Get Set for the Super (or Do You Say Harvest?) Full Moon 3 of 3 for 2014

Time to dust off those ‘what is a perigee Full Moon’ explainer posts… the supermoon once again cometh this weekend to a sky near you.

Yes. One. More. Time.

We’ve written many, many times — as have many astronomy writers — about the meme that just won’t die. The supermoon really brings ‘em out, just like werewolves of yore… some will groan, some will bemoan the use of a modernized term inserted into the common astronomical vernacular that was wrought by an astrologer, while others will exclaim that this will indeed be the largest Full Moon EVER…

But hey, it’s a great chance to explain the weird and wonderful motion of our nearest natural neighbor in space. Thanks to the Moon, those astronomers of yore had some great lessons in celestial mechanics 101. Without the Moon, it would’ve been much tougher to unravel the rules of gravity that we take for granted when we fling a probe spaceward.

The Moon reaches Full on Tuesday, September 9th at 1:38 Universal Time (UT), which is 9:38 PM EDT on the evening of the 8th. The Moon reaches perigee at less than 24 hours prior on September 8th at 3:30 UT — 22 hours and 8 minutes earlier, to be precise — at a distance 358,387 kilometres distant. This is less than 2,000 kilometres from the closest perigee than can occur, and 1,491 kilometres farther away than last month’s closest perigee of the year, which occurred 27 minutes prior to Full Moon.

A Proxigean or Perigee Full “Supermoon” as reckoned by our preferred handy definition of “a Full Moon occurring within 24 hours of perigee” generally occurs annually in a cycle of three over two lunar synodic periods, and moves slowly forward by just shy of a month through the Gregorian calendar per year. The next cycle of “supermoons” starts on August 30th, 2015, and you can see our entire list of cycles out through 2020 here.

What’s the upshot of all this? Well, aside from cluttering inboxes and social media with tales of the impending supermoon this weekend, the rising Moon will appear 33.5’ arc minutes in diameter as opposed to its usually quoted average of 30’ in size. And remember, that’s in apparent size as seen from our Earthly vantage point… can you spy a difference from one Full Moon to the next? Fun fact: the rising Moon is actually farther away from you to the tune of about one Earth radius than when it’s directly overhead at the zenith.

Fed up with supermoon-mania? The September Full Moon also has a more pedestrian name: The Harvest Moon. Actually, this is the Full Moon that falls nearest to the September Equinox, marking the start of the astronomical season of Fall in the northern hemisphere and Spring in the southern. In the current first half of the 21st century, the September Equinox falls on the 22nd or 23rd, meaning that the closest Full Moon (and thus the Harvest Moon) can sometimes fall in October, as last happened in 2009 and will occur again in 2017. In this instance, the September Full Moon would then be referred to as the Corn Moon as reckoned by the Algonquins, and is occasionally referred to as the Drying Grass Moon by Sioux tribes. In 2014, the Harvest Full Moon “misses” falling in October by about 32 hours!

July 14th
The waning gibbous Moon of July 14th, 2014- shortly after the first supermoon of the year. Credit: Blobrana.

So, why is it known as the Harvest Moon? Well, in the age before artificial lighting (and artificial light pollution) the rising of the Full Moon as the Sun sets allowed for a few hours of extra illumination to bring in crops. In October, the same phenomenon gave hunters a few extra hours to track game by the light of the Full Hunters Moon, both essential survival activities before the onset of the long winter.

And that Full Harvest Moon seems to “stick around” on successive evenings. This is due to the relatively shallow angle of the evening ecliptic to the eastern horizon as seen from mid-northern latitudes in September.

September 8th
The rising Full Moon on the evening of September 8th as seen from latitude 40 degrees north. Note the shallow angle of the ecliptic. Created using Stellarium.

