Phew! Our eyes and thoughts have been cast so far out into the outer reaches of the solar system following New Horizons and Pluto this week, that we’re just now getting to the astronomical action going on in our own backyard.
This week leading up to the September equinox offers you a fine chance to catch an elusive phenomenon in the pre-dawn sky.
We’re talking about the zodiacal light, the ghostly pyramid-shaped luminescence that heralds the approach of dawn. Zodiacal light can also be seen in the post-dusk sky, extending from the western horizon along the ecliptic.
September is a great time for northern hemisphere observers to try and sight this glow in the early dawn. This is because the ecliptic is currently at a high and favorable angle, pitching the zodiacal band out of the atmospheric murk low to the horizon. For southern hemisphere observers, September provides the best time to hunt for the zodiacal light after dusk. In March, the situation is reversed, with dusk being the best for northern hemisphere observers and dawn providing the best opportunity to catch this elusive phenomenon for southern observers.
Cory Schmitz’s recent outstanding photos taken from the Nevada desert brought to mind just how ephemeral a glimpse of the zodiacal light can be. The glow was a frequent sight for us from dark sky sites just outside of Tucson, Arizona—but a rarity now that we reside on the light-polluted east coast of the U.S.
In order to see the zodiacal light, you’ll need to start watching before astronomical twilight—the start of which is defined as when the rising Sun reaches 18 degrees below the local horizon—and observe from as dark a site as possible under a moonless sky.
The Bortle dark sky scale lists the zodiacal light as glimpse-able under Class 4 suburban-to-rural transition skies. Under a Class 3 rural sky, the zodiacal light may extend up to 60 degrees above the horizon, and under truly dark—and these days, almost mythical—Class 1 and 2 skies, the true nature of the zodiacal band extending across the ecliptic can become apparent. The appearance and extent of the zodiacal light makes a great gauge of the sky conditions at that favorite secret dark sky site.
The source of the zodiacal light is tiny dust particles about 10 to 300 micrometres in size scattered across the plane of the solar system. The source of the material has long been debated, with the usual suspects cited as micrometeoroid collisions and cometary dust. A 2010 paper by Peter Jenniskens and David Nesvorny in the Astrophysical Journal cites the fragmentation of Jupiter-class comets. Their model satisfactorily explains the source of about 85% of the material. Dust in the zodiacal cloud must be periodically replenished, as the material is slowly spiraling inward via what is known as the Poynting-Robertson effect. None other than Brian May of the rock group Queen wrote his PhD thesis on Radial Velocities in the Zodiacal Dust Cloud.
But even if you can’t see the zodiacal light, you still just might be able to catch it. Photographing the zodiacal light is similar to catching the band of the Milky Way. In fact, you can see the two crossing paths in Cory’s images, as the bright winter lanes of the Orion Spur are visible piercing the constellation of the same name. Cory used a 14mm lens at f/3.2 for the darker image with a 20 second exposure at ISO 6400 and a 24mm lens at f/2.8 with a 15 second exposure at ISO 3200 for the brighter shot.
Under a truly dark site, the zodiacal light can compete with the Milky Way in brightness. The early Arab astronomers referred to it as the false dawn. In recent times, we’ve heard tales of urbanites mistaking the Milky Way for the glow of a fire on the horizon during blackouts, and we wouldn’t be surprised if the zodiacal light could evoke the same. We’ve often heard our friends who’ve deployed to Afghanistan remark how truly dark the skies are there, as military bases must often operate with night vision goggles in total darkness to avoid drawing sniper fire.
Another even tougher but related phenomenon to spot is known as the gegenschein. This counter glow sits at the anti-sunward point where said particles are approaching 100% illumination. This time of year, this point lies off in the constellation Pisces, well away from the star-cluttered galactic plane. OK, we’ve never seen it, either. A quick search of the web reveals more blurry pics of guys in ape suits purporting to be Bigfoot than good pictures of the gegenschein. Spotting this elusive glow is the hallmark of truly dark skies. The anti-sunward point and the gegenschein rides highest near local midnight.
And speaking of which, the September equinox occurs this weekend on the 22nd at 4:44 PM EDT/20:44 Universal Time. This marks the beginning of Fall for the northern hemisphere and the start of summer for the southern.
The Full Harvest Moon also occurs later this week, being the closest Full Moon to the equinox occurring on September 19th at 7:13AM EDT/11:13 UT. Said Moon will rise only ~30 minutes apart on successive evenings for mid-northern latitude observers, owing to the shallow angle of the ecliptic. Unfortunately, the Moon will then move into the morning sky, drowning out those attempts to spy the zodiacal light until late September.
