Watching the heavens on a nightly, or even casual basis? The web and modern technology has certainly altered the landscape of modern astronomy, (mostly) for the better. Once, we all huddled around cardboard planispheres, illuminated by red flashlights; now, it’s now a common sight to see illuminated smartphone apps accompanying telescopes at star parties, all waving skyward with virtual planetarium programs guiding users around the night sky.
A periodic comet may put on a fine show for northern hemisphere viewers over the next few months.
Comet 21/P Giacobini-Zinner is currently a fine binocular comet, shining at +8th magnitude as it cruises across the constellation Cassiopeia. This places it above the horizon for the entire night for observers north of the equator in August, transiting the local meridian at dawn. And unlike most comets that get lost in the Sun’s glare (like the current situation with C/2017 S3 PanSTARRS), we’ll be able to track Comet 21/P Giacobini-Zinner right through perihelion on September 10th.
This is because the comet is on a short period, 6.6 year orbit around the Sun that takes it from an aphelion of 6 Astronomical Units (AU) exterior to Jupiter’s orbit, to a perihelion of 1.038 AU, just 3.3 million miles (5.2 million kilometers) exterior to Earth’s orbit. The 2018 apparition sees the comet pass 0.392 AU (36.5 million miles/58.3 million kilometers) from the Earth on September 11th.
This is the closest passage of the comet near Earth since September 14th, 1946, and won’t be topped until the perihelion passage of September 18th, 2058. Its next cycle of passes to Earth closer than 0.1 AU aren’t until next century in the years 2119 and 2195, respectively.
Discovered by astronomer Michel Giacobini at the Côte d’Azur Observatory in Nice, France on the night of December 20th, 1900 as it was crossing the constellation Aquarius, the 21st periodic comet was recovered two orbits later by Ernest Zinner on October 23rd, 1913 as it passed a series of variable stars near Beta Scuti.
Though the comet generally tops out at +8th magnitude, it has been known to undergo periodic outbursts near perihelion, bringing it up about 3 magnitudes (about 16 times) in brightness. This occurred most notably in 1946.
Comet 21/P Giacobini-Zinner is also the source of the Draconid (sometimes referred to as the Giacobinid) meteors, radiating from the constellation Draco the Dragon on and around October 7th and 8th. Feeble on most years, this shower can produce surprises, such as occurred in 1998, 2005 and most recently in 2011, when a Draconid outburst topped a zenithal hourly rate of 400 meteors per hour, flirting with ‘meteor storm’ status. And while we’re not expecting a meteor storm to accompany the 2018 perihelion passage of Comet 21/P Giacobini-Zinner, you just never know… it’s always worth keeping an eye out on early October mornings for the “Tears of the Dragon,” just in case. Note that the Moon reaches New phase on October 9th, just a few days after the meteor shower’s expected annual peak, a fine time to watch for any unheralded Draconid outbursts.
Prospects for Comet 21P
The comet is visible from the northern hemisphere through the remainder of August and all through September as it glides across Auriga, Taurus and Gemini and visits several well known celestial sights. In fact, it actually transits in front of several deep sky objects, including Messier 37 (Sept 10th), and Messier 35 (Sept 15th).
The comet will be moving at about two degrees per day when it’s nearest to the Earth, on and around September 11th.
We begin to lose the comet, as it heads southward in late October. Still, the comet is over 50 degrees above the eastern horizon at dawn come October 1st as seen from latitude 30 degrees north, having maintained a similar elevation throughout most of September. Not bad at all.
Here are some upcoming dates with destiny for Comet 21/P Giacobini-Zinner:
August 19: Crosses into the constellation Camelopardalis.
August 29: Crosses into the constellation Perseus.
August 30th: Crosses into the constellation Auriga.
September 2: Passes one degree from the bright star Capella.
Sept 7-8: Grouped 2 degrees from the open clusters M36 and M38.
Sept 10: Photo-Op: Skirts very near the open cluster M37. Also reaches perihelion on this date, at magnitude +7.
Sept 11: Passes closest to the Earth, at 0.392 AU distant.
Sept 13: Nicks the corner of the constellation Taurus.
Sept 14th : Enters the constellation Gemini.
Sept 15th: Photo-Op: crosses in front of the open cluster M35.
Sept 16: Crosses the ecliptic southward and near the +3.3 magnitude star Propus (Eta Geminorum).
Sept 17: Crosses into Orion.
Sept 21: Crosses into Gemini.
