Comet 21P/Giacobini-Zinner Shines in September

Watch a binocular-bright comet leapfrog across Auriga in the next few weeks before a remarkable conjunction with the bright star cluster, M35.

Pretty package

Periodic comet 21P/Giacobini-Zinner displays a bright coma and a dust tail at least ½° long in this photo taken on August 22nd.
Michael Jäger

Every season, there are something like a dozen comets visible throughout the night. Most are faint, usually around magnitude 11–13, and require larger amateur instruments to see. This time around, we have a wonderful exception — 21P/Giacobini-Zinner.

With an orbital period of 6.6 years, it's a regular visitor to our planet's night skies. G-Z's making a favorable approach at this apparition, passing just 58.6 million kilometers over the tops of our heads on September 10–11, the same date it reaches perihelion. That's just a few million kilometers shy of Mars's close brush with Earth this past July.

In late August, the comet glows at magnitude 7.5 and is expected to peak at around 7 next month. I easily saw it two weeks ago in 50-mm binoculars from a semi-rural sky when it crossed into 8th-magnitude territory. At that time, the comet appeared near the "W" of Cassiopeia and looked like a small fuzzy patch with a brighter center. Through my 15-inch Dob at low magnification, it was a beautiful sight with a compact, 3-arcminute-wide coma housing a bright "false nucleus" and a tail more than ½° long fanning to the west. At 357× I detected a small density enhancement within the coma sunward of the nucleus.

Slicing Auriga

Use this finder chart to track 21P/G-Z through mid-September. The comet's location is shown daily at 0h UT with positions marked every 5 days. Stars are plotted to magnitude 7.5 as are the brighter Messier objects. North is up. Click for a large version and print out.
SkyMap software with additions by the author

Comet 21P/G-Z has since moved east. This week (August 29–September 2), it crosses from Perseus into Auriga and glows low in the northern sky at nightfall. At twilight's end from 40° N, the comet will be only 5–7° high, not exactly the best viewing circumstances.

Midnight's better, when it climbs to about 15° altitude just 1° south of Auriga's brightest star Capella on September 2nd.  Another option, perhaps the best, is to wait until September 5th, when the Moon thins to a crescent and Auriga stands more than 50° high shortly before the start of morning twilight.

Chasing G-Z

Three scenarios for Comet 21P/Giacobini-Zinner: early evening in late August; midnight before moonrise in early September; and high up before dawn from September 6–23.
Stellarium with additions by the author

For more than two weeks, from September 5th through about the 23rd, the comet will hold court in a dark sky as it moves from Auriga through Gemini and into Monoceros. Getting up early to observe takes more effort, but it always pays off. This time of year it means the welcome sight of Orion and the shimmer of Sirius. Besides, you have to if you want to see a comet that really looks like one with a nice tail and bright head. How many of those have flown by this year? For most of us, 21P/G-Z will be the first.

Cluster-comet conjunction

Next month, Comet 21P/Giacobini-Zinner will fly across the rich open cluster M35 in Gemini before dawn for skywatchers in the Americas. This view simulates the rare event as seen through a small telescope. North is up.
Stellarium with additions by the author

Fortuitously, 21P/G-Z is passing through the winter Milky Way and will make scenic pit stops near several bright, well-known star clusters. Watch for it to pass close to M37 on the night of September 9th and directly across M35 in Gemini on the morning of September 15th for the Americas.

The M35 crossing will be amazing to watch for several reasons. The comet enters the outskirts of the cluster around 7:30 UT (2:30 a.m. CDT) and exits about 11:30 UT (6:30 a.m.). Traveling at an apparent speed of 4.4′ per hour, anyone with a small telescope can watch it slowly slide across this famous Messier object as the wee hours tick by. With 120 stars brighter than magnitude 13 it's quite possible that 21P/G-Z's false nucleus — the bright condensation near the coma's center — will occult one or more of them along the way. The binocular view should be unique with the rich cluster appearing to sprout a tail!