Here’s a sample of rising times for the Moon this month as seen from Baltimore, Maryland at 39.3 degrees north latitude:

Saturday, September 6th: 5:43 PM EDT

Sunday, September 7th: 6:23 PM EDT

Monday, September 8th: 7:05 PM EDT

Tuesday, September 9th: 7:44 PM EDT

Wednesday, September 10th: 8:22 PM EDT

Note the Moon rises only ~40 minutes later on each successive evening.

Stephen Rahn
The Full Harvest Moon of 2013 plus aircraft. Credit: Stephen Rahn.

We’re also headed towards a “shallow year” in 2015, as the Moon bottoms out relative to the ecliptic and only ventures 18 degrees 20’ north and south of the celestial equator at shallow minimum. This is due to what’s known as the Precession of the Line of Apsides as the gravitational pull of the Sun slowly drags the orbit of the Moon round the earth once every 8.85 years. The nodes where the ecliptic and path of the Moon meet — and solar and lunar eclipses occur — also move slowly in an opposite direction of the Moon’s motion, taking just over twice as long as the Precession of the Line of Apsides to complete one revolution around the ecliptic at 18.6 years. This is one of the more bizarre facts about the motion of the Moon: its orbital tilt of 5.1 degrees is actually fixed with respect to the ecliptic as traced out by the Earth’s orbit about the Sun, not our rotational axis. Native American and ancient Northern European knew of this, and the next “Long Night’s Moon” also called a “Lunar Standstill” when the Moon rides high in the northern hemisphere sky is due through 2024-2025.

The footprint of the September 11th occultation of Uranus. Credit: Occult 4.0.

And to top it off, the Moon occults Uranus just two days after Full on September 11th as seen from northeastern North America, Greenland, Iceland and northern Scandinavia. We’re in a cycle of occultations of Uranus by the Moon from late 2014 through 2015, and this will set the ice giant up for a spectacular close pass, and a rare occultation of the planet for a remote region in the Arctic during the October 8th total lunar eclipse…

More to come!



Observing Alert: Watch the Moon Cross the Hyades This Week

A photogenic grouping greets evening sky watchers this week providing a fine teaser leading up to a spectacular eclipse.

On the evening of Thursday, April 3rd headed into the morning of the 4th, the waxing crescent Moon crosses in front of the Hyades open star cluster.  This is the V-shaped asterism that marks the head on Taurus the Bull, highlighted by the brilliant foreground star Aldebaran as the bull’s “eye”.  Viewers across North America will have a ring-side seat to this “bull-fight” as the 20% illuminated Moon stampedes over several members of the Hyades in its path.

Starry Night
The passage of the Moon through the Hyades over a three hour span on the night of April 3rd (April 4th in Universal Time) comparing the North American locales of Tampa, Florida and Seattle, Washington. (Credit: Starry Night Education Software).

The brightest stars to be occulted are the Delta Tauri trio of stars ranging in magnitudes from +3.8 (Delta Tauri^1) to +4.8(2) and +4.3(3). Such occlusions – known in astronomy as occultations – are fun to watch, and can reveal the existence of close binary companions as they wink out behind the lunar limb. Several dozen occultations of stars brighter than +5th magnitude by the Moon happen each year, and the best events occur when the Moon is waxing and the stars disappear against its dark leading edge. We recently caught one such event last month when the Moon occulted the bright star Lambda Geminorum:

We are currently seeing the Moon cross the Hyades during every lunation until the year 2020, though it’s a particularly favorable time to catch the event in April 2014 as the Moon is a slender crescent. Notice that you can just make out the dark limb of the Moon with the naked eye? What you’re seeing is termed Earthshine, and that’s just what it is: the nighttime side of the Moon being illuminated by sunlight that is reflected off of the Earth. Standing on the Earthward side of the Moon, an observer would see a waning gibbous Earth about two degrees across. Yutu has a great view!