Sept 23: Crosses into Monoceros.
Sept 24: Passes near the Christmas Tree Cluster, NGC 2264.
Oct 1: Crosses the galactic plane and the celestial equator southward.
Oct 7: Crosses in front of the open cluster M50.
Oct 10: Crosses into Canis Major.
Oct 31st: Passes near the bright star Aludra and may drop below +10th magnitude.
Binoculars are your best friend when you’re looking for comets brighter than +10th magnitude. With a generous field of view, binoculars allow you to sweep a suspect area until the faint fuzzball of a comet snaps into view. I like to ‘ambush’ a comet as it passes near a bright star, and a good time to spot comet 21/P Giacobini-Zinner is coming right up on September 2nd when it passes less than one degree from the bright +0.1 magnitude star Capella.
Don’t miss this year’s fine apparition of Comet 21/P Giacobini-Zinner, coming to a night sky near you.
Now, watch for a rare event this weekend, with the final eclipse for 2018 coming up on Saturday, August 11th, with a partial solar eclipse spanning northern Europe and the Arctic.
What’s so unique about this eclipse? Well, not only is it the last one for 2018, but it’s part of three eclipses in the second eclipse season of the year. Most seasons only feature two eclipses (one lunar and one solar) but every few years or so, it is possible to have a season with three: either lunar-solar-lunar (such as occurred in 2013) or solar-lunar-solar.
This is only possible when the middle eclipse occurs very near ascending or descending node along the ecliptic. The nodes are where the path of the Moon, inclined 5.1 degrees relative to the ecliptic plane intersect it—when these nodes are occupied by an alignment of the Earth, Sun and Moon (known as a syzygy, a fine word in Scrabble to land on a triple word score, though you’ll need a blank tile for the third ‘y’) a solar or lunar eclipse occurs. For an eclipse triple play, the middle eclipse needs to happen very near a node crossing, producing a fairly long eclipse. That’s exactly what happened on July 28th, when the Moon crossed through descending node just over an hour after crossing out of the Earth’s umbral shadow after the longest lunar eclipse for the 21st century.
This also leaves the Moon close enough to the opposite ascending node two weeks post and prior to July 28th on July 13th and August 11th to just nick the Sun for a partial solar eclipse, one over the Antarctic and one over the Arctic.
Saturday’s partial eclipse touches down over the eastern coast of Canada at sunrise. From there, it sweeps eastward over Greenland, Iceland and the North Atlantic, with the Moon’s penumbra just grazing the northern United Kingdom before crossing over Scandinavia. Then, the shadow crosses over Asia, with a photogenic partial solar eclipse wrapping up at sunset over eastern China, the Koreas and the Russian far east.
Note that this eclipse is also a relative newcomer for its particular saros 155, as it is member 6 of a series of 71 eclipses. The saros just began less than a century ago on June 17th, 1928, and won’t produce its first total solar eclipse until September 12th, 2072 AD.
As of this writing, we’ve yet to see evidence of anyone carrying the eclipse live, though we’ll note it here if any webcast(s) surface.
When is the next one? Well, the next partial solar eclipse is on January 6th 2019, and the next total solar eclipse occurs on July 2nd, 2019.
Enter the Perseids
This weekend’s eclipse at New Moon also sets us up for a fine display of the Perseid meteors for 2018. This year, the Perseids are expected to peak on the morning of August 12th and August 13th. Watch for a zenithal hourly rate of 100 meteors per hour at the peak. A dependable annual favorite, the Perseids are debris remnants of period comet 109/P Swift-Tuttle.
Astronomer Gianluca Masi and the Virtual Telescope Project 2.0 will host a live webcast for the 2018 Perseids on August 12th starting at 20:30 UT.
Don’t miss the astronomical action worldwide this weekend, either live or online.
Comets are one of those great question marks in observational astronomy. Though we can plot their orbits thanks to Newton and Kepler, just how bright they’ll be and whether or not they will fizzle or fade is always a big unknown, especially if they’re a dynamic newcomer from the Oort Cloud just visiting the inner solar system for the first time.
We had just such a surprise from a cosmic visitor over the past few weeks, as comet C/2017 S3 PanSTARRSerupted twice, brightening into binocular visibility. Discovered on December 23rd 2017 during the PanSTARRS survey based on Haleakala, Hawai’i, S3 PanSTARRS is on a long-period, hyperbolic orbit and is most likely a first time visitor to the inner solar system.
