Archive for September, 2015

The Sky This Week, September 22 – 29, 2015 !

The Sky This Week, 2015 September 22 – 29

Equinox, Harvest Moon, and a total lunar eclipse!
Total Lunar Eclipse, 2014 October 10, 10:38 UT
Imaged with a Canon EOS Rebel T2i DSLR, 250mm f/8, 1/4s @ ISO 1600

The Moon brightens the evening sky this week, passing through the faint autumnal constellations as she waxes to Full Moon on the 27th at 10:51 pm Eastern Daylight Time. At this time Luna will undergo a total eclipse by the shadow of the Earth; more on this below. September’s Full Moon is known almost universally in the northern hemisphere as the “Harvest Moon” due to the peculiar geometry of its orbit with respect to the eastern horizon that occurs each year at this time. At northern temperate latitudes the plane of Luna’s orbit intersects the eastern horizon at a shallow angle that becomes even shallower at higher latitudes. This causes the Moon to rise at around the same time on the nights immediately before and after Full Moon. Before the invention of artificial lighting the extra light provided by the rising Full Moon gave farmers a little extra light to bring in their crops. Typically the interval between successive moonrises averages about an hour later on successive nights. Here in Washington the difference is about 40 minutes at this time of the year. In Scotland the difference is about 25 minutes, while the northernmost regions of Norway see successive moonrises that are just three minutes later each night. Venture above 75 degrees north latitude and the Moon will actually rise earlier each night around the full phase!

This year’s Harvest Moon will also provide us with a very nice total eclipse that will be a “prime time” event for the eastern United States. Luna enters Earth’s penumbral shadow at 8:10 pm EDT, and over the course of the next 45 minutes you may begin to notice a slight darkening of the Moon’s upper left limb. At 9:07 pm the Moon enters the umbral shadow and is completely eclipsed beginning at 10:11 pm. Mid-eclipse occurs at 10:47 pm, and totality ends at 11:23 pm. Luna exits the umbra at 12:27 am and the penumbra at 1:24 am. Much ado has been trumpeted about this eclipse occurring in conjunction with the closest lunar perigee of the year, but rest assured that there’s very little substance in this phenomenon. Luna’s disc will be about 7% bigger than average, but most of us would have a hard time detecting this with the unaided eye. There is also a fair amount of nonsense circulating on social media calling the eclipse a “blood moon” with the implication of dire effects of earth-shattering import. Rest assured that there have been thousands of total lunar eclipses across recorded human history, and whatever calamities that have been associated with them have usually been the fault of poor human judgment. Enjoy this eclipse for what it is: one of Nature’s wonderful treats for the eye.

The autumnal equinox falls on the 23rd at 4:21 am EDT. At that instant the center of the Sun’s disc stands directly over the Equator at a point in the Indian Ocean about 1000 kilometers east of southern Somalia. If Old Sol were a point source of light and Earth didn’t have an atmosphere the length of day and night would be exactly 12 hours. However, the Sun subtends a disc that’s half a degree in apparent diameter and we have a significant atmosphere that sustains us, so our day of “equal night” falls on the 26th.

Our only bright evening planet remains Saturn, who may be found in the southwestern sky as evening twilight deepens. You have a limited window to observe him, however, as he sets well before 10:00 pm. His rings can be spotted in just about any telescope, but your chance of seeing a nice crisp image will be severely limited by the dense layers of our atmosphere that you’ll have to peer through.

The best planet viewing is before dawn in the eastern sky. Here you’ll find the dazzling glimmer of Venus well up as twilight gathers. You should also see bright Jupiter rising out of the horizon haze by 6:00 am. Halfway between them you’ll find the much fainter glow of Mars close to the bright star Regulus in Leo. These planets will gradually move toward the evening sky over the next several months, promising to give us an interesting springtime sky to look forward to.


The Sky This Week, September 15 – 22, 2015 !

The Sky This Week, 2015 September 15 – 22

Share the Moon with friends and neighbors.
Messier 11, the “Wild Duck Cluster”
Imaged 2015 August 15 with a Canon EOS Rebel T2i DSLR, 80mm f/6 Antares Sentinel refractor,
25 90-second subframes @ ISO 3200 at Fishers Island, New York

The Moon returns to the evening sky this week, waxing from a slender crescent to First Quarter, which occurs on the 21st at 8:59 am Eastern Daylight Time. Luna courses through the southern reaches of the ecliptic, passing just over two degrees north of the second-magnitude star Zubenelgenubi on the 17th and a similar distance northwest of golden Saturn on the 18th. On the 19th skywatchers around the world will take part in the annual International Observe the Moon Night, an evening set aside each year to promote observation and appreciation of our natural satellite and its connection to planetary science. I often say that the Moon is “looked over, then overlooked” by novice telescope users, but studying the various formations on Luna’s battered surface can become a lifelong passion, and almost any telescope will provide stunning views of the only other place in the solar system that humans have visited. There are several places to participate in this event in the Washington area. The National Air & Space Museum’s observatory will be open that evening, and astronomers will have telescopes set up to observe at the Arlington County Planetarium. Farther afield, the Northern Virginia Astronomy Club will host public observing sessions at C.M. Crockett Park near Midland, Virginia and Sky Meadows State Park near Paris, Virginia. If you own a telescope, set it up in your neighborhood and hold an impromptu Moon party for all of your neighbors!