The comet has been seen at nearly every return since its discovery in 1900. In 1946, it passed only 0.26 a.u. from Earth in late September, when it reached magnitude 7, its brightest ever observed up until that time. Just day after, it underwent a bright outburst, rising to magnitude 6 in early October. Several smaller outbursts were observed during the 1959 apparition. Given its eruptive nature, amateurs should keep track of it each clear night. If you do see a sudden and unexpected increase in brightness, please let us know by leaving a comment.

Two tails, two nebulae

The comet displays two tails — a broader dust tail and a partially overlapping, fainter ion tail — in this photo from August 22nd, when it passed near the reflection nebulae vdB 14 (top) and vdB 15 in Camelopardalis. When viewing comets, explore the full range of magnification. Low powers are great for the coma and tail, but if seeing conditions allow, explore the nucleus region for signs of tiny dust fans and other emissions using 250× or higher.
Michal Kaluzny

Although 21P/Giacobini-Zinner is the parent comet of the annual Draconid meteor shower, which peaks on October 9th, it's normally a weak shower that generates only a handful of meteors per hour except during rare outbursts, the most recent of which occurred in 2011. Meteor counts increase over the normal when Earth ducks inside the comet's orbit shortly after its passing. That's not expected to happen this year, but some models predict a somewhat higher than average count (maybe up to 15 per hour) during the comet's passage through an older dust trail left behind in 1953.

If you don't mind setting aside an hour before dawn, get aboard 21P and let it transport you across the winter sky as summer fades.

15 thoughts on “Comet 21P/Giacobini-Zinner Shines in September

  1. OwlEyeOwlEye


    I love that last paragraph! It is fortunate that the moon will not interfere on the mornings of the 9th and 15th of September for the passage of M 37 and M 35!!

    I have not seen this comet since 1985 – 33 years ago – when it was 0.47 AU from Earth On September 6th, and I am looking forward to seeing it again!

    Doug Z

  2. Bob KingBob King Post author

    Hi Doug,
    Thanks! You’re going to like this comet. Comet tails have been few and far between. I’m hoping for one of its classic outbursts. We’ll see!

  3. Joe StieberJoe Stieber

    I’ve already seen 21P a number of times this month, mostly with my 15×56 binoculars, under both dark skies in the NJ Pines, as well as my suburban backyard with a thick moon in the sky. The former was a piece of cake, the latter was at the threshold of visibility. Before the moon interfered, i also had my 12.5-inch dob out to the Pines for a look. Nice! It’s a treat to have such an easily asscessible comet to view! Can’t wait for the moon to clear out.

    And thanks for the heads-up on the M35 crossing. I had only looked very casually beyond the close pass by M37.

    1. Bob KingBob King Post author

      You’re welcome, Joe! Sounds like you’ve been following this one like I have. I’ve given the comet a rest with the full moon and (unfortunately) forest fire smoke, but I’m eager to be out again when the moon thins a bit more.

  4. RodRod

    Good comet report and charts here Bob. I will try and view with my 90-mm refractor and 10×50 binoculars – later. Early this morning and now, we have t-storms and plenty of clouds in the area where I am at. I did plenty of grass mowing and weed whacking too, clearing out an area on my neighbor’s large horse farm that has great views of NE and E skies (trees far away). Hopefully I can join the fun bunch here and view this comet too 🙂

  5. Tom-Reiland

    This is at least the fourth apparition that I’ve been able to observe Comet G-Z, including several times in July. It was not easy to see it tonight because of Moon, just a day before Last Quarter, neighbors’ lights and the humidity. Thanks to your charts I was able spot it near Capella forming a triangle with the 6 mag star to the NE of Capella. It appeared as a small, faint, diffuse white patch through my 10 X 50 binoculars. Not sure of the magnitude, maybe 7.2? I hope I can get a good look at it Sunday night when it’s at its closest to Capella, but knowing the weather in Western Pa, it will probably be cloudy. Aside from a four night stretch from July 6 to 9, this might be the worst observing conditions for any Summer in my 45+ years of viewing celestial wonders.