S3 PanSTARRS was not only rocked by two new outbursts in quick succession, but seems to have undergone a tail disconnection event just last week, leveling off its brightness at around +8 magnitude and holding. This puts it in the range of binoculars under dark skies, looking like a fuzzy globular that refuses to snap into focus as it currently glides through the constellation of Camelopardalis the Giraffe the dawn sky.
As July closes out, the time to catch sight of Comet S3 PanSTARRS is now, before it’s lost in the Sun’s glare. From latitude 40 degrees north, the comet sits 20 degrees above the northeastern horizon, about an hour before sunrise. By August 7th however, it drops below 10 degrees altitude. From there, the comet begins to circle the Sun as seen from the Earth beginning to favor southern hemisphere observers at dawn, who may be able to track it straight through perihelion on August 16th, if its brightness holds up. From there, northern hemisphere viewers may get a second view at dawn in September, again, if its brightness holds.
You never know when it comes to comets. Here’s a brief rundown of the celestial happenings for comet C/2017 S3 PanSTARRS:
3- Crosses into the constellation Gemini.
4- Passes near the bright star Castor.
5- Passes near the bright star Pollux.
7- Crosses into the constellation Cancer.
7- Passes closest to the Earth, at 0.758 Astronomical Units (AU) distant.
8- Crosses southward over the ecliptic plane.
9- Passes just 4 degrees from the Beehive cluster, M44.
11- Passes 2 degrees from the open cluster M67.
12- Passes 10.5 degrees from Sun (1st apparent close pass as seen from the Earth)
13- Crosses into the constellation Hydra.
15- Reaches maximum brightness: the comet may top +2nd magnitude in mid-August.
16- Reaches perihelion at 0.21 AU from the Sun.
18- Crosses into the constellation Sextans.
30-Crosses into the constellation Leo.
31-Crosses the ecliptic plane northward.
3- passes 4 degrees from the Sun.
25- Crosses into the constellation Coma Berenices.
From there, Comet C/2017 S3 PanSTARRS drops back below 6th magnitude in September, then below 10th magnitude in October as it heads back off into the icy realms of the outer solar system.
Be sure to nab this icy interloper why you can. The quote comet hunter David Levy, “Comets are like cats… they have tails, and they do exactly what they want.”
One of the top astronomy events of 2018 occurs on the evening of Friday, July 27th, when the Moon enters the shadow of the Earth for a total lunar eclipse. In the vernacular that is the modern internet, this is what’s becoming popularly known as a “Blood Moon,” a time when the Moon reddens due to the refracted sunlight from a thousand sunsets falling upon it. Standing on the surface of the Moon during a total lunar eclipse (which no human has yet to do) you would see a red “ring of fire” ’round the limb of the eclipsed Earth.
This is the second total lunar eclipse for 2018, and the middle of a unique eclipse season bracketed by two partial solar eclipses, one on July 13th, and another crossing the Arctic and Scandinavia on August 11th.
The July 27th total lunar eclipse technically begins around 17:15 Universal Time (UT), when the Moon enters the bright penumbral edge of the Earth’s shadow. Expect the see a slight shading on the southwest edge of the Moon’s limb about 30 minutes later. The real action begins around 18:24 UT, when the Moon starts to enter the dark inner umbra and the partial phases of the eclipse begin. Totality runs from 19:30 UT to 21:13 UT, and the cycle reverses through partial and penumbral phases, until the eclipse ends at 23:29 UT.
Centered over the Indian Ocean region, Africa, Europe and western Asia get a good front row seat to the entire total lunar eclipse. Australia and eastern Asia see the eclipse in progress at moonset, and South America sees the eclipse in progress at moonrise just after sunset. Only North America sits this one out.
Now, this total lunar eclipse is special for a few reasons.
First off, we’ll have the planet Mars at opposition less than 15 hours prior to the eclipse. This means the Red Planet will shine at a brilliant magnitude -2.8, just eight degrees from the crimson Moon during the eclipse, a true treat and an easy crop to get both in frame. We fully expect to see some great images of Mars at opposition along with the eclipsed Moon.
How close can the two get? Well, stick around until April 27th, 2078 and you can see the Moon occult (pass in front of) Mars during a penumbral lunar eclipse as seen from South America.
And speaking of occultations, the Moon occults some interesting stars during totality Friday, the brightest of which is the +5.9 magnitude double star Omicron Capricorni (SAO 163626) as seen from Madagascar and the southern tip of Africa. Omicron Capricorni has a wide separation of 22″.