You can still enjoy relatively dark skies early in the week before the Moon begins to blot out the fainter stars. The summer Milky Way is prominently placed overhead by 9:00 pm, offering an array of targets for binoculars and small telescopes. It is hard to describe the sight of vast numbers of stars that reveal themselves in the misty glow of the Galaxy’s softly glowing clouds. Some of my favorite views are those I’ve had with my 80-millimeter (3.1-inch) wide-field telescope at a modest magnification of about 20X. In addition to the clouds of stars, there are also voids where scarcely a star can be found. Interspersed are bright knots of light betraying the location of a star cluster or glowing gaseous nebula. One of my favorite targets can be found halfway between Altair, the southernmost star in the Summer Triangle, and Kaus Borealis, the star that marks the top of the “Teapot” asterism in Sagittarius. In the midst of one of the Milky Way’s densest star clouds is a fuzzy patch of light which a small telescope reveals as a wonderful star cluster known as Messier 11. This cluster is popularly known as the “Wild Duck Cluster” thanks to a wonderful description penned by British amateur astronomer W.H. Smyth in his “A Cycle of Celestial Objects” published in 1844. This book, with descriptions of over 850 double stars, star clusters, and nebulae, was the first observing guide intended for amateur astronomers. It has been reprinted many times, most recently in 1986, and is still a wonderful companion at the eyepiece.

Saturn gets a visit from the waxing crescent Moon on the evening of the 18th, and the ringed planet still puts on a good show in the early evening hours. Lately I’ve been observing Saturn in twilight about half an hour after sunset. This gives me about an hour of relatively stable air before Saturn begins to slip into the thermal currents near the southwest horizon. Saturn is the only bright planet currently visible in the evening sky, so take advantage of him while you still can.

The rest of the naked-eye planets may be found in the east before dawn. You’ll have no trouble spotting Venus, who reaches her greatest brilliancy on the 21st. Closer to the horizon is Jupiter, gradually emerging from his recent conjunction with the Sun. In between you’ll find ruddy Mars, currently a second-magnitude object. The red planet spends the week closing in on the bright star Regulus in Leo, the Lion. He’ll pass just south of the star on the mornings of the 24th and 25th. Watch these planets over the next month. They will be joined by fleet Mercury and undergo a number of interesting conjunctions during that time.

The Sky This Week, September 8 – 15, 2015 !

The Sky This Week, 2015 September 8 – 15

Count stars in the evening, take pictures in the morning.
Messier 31, the Great Andromeda Galaxy
Imaged 2015 August 8 with a Canon EOS Rebel T2i DSLR, 80mm f/6 Antares Sentinel refractor,
20 90-second subframes @ ISO 3200 at Fishers Island, New York

The Moon graces the pre-dawn sky early in the week, then disappears in the glare of morning twilight before New Moon, which occurs on the 13th at 2:41 am Eastern Daylight Time. This particular New Moon will produce a partial solar eclipse that won’t be visible to us here in the US. If you’d still like to travel to see it, you’ll need to book passage to southern Africa, the Kerguelen Island archipelago in the southwestern Indian Ocean, or Antarctica. Maximum eclipse will be best seen from the latter, where penguins and a few research scientists will see some 70% of the Moon’s disc obscured by the Sun. This will lead up to a spectacular total lunar eclipse that will be visible from our part of the world on the night of September 27-28. You’ll have a wonderful photo opportunity before dawn on the 10th, when the waning crescent Moon passes just three degrees from dazzling Venus, which has vaulted into the morning sky after last month’s conjunction with the Sun. On the following morning, see if you can spot the bright star Regulus just to the north of the very slender lunar crescent. If it’s really clear you may also spy the ruddy glow of Mars just 8 degrees above Luna and the star.

A waning crescent in the morning sky means that it’s “dark sky” time for all of us. If you’re living in a place with no streetlights or urban areas nearby you’ll have a ringside seat to the summer Milky Way, which seems to bisect the sky by the end of evening twilight. Even if you live in a city you can still find the summer’s brighter stars, and by doing so you can help astronomers study the effects of light pollution around the world. The citizen-science Globe at Night program encourages skywatchers everywhere to count the number of stars they see in particular areas of the sky as the seasons progress. For September your target constellation is Cygnus, the Swan, whose brightest star Deneb is the faintest of the three bright stars that make up the Summer Triangle asterism. This distinctive feature is now directly overhead between 9:00 and 10:00 pm. From the Globe at Night website select the link to Cygnus and compare your view with the star maps that depict different limiting magnitudes. From my home in the suburbs I’m lucky to see third-magnitude stars, but on my recent visit to Belle Isle State Park in Virginia’s Northern Neck stars of sixth-magnitude were visible. How many can you see from your favorite observing site?