Credit Occult 4.0
The occultation footprint for Delta Tauri^1. Credit: Occult 4.0

The Moon will cross its descending node where its apparent path intersects the ecliptic on April 1st (no joke, we swear) at 2:30 Universal Time or 10:30 PM EDT on March 31st. The next nodal crossing now occurs in just two weeks, and the Earth’s shadow will be there to greet the Moon on the morning of April 15th in the first of four total lunar eclipses that span 2014 and 2015. The month of April also sees the Moon’s orbit at its least eccentric, a time at which perigee – the Moon’s closest point to Earth – is at its most distant and apogee – its farthest point – is at its closest. This currently happens near the equinoxes, through the nodes slowly travel across the ecliptic completing one revolution every 18.6 years. Perigee can vary from 356,400 to 370,400 kilometres, and apogee can span a distance from 404,000 to 406,700 kilometres.

Looking west from the US SE at about 10PM local on the evening of April 3rd. Credit: Stellarium.

We’re also headed towards a “shallow year” in 2015 when the Moon has the least variability in respect to its declination. This trend will then reverse, climaxing with a “Long Nights Moon” riding high in the sky in 2025, which last occurred in 2006. The Moon will inch ever closer to Aldebaran on every successive lunation now, and begins a series of occultations of Aldebaran on January 29th, 2015 through the end of 2018. Occultations of Aldebaran always occur near these shallow years, and will be followed by a cycle of occultations of Regulus starting in 2017. We caught an excellent daytime occultation of Aldebaran by the Moon from North Pole, Alaska during the last cycle in the late 1990s.

Photos by Author
The Moon passing between the Hyades and Pleiades in 2011 with Earthshine highlighted. Photos by author.

Now for the wow factor. Our Moon is 3,474 kilometres across and located just over one light second away. The Hyades star cluster covers about 6 ½ degrees of sky – about 7 times the size of the Full Moon – but is the closest open cluster to the Earth at 153 light years distant and has a core diameter of about 18 light years across. As mentioned previous, Aldebaran isn’t physically associated with the Hyades, but is merely located in the same direction at 65 light years distant.

The Hyades star cluster also provided early 20th astronomers with an excellent study in galactic motion. At an estimated 625 million years in age, the Hyades are slowly getting disbanded and strewn about the Milky Way galaxy in a process known as evaporation. The Hyades are also part of a larger stellar incorporation known as the Taurus Moving Cluster. Moving at an average of about 43 kilometres a second, the members of the Hyades are receding from us towards a divergent point near the bright star Betelgeuse in the shoulder of Orion. 50 million years hence, the Hyades will be invisible to the naked eye as seen from Earth, looking like a non-descript open cluster and providing a much smaller target for the Moon to occult at 20’ across. Astronomer Lewis Boss was the first to plot the motion of the Hyades through space in 1908, and the cluster stands as an essential rung on the cosmic distance ladder, with agreeing measurements independently made by both Hubble and Hipparcos and soon to be refined by Gaia.

Photographing and documenting this week’s passage of our Moon across the Hyades is easy with a DSLR camera: don’t be afraid to vary those ISO and shutter speeds to get the mix of the brilliant crescent Moon, the fainter earthshine, and background stars just right. The more adventurous might want to try actually catching the numerous occultations of bright stars on video. And U.S. and Canadian west coast observers are well placed to catch the Moon cross right though the core of the Hyades… a video animation of the event is not out of the question!

And from there, the Moon heads on to its date with destiny and a fine total lunar eclipse on April 15th which favors North American longitudes. We’ll be back later this week with our complete and comprehensive eclipse guide!

Now is a Great Time to Try Seeing Venus in the Daytime Sky

Here’s a feat of visual athletics to amaze your friends with this week. During your daily routine, you may have noticed the daytime Moon hanging against the azure blue sky. But did you know that, with careful practice and a little planning, you can see Venus in the broad daylight as well?

This week offers a great chance to try, using the daytime Moon as a guide. We recently wrote about the unique circumstances of this season’s evening apparition of the planet Venus. On Friday, December 6th, Venus will reach a maximum brilliancy of magnitude -4.7, over 16 times brighter than Sirius, the brightest star in the sky. And just one evening prior on Thursday December 5th, the 3-day old crescent Moon passes eight degrees above it, slightly closer together than the span of your palm held at arm’s length.