The second unique fact surrounding this eclipse is one you’ve most likely already heard: it is indeed the longest one for this century… barely. This occurs because the Moon reaches its descending node along the ecliptic on July 27th at 22:40 UT, just 21 minutes after leaving the umbral shadow of the Earth. This makes for a very central eclipse, nearly piercing the umbral shadow of the Earth right through its center.
Totality on Friday lasts for 1 hour, 42 minutes and 57 seconds. This was last beat on July 16th, 2000 with a duration of 1 hour, 46 minutes and 24 seconds (2001 is technically the first year of the 21st century). The duration for Friday’s eclipse won’t be topped until June 9th 2123 (1 hour 46 minutes six seconds), making it the longest for a 123 year span.
The longest total lunar eclipse over the span of 5,000 years from 2000 BC to 3000 AD was on May 31st, 318 AD at 106.6 minutes in duration.
A Minimoon Eclipse
Finally, a third factor is assisting this eclipse in its longevity is the onset of the MiniMoon: The Moon reaches apogee at July 27th, 5:22 UT, 14 hours and 37 minutes prior to Full and the central time of the eclipse. This is the most distant Full Moon of the year for 2018 (406,222 km at apogee) the 2nd most distant apogee for 2018. Apogee on January 15th, beats it out by only 237 kilometers. This not only gives the Moon a slightly smaller size visually at 29.3′, versus 34.1′ near perigee, less than half of the 76′ arcminute diameter of the Earth’s shadow. This also means that the Moon is moving slightly slower in its orbit, making a more stately pass through the Earth’s shadow.
What will the Moon look like during the eclipse? Not all total lunar eclipses are the same, but I’d expect a dark, brick red hue from such a deep eclipse. The color of the Moon during a eclipse is described as its Danjon number, ranging from a bright (4) to dark murky copper color (0) during totality.
Tales of the Saros
This particular eclipse is member 38 of the 71 lunar eclipses in saros series 129, running from June 10th, 1351 all the way out to the final eclipse in the series on July 24th, 2613 AD. If you caught the super-long July 16th, 2000 eclipse (the longest for the 20th century) then you saw the last one in the series, and the next one for the series occurs on August 7th, 2036. Collect all three, and you’ve completed a triple exeligmos series, a fine word in Scrabble to land on a triple word score.
Photographing the Moon
If you can shoot the Moon, you can shoot a total lunar eclipse, though a minimum focal length lens of around 200mm is needed to produce a Moon much larger that a dot. The key moment is the onset of totality, when you need to be ready to rapidly dial the exposure settings down from the 1/100th of a second range down to 1 second or longer. Be careful not to lose sight of the Moon in the viewfinder all together!
Are you watching the eclipse during moonrise or moonset? This is a great time to shoot the eclipsed Moon along with foreground objects… you can also make an interesting observation around this time, and nab the eclipsed Moon and the Sun above the local horizon at the same time in what’s termed a selenelion. This works mainly because the Earth’s shadow is larger than the apparent diameter of the Moon, allowing it to be cast slightly off to true center after sunrise or just before sunset. Gaining a bit of altitude and having a low, flat horizon helps, as the slight curve of the Earth also gives the Sun and Moon a tiny boost. For this eclipse, the U2-U3 umbral contact zone for a selenelion favors eastern Brazil, the UK and Scandinavia at moonrise, and eastern Australia, Japan and northeastern China at moonset.
Incidentally, a selenelion is the second visual proof you see during a lunar eclipse that the Earth is indeed round, the first being the curve of the planet’s shadow seen at all angles as it falls across the Moon.
Another interesting challenge would be to capture a transit of the International Space Station during the eclipse, either during the partial or total phases… to our knowledge, this has never been done during a lunar eclipse. This Friday, South America gets the best shots at a lunar eclipse transit of the ISS:
Live on the wrong continent, or simply have cloudy skies? Gianluca Masi and the Virtual Telescope Project 2.0 have you covered, with a live webcast of the eclipse from the heart of Rome, Italy on July 27th starting at 18:30 UT.
Be sure to catch Friday’s total lunar eclipse, either in person or online… we won’t have another one until January 21st, 2019.
Learn about eclipses, occultations, the motion of the Moon and more in our new book: Universe Today’s Guide to the Cosmos: Everything You Need to Know to Become an Amateur Astronomer now available for pre-order.