As the night progresses another geometric figure rises in the eastern sky. The asterism known as the “Great Square” looks like a giant diamond suspended over the eastern horizon as twilight ends, and by midnight it’s nearing the meridian high in the south. The Square is a part of the larger constellation of Pegasus, the winged horse which carried the hero Perseus to rescue Andromeda in a famous legend from mythology. Andromeda herself can be traced out as two diverging chains of stars, one relatively bright, the other rather faint, which seem to join at the upper left-hand corner of the Square at the star Alpheratz. If you’re in a dark location, follow the brighter chain of stars to its second member, then scan northward past two fainter stars. On a good night you should see a smallish smudge of diffuse light that resembles a detached bit of the Milky Way. This is actually the famous Andromeda Galaxy, our closest large galactic neighbor some 2.5 million light-years distant. Binoculars begin to show its true nature, and in a medium-sized telescope the eyepiece is filled with a diaphanous glow that defies resolution. It is the most distant object we can see with our unaided eyes!

Saturn still lingers in the southwestern sky as evening twilight deepens. The ringed planet is gradually moving eastward toward the stars that form the “head” of Scorpius, and you still have an hour or so once he becomes visible to get a decent view of him through a telescope. His eastward plod is very slow, however, and he is gradually losing the battle with the approaching Sun.

All of the other planets are congregating in the morning sky. By mid-October Mercury, Venus, Mars, and Jupiter will all appear before the rising Sun. Stay tuned…

The Sky This Week, September 1 – 8, 2015 !

The Sky This Week, 2015 September 1 – 8

Spend the last weekend of summer touring the Galaxy.
Summertime Milky Way in the Summer Triangle
Imaged 2015 August 19 with a Canon EOS Rebel T2i DSLR, 24mm @f/5.6, 90 seconds
@ ISO 3200 at Fishers Island, New York

The Moon moves into the morning sky this week, climbing from the autumnal constellations to join the rising winter constellations. Last Quarter occurs on the 5th at 5:54 am Eastern Daylight Time. If you’re up before the Sun look for the Moon close to the reddish tint of the bright star Aldebaran. On the following morning Luna stands above the rising figure of Orion, the Hunter. On the morning of the 7th the Moon passes one degree north of the 2nd-magnitude star Alhena in the constellation Gemini.

As the Moon drifts into the morning sky evening skywatchers can begin to enjoy the best of the summertime sky, just in time for the long holiday weekend. If you’re venturing away from the city to the mountains or the shore, take some time in the evening to look for the fuzzy glow of the Milky Way, our home galaxy. Few urban dwellers are aware of this softly glowing ribbon of light, which bisects the sky at around 10:00 pm, so it’s a treat to be able to find a place to enjoy it. The brightest parts of the Milky Way lie to the south, just west of the “teapot” asterism of Sagittarius. When we look in this direction we are viewing the densest portions of the galaxy, gazing toward its massive core. We can’t actually see the galactic center since it is obscured by vast clouds of dust and gas as well as billions of intervening stars. If you point a pair of binoculars or a small telescope in this direction you’ll see some of these vast star-clouds as well as concentrated smudges of light betraying the presence of star clusters and glowing gaseous nebulae. Following the Milky Way toward the zenith will take you to the bright stars of the Summer Triangle, where you’ll notice a dark “rift” in the Milky Way. This dark patch is actually a vast cloud of cold interstellar gas and dust that’s blocking the light of more distant star clouds. From very dark locations you can trace out a number of intricate features in these dark regions. They were so prominent in pre-Colombian skies that the Inca and related native tribes made constellations out of them instead of the bright stars! Here again binoculars and small telescopes will show bright knots of star clusters, many of which have faint background haze from dimly glowing gas clouds. Continuing to the northeast, you’ll see the “W”-shaped constellation of Cassiopeia, whose Milky Way star clouds are littered with star clusters, including the famous “Double Cluster”, one of the best binocular targets in the heavens. Take some time over the weekend to examine some of these sights. On the cosmic scale they are practically in our back yard.

Most of the bright objects you’ll see in the evening sky are stars. High in the west as twilight ends you’ll find the rosy glow of the star Arcturus, the brightest star in the northern hemisphere of the sky. Overhead you’ll find the blue-hued stars of the Summer Triangle, Vega, Deneb, and Altair. Rising in the southeast is the solitary star Fomalhaut, the most isolated of the first-magnitude stars.

The only bright planet left in the evening sky is Saturn, which glows with a yellowish tint in the southwest as evening twilight fades. The ringed planet is still a good target for the small telescope, but his time in the sky is brief, though, since he sets at around 11:00 pm. Just over 10 degrees to the east is the bright ruddy star Antares, the heart of Scorpius, the Scorpion. Saturn will spend next summer in the company of this star.

173 years ago this week President John Tyler signed S.285, the bill that established the first permanent Depot of Charts and Instruments, which ultimately became the U.S. Naval Observatory. The bill appropriated an initial sum of $10,000 for the purchase of instruments and books to establish our Library. A later appropriation of $15,000 provided for buildings and staff to operate the newly purchased instruments. By 1845 we were off and running, and we haven’t looked back since!