  6. RodRod

    I did view the comet this morning near 0400 EDT (better elevation angle). I used my trusty, 90-mm refractor with 40-mm and 32-mm plossl eyepieces for 25x and 31x views. Some notes from my stargazing log: [Waning crescent Moon 35.63% illuminated at 0445 EDT. Virtual Moon Atlas shows 36.2% illuminated. The Moon is in Orion constellation, mv -11.91 at altitude 34 degrees and 91 degrees azimuth at 0400 EDT. I did observe comet 21P tonight using the 90-mm telescope. Good views using TeleVue 40-mm plossl and the TeleVue 32-mm plossl provided nice views too. The comet had brighter nucleus and hint of small or short tail in the telescope view. The comet at 25x and 31x views was not large but distinct in the field. I have seen more exciting comets using the telescope than 21P but observing the comet was fun. A meteor flashed by moving from Auriga towards ESE in the direction of Orion or where the Moon was about 0330 EDT. The meteor was brighter than Capella but not as bright as the Moon. The meteor left a train or trail as it flashed by. Capella’s position at 0400 EDT, altitude = 51 degrees and 61 degrees azimuth for my location.]

  7. OwlEyeOwlEye

    Hi Bob,

    It hasn’t been clear around here for over a week, what with the relentless – but much-needed – rain, followed by more rain from the remnants of TS Gordon. However, I was able to get out this morning for a look at 21P. My skies are not as dark as they were 17 years ago, and while I could see M 36, 37 and 38 in a 7 X 50 finder, the comet was not visible.

    In a 6-inch reflector, however, 21P was easily visible, and just fit with M 36 in the same field at 30.5 X, showing a bright inner coma and nice stub of tail. As I observed it, a Barred Owl gave a single whooo and a few minutes later, as the eastern horizon took on a pale glow, a pack of Coyotes began barking and howling in the distance!

    Our local forecast is looking pretty good at this point for the passage through M 35 on the morning of the 15th!

    Doug Z

  8. TJW1012

    Hey Bob,
    I am so excited to get to look at this comet again on the 15th. I saw it about a month ago at a nice dark site and it presented a nice tail and false nucleus through a 20 inch. I have a concern though. I noticed some other charts have been putting 21/P a couple of degrees away from M35. I don’t know why there would be a shift in the data. I am very excited though to see this comet again. Great article!

    1. Bob KingBob King Post author


      I just saw 21P this morning under beautiful skies in my 15-inch. Wow, what a nice tail, and I picked up some extra “fuzz” (jetting material) sunward of the nucleus using 412x.
      I double-checked and the comet will pass through the cluster. Not the exact center but just a little northeast of the center during early morning hours Saturday across the Americas. I don’t know your location but closest approach to the center is around 4 a.m. Central Daylight. From Europe and Asia, the comet will be near but not on top of the cluster.

  9. Tom-Reiland

    When I arrived at Wagman Obs conditions were decent, but started to deteriorate shortly after I arrived because of clouds from Hurricane Florence.  That was about 11:30 PM. I decided to wait it out because first contact with M35 was not for several more hours. It started clearing out about 3 AM.  I was able to locate M35 and Comet Giacobini-Zinner a few minutes later.  The Comet was on the an edge of the cluster at that time and I watched it until 3:30 AM when conditions worsened.  The weather did not match any of the forecasts of several forecasters. I could see a short, faint tail through the 21″ Reflector at 127X, with a compact, though not stellar, nucleus. I estimate the Comet G-Z has faded by at least 0.5 magnitude, It was not easy to judge, especially because it’s been 10 nights since I last observed it.

  10. miken6mz

    Thanks for the article. Spotted the comet this morning in my 20x80s in the murky light-polluted Seattle suburb skies, my eyes not even beginning to become dark-adapted. All I could see was a smudge of light, but it’s still exciting nonetheless.

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