Created using Starry Night Education software.
The orientation of Venus and the Moon on Thursday, December 5th as it crosses the local meridian at 3PM EST. Created using Starry Night Education software.

The Moon will thus make an excellent guide to spot Venus in the broad daylight. It’s even possible to nab the pair with a camera, if you can gauge the sky conditions and tweak the manual settings of your DSLR just right.

The best time to attempt this feat on Thursday will be when the pair transits the local meridian due south of your location. Deep in the southern hemisphere, the Moon and Venus will appear to transit to the north.  This occurs right around 3:00 PM local. The fingernail Moon will be easy to spot, then simply begin scanning the sky to the south of it with the naked eye or binoculars for the brilliant diamond of Venus. High contrast and blocking the Sun out of view is key — Venus will easily pop right out against a clear deep blue sky, but it may disappear all together against a washed out white background.

The Moon will be at a 10% illuminated phase on Thursday, while Venus presents a slimming crescent at 27% illumination. Though tougher to find, Venus is actually brighter than the Moon in terms of albedo… expand it up to the apparent size of a Full Moon and it would be over four times as bright!

Photo by author.
Church and Venus as seen from Westgate River Ranch, Florida. Photo by author.

You’ll be amazed what an easy catch Venus is in the daytime once you’ve spotted it — we’ve included views of Venus in the daytime when visible during sidewalk star parties for years.

Due to its brilliancy, Venus has also been implicated in more UFO sightings than any other planet, and even caused the Indian Army to mistake the pair for snooping Chinese drones earlier this year when it was in conjunction with the planet Jupiter. A daytime sighting of the planet Venus near the Moon was almost certainly the “curious star” reported by startled villagers observing from Saint-Denis, France on January 13th, 1589.

Venus can also cast a noticeable shadow near greatest brilliancy, an effect that can be discerned against a fresh snow-covered landscape. Can’t see it? Take a time exposure shot of the ground and you may just be able to tease it out… but hurry, as the waxing Moon will soon be dominating the early evening night sky show!

Another phenomenon to watch for this week on the face of the waxing crescent Moon is known as Earthshine. Can you just make out the dark limb of the Moon? This is caused by the Earth acting as a “mirror” reflecting sunlight back at the nighttime side of the Moon. And don’t forget, China’s Chang’e-3 lander plus rover will be landing on the lunar surface in the Sinus Iridum region later this month on December 14th, the first lunar soft landing since 1976!

The imaginary line of the ecliptic currently bisects the Moon and Venus, as Venus sits at an extreme southern point 2.5 degrees below the ecliptic — in fact, 2013 the farthest south it’s been since 1930 — and the Moon sits over four degrees above the ecliptic this week. The Moon also reached another notable point today, as it reached its most northern “southerly point” for 2013 at a declination of -19.6 degrees. The Moon’s apparent path is headed for a “shallow year” in 2015, after which it’ll begin to slowly widen over its 18.6 year cycle out to a maximum declination range in 2024. It’s a weird but true fact that the motion of the Moon is not fixed to the Earth’s equatorial plane, but to the path of our orbit traced out by the ecliptic, to which its orbit is tilted an average of five degrees.

The view looking west tonight from latitude 30 degrees north. Created using Stellarium.

And speaking of the Moon, there’s another fun naked-eye feat you can attempt tonight. At dusk, U.S. East Coast observers might just be able to pick up the razor thin crescent Moon hanging low to the West, only 23 hours past New. Begin scanning the western horizon about 10 minutes after sunset. Can you see it with binoculars? The naked eye? Chances get better for sighting the slim crescent Moon the farther west you go. North American observers will have a chance at a “personal best” during next lunation in the first few days of 2014… more to come!

Be sure to send those Venus-Moon conjunction pics in to Universe Today!