This is the Saint Louis Science Center’s NIGHT SKY UPDATE for Comet Leonard.
December 16, 2021 Update for Comet Leonard
Position of Comet Leonard at 5:11 p.m. on December 16, 2021. Image created using Stellarium.
Over the past few weeks, what was supposed to be a decently bright comet, appeared to be going down the road of another disappointment. Comet Leonard was underperforming. That all changed on December 15, 2021, when Comet Leonard brightened significantly becoming a 3rd magnitude object. Comets are fickle objects. Often a comet is hyped to be the next bright comet and then never achieves what observers are hoping for. As Comet Leonard has shown, we should never give up on a comet since conditions can change over night. The significant increase in brightness is likely due an outburst causing the coma of the comet to brighten.
A brightness change due to an outburst can fade quickly. The last visual estimate I have seen puts the comets at 3.3 magnitude. If you want to take advantage of this, you will want to look for the comet over the next few days. Looking at the weather forecast, the only night with decent weather conditions this week looks to be December 16, 2021. Your best chance to find Comet Leonard will come as civil twilight ends at 5:11 p.m. on December 16. At this time, Comet Leonard will be about 10° above the southwest horizon at azimuth 229°.
To find Comet Leonard start with Venus in the southwest. It will be the brightest object you see in that direction. Once you can see Venus look about 8° southwest of Venus to find Comet Leonard. You will likely need binoculars to see the comet. Also keep an eye on the weather conditions. A great tool for this is the Clear Sky Chart posted by A. Danko and forecasted by A. Rahill using data from Environment Canada. You can find their clear sky chart for St. Louis, Missouri here.
Comet Leonard C/2021 A1 (Leonard)
Comet Leonard C/2021 A1 (Leonard) imaged on November 28, 2021. Image credit: University of Hertfordshire Observatory
The night sky has long been a point of interest for humanity, beckoning us to look up into its inky black void. The moon and its cyclic behaviors along with the Sun, planets and seasonal constellations revealed to us that in nature, we find consistency and predictability. Born from this knowledge are ideas such as time keeping, navigation and other innovations that aided civilization’s growth. As predictable as nature can be, it also presents things that can seem unpredictable. Objects can suddenly appear and behave in ways we do not understand. For ancient astronomers, comets were one of these unpredictable things. Historically, seen as foreboding omens, today we know that these icy denizens from the distant reaches of the solar system brighten as they begin to move sunward.
What are comets and where do they come from
Comets are icy bodies leftover from the formation of our solar system. Comets come from great distances where it is cold enough that volatiles such as water remain frozen. The two sources of comets are the Kuiper Belt and Oort Cloud. The Kuiper Belt is a belt of objects that orbit the Sun beyond Neptune. The Oort Cloud has been proposed to explain long period comets that can take hundreds of thousands of years to orbit the Sun. The Oort Cloud is thought to be a spherical cloud of debris as opposed to being restricted to the plane of our solar system. To help appreciate how far away these structures are, the average distance between the Earth and Sun is roughly 93 million miles. This is an astronomical unit (AU). Pluto, whose orbit lies within the Kuiper Belt is 30 AU when nearest the Sun and 50 AU when at its greatest distance. The inner edge of the Oort Cloud is thought to begin somewhere between 2000 and 5000 AU.
Comet 67P/Churyumov-Gerasimenko image by the Rosetta spacecraft. The false color image shows the large Icy comet outgassing. Image credit: ESA/Rosetta/NAVCAM
When a comet begins to head sunward, higher temperatures heat the icy body causing it to sublimate. As solids turn to gas, the gas is released creating the coma of a comet. This is what we see when we observe a comet. As it gets closer, various tails begin to form; a dust tail points opposite the direction it travels and an ion tail that points away from the Sun. For backyard observers, most comets remain a faint fuzzy ball when seen.
