases) = 29.531 days.

Note that:

239 Anomalistic months = 239×27.555= 6585.645 days.

242 Draconic months = 242×27.212=6585.304 days.

223 Synodic months = 223×29.531=6585.413 days.

There’s that mis-alignment of a third of a day again (8 hours) for every 18 years and 11 days. This also causes the node of each eclipse in the cycle to drift eastward by 0.5° along the ecliptic. Thus, each eclipse isn’t exactly the same. A lunar saros series starts with a very brief penumbral like this weekend’s, becomes deeper and deeper every 18+ year period until partial and total eclipses begin centuries down the road. Thereafter, the cycle reverses, until a final faint penumbral marks the end of the lunar saros.

diagram — The progression of selected eclipses of the same saros cycle. (Credit: Matthew Zimmerman. Wikimedia Commons graphic in the Public Domain).

After this weekend’s eclipse, the next start of a lunar saros won’t occur until November 8^th 2060 with the start of saros 156. The last new saros series (number 149) began on June 13^th, 1984.

There are numbered saros series for both lunar and solar eclipses. There are currently 41 saroses (the plural of saros) active with the inclusion of this weekend’s start of lunar saros 150.

Saros 150, of which this eclipse is the 1^st of 71, will last for just over 1,262 years. It will begin to produce partial eclipses on August 20^th, 2157 and produce its 1^st total on its 32^nd lunar eclipse on April 29^th, 2572.

It amazes me that ancient cultures such as the Chaldeans new of saros cycles and could predict eclipses. Being geographically isolated, lunar eclipse cycles would have been easier to decipher than solar ones, as you only have to be on the Moonward facing hemisphere of the Earth to witness the eclipse. They may well have stumbled upon the saros while attempting to calculate a slightly longer 19 year period known as a Metonic cycle to align ancient luni-solar calendars.

And yes, that 8 hour offset also means that after a triple saros period, lunar and solar eclipses of the same saros series do return to roughly the same longitude every 54 years & 34 days. This is known as an exeligmos, and if you get this on a triple-word score in Scrabble, you can safely retire from the game.

The theoretical visibility circumstances for this week’s penumbral eclipse. (Credit: F. Espenak/NASA/GSFC).

And while this eclipse is more of academic than observational interest, you can always enjoy the light of a brilliant Full Moon. The May Full Moon is referred to as the Flower, Milk, and Corn Planting Moon by the Algonquian Indians of North America, alluding the latent season of Spring.

Also, keep an eye out for several conjunctions and occultations this week by the Moon with bright stars and planets.

The first up is the bright star Spica (Alpha Virginis) which gets occulted by the waxing gibbous Moon around ~11:00 UT on Wednesday, May 22nd for viewers across northern Australia, southern Asia and the South Pacific. Spica is one of four stars brighter than magnitude +1.5 that the Moon can occult, the others being Antares, Aldebaran and Regulus. This is the 6th occultation in a cycle of 13 of Spica by the Moon spanning 2013.

The planet Saturn will lie about 4° north of the waxing gibbous Moon on the following evening of May 23^rd.

Also, watch for an occultation of the +2.6^th magnitude star Beta Scorpii on the evening of May 24^th around the time of the lunar eclipse. This will be a difficult one, as the Moon will be near 100% illumination. Conjunction of the Moon and Beta Scorpii in right ascension occurs at 3:04 UT on May 25^th, about 2.5 hours after Full. The occultation will span the southeastern US, Caribbean, northern South America and western Africa.

Created by Author — Visibility path of the occultation of Beta Scorpii by the Moon. (Credit: Occult 4.1.0.2).

2013 isn’t a grand year for eclipses. We’ve got two more in the late season of the year, another slightly deeper penumbral on October 18^th and a hybrid solar eclipse on November 3^rd. And when, may you ask, will we FINALLY have another total lunar eclipse? Stick around ‘til U.S. Tax Day next year (April 15^th 2014) for a total lunar eclipse spanning the Americas!

Tag: bright star occultations

A Spectacular Grazing Occultation for Aldebaran at Dawn

The Birth of a Saros – This Weekend’s Hidden Eclipse