Be sure to get out there on these coming mornings and check out the zodiacal light, and send in those pics in to Universe Today!
Sky watchers worldwide are in for a treat Sunday evening September 8, 2013 as the waxing crescent Moon passes near the dazzling planet Venus. And for a select few, the Moon will actually pass in front of Venus, in what is known as an occultation.
The action has already started this week, as the Moon reached New phase earlier today at 7:36 AM EDT/11:36 UT. The appearance of the slim crescent Moon nearest to the September equinox marks the start of the Jewish New Year with the celebration of Rosh Hashanah, which this year began as early as it possibly can at sundown on September 4th. As per tradition, Rosh Hashanah formally begins when the sky is dark enough for three stars to be seen. The convention established by Hillel II in 363 A.D. uses the mean motion of the Moon to fix the start dates of the Jewish luni-solar calendar, which means that occasionally Rosh Hashanah can start a day early. This also occurred in 2002.
The New Moon has also been historically an opportune time for nighttime military operations to commence —Desert Storm in 1991 and the raid against Bin Laden in 2011 were both conducted under the darkness afforded by the absence of moonlight around a New Moon. It’s yet to be seen if planners looking to conduct airstrikes on Syria are planning on taking advantage of the same conditions to begin operations soon.
Tonight, you can see the +1st magnitude star Spica less than two degrees away from -4th magnitude Venus. This places Venus at 100 times brighter than Spica and visible before sunset if you know exactly where to look for it.
The brightest star in the constellation Virgo, Spica is 260 light years distant and on the short list of nearby stars that will eventually go supernova. Fortunately for us, Spica is well outside of the ~100 light year radius “kill zone”.
You might just be able to spy the Moon and the -1st magnitude planet Mercury low to the west at dusk for the first time for this lunation tonight or (more likely) Friday night. This is also a great time to check out LADEE’s future home as it departs for lunar orbit from Wallops Island in Virginia on Friday night.
Hey, LADEE sitting on the pad atop its Minotaur V rocket with the slim crescent Moon in the background at dusk Friday night would be a great money shot, I’m just sayin’…
This weekend will see the Moon increase in illumination and elevation above the western horizon each evening until Spica, Venus, and the waxing crescent Moon fit within a four degree circle on Sunday night. The Moon will be 12% illuminated, while Venus is currently at a gibbous phase and 72% lit.
This will also present a good chance to see Venus during the daytime, using the nearby crescent Moon as a guide. This is a fun thing to try, and no gear is required! Though Venus may seem tough to find against the bright daytime sky, appearances are deceptive. With an albedo of 67% versus the Moon’s average of 14% Venus is actually brighter than the Moon per square arc second of size!
The Moon will also occult Spica on the evening of September 8th for observers in the Middle East and Europe right around sunset. Spica is one of four bright stars that the Moon can occult in the current epoch, along with Antares, Aldebaran, and Regulus. This is also part of a series of fine occultations of Spica by the Moon ongoing from 2012 to 2014.
Sundown on September 8th offers a special treat, as the 3-day old Moon passes less than a degree from Venus worldwide. The pair will fit easily into the field of view of binoculars or a telescope at low power and present an outstanding photo op.
And for observers based in Argentina and Chile, the Moon will actually occult Venus. Occultations are grand events, a split-second astronomical event in a universe that seems to usually move at a glacial pace. This particular occultation occurs for South American observers just before & after sunset.
We witnessed and recorded a similar pairing of Venus and the daytime Moon from the shores of our camp on Saint Froid Lake in northern Maine back in 2007:
Also, keep an eye out for a ghostly phenomenon known as the ashen light on the dark limb of the Moon. Also known as Earthshine, what you’re seeing is the reflection of sunlight off of the Earth illuminating the (cue Pink Floyd) dark side of the Moon. When the Moon is a crescent as seen from the Earth, the Earth is at gibbous phase as seen from the nearside of the Moon. Remember, the lunar farside and darkside are two different things! Earthshine can vary in brightness, based on the amount of cloud and snow cover present or absent on the Earth’s moonward side. My Farmer’s Almanac-consulting grandpappy would call ashen light the “Old Moon in the New Moon’s arms,” and reckon rain was a comin’…
Be sure to check out these astronomical goings on this weekend, and send those pics in to Universe Today!