On rare occasions, a comet will become bright enough we can see it naked eye. Comet Halley, Comet Hyakutake and Comet Hale-Bopp were the best comets seen by northern hemisphere viewers in the last 120 years. Their last apparitions occurred in 1986, 1996 and 1997 respectively. For comet sleuths, 2021 has been a slow year. The last bright comet seen in the northern hemisphere was C/2020 F3 (NEOWISE). This comet became visible to the unaided eye for a short time in 2020 making the best comet seen in the northern hemisphere since Comet Hale-Bopp in 1997. As 2021 comes to an end, we are finally being treated to a reasonably bright comet.
Comet Leonard C/2021 A1 (Leonard) predicted behavior
Animation of C/2021 A1 (Leonard) orbit. Image credit: Phoenix7777; HORIZONS System, JPL, NASA
On January 3, 2021, a comet was discovered by astronomer G. J. Leonard at the Mount Lemmon Observatory. The comet was given the designation C/2021 A1 (Leonard) and most today call it Comet Leonard after its discoverer. At the time of discovery, Comet Leonard was about 5 AU from the Sun. Once its orbit was determined, it was found that Comet Leonard has been on a sunward path for the last 35,000 years from roughly 3500 AU away (325.5 billion miles). After Comet Leonard reaches perihelion on January 3, 2022, it is expected that its orbit will become hyperbolic and as such will be ejected out of the solar system.
As of Early December 2021, Comet Leonard is found in the east before dawn. As it heads sunward, we will see the comet will pass through the constellations Boötes, Hercules, Serpens Caput, Ophiuchus and Sagittarius. It is estimated to reach 4th magnitude from December 8 through December 20. We lose the comet to the Sun’s glare around December 12, 2021, after which it will become visible low in the southwest after sunset around December 15, 2021. A few ideal dates to try and find Comet Leonard are December 12, 2021, and December 18, 2021. December 12 is when the comet will be closest to Earth and December 18 is when it will be nearest Venus. The last time I observed Comet Leonard, it was near the bright star Arcturus in Boötes on December 6, 2021. The comet still appeared faint, and I estimated a magnitude of 6.8 to 7.0. A few reports of the comet being seen naked eye have started being posted by observers in dark locations.
How bright a comet will become is notoriously difficult to predict. A few comets that famously enticed observers with great expectations but in the end failed to impress are Comet Kahoutek and Comet ISON. For St. Louis viewers, Comet Leonard will likely remain visible only through binoculars or telescopes. There is an off chance that forward scattering of sunlight by dust particles in the Comet Leonard’s dust tail, might enhance what we see when the comet is lower to the horizon.
How to find Comet Leonard
Maps showing where to find Comet Leonard through December 20, 2021. Maps created using Stellarium, EG. Comet image credit University of Hertfordshire Observatory.
For those interested in spotting Comet Leonard, you will need a few things. A good map of the sky is always good. To find the comet amongst the background stars, you will need a star chart with stars down to magnitude 7.5. A tool I often use is the free planetarium software Stellarium. This is freely available to all operation systems. Once you set Stellarium up for your viewing location, you will need to update its visible comet database. There are several tutorials on how to do this available online. Once this is done, use the search function to find C/2021 A1. Once it takes you to where the comet is found, zoom in until you get enough background stars to help identify the comet.
The other thing you will need to find Comet Leonard is binoculars. I have been able to find the comet in small binoculars, but it was a struggle. The smallest binocular I used was a 6×30 binocular. With these the comet swam in and out of view making it tough to see. Moving up in size to a 10×50 binocular, I had no problem spotting the comet but at times I needed to use averted vision. As things change, we will update this post with relevant information. As things are now, Comet Leonard will be a challenge to find. This will hopefully change as the comet continues to head sunward.
Resources for current information on Comet Leonard and other comets can be found at The Sky Live and Comet Chasing-Sky Hound.
James S. McDonnell Planetarium
Night Sky Update: December 8, 2021, Comet Leonard