3.12.2021

It is unusual to see a comet at its best: Most comets are brightest nearest the sun, just when they're most difficult to spot against the sun's glare or hidden below the horizon.  

Comet C/2021 A1 (Leonard) is, alas, no exception. But it should be visible in small telescopes and binoculars and quite possibly even dimly to the unaided eye in the Northern Hemisphere's pre-dawn sky as it increases in brightness for at part of December, and later in the month perhaps even briefly in the evening sky, just after sunset.

Astronomer Gregory J. Leonard, a senior research specialist, discovered the comet that now bears his name on Jan. 3 at the Mount Lemmon Infrared Observatory, located in the Santa Catalina Mountains, approximately 17 miles northeast of Tucson, Arizona. 

Since its discovery, the comet has been approaching both the sun and Earth but has been a very dim object. Most comets remain too faint for amateur telescopes to catch, but for a short while in December, Comet Leonard should rise out of the ordinary.  

The icy ball has come a long way to make this pre-Christmas appearance to us. About 35,000 years ago, the comet was at the far end of its elongated elliptical orbit — aphelion — some 3,500 astronomical units from the sun. One astronomical unit (AU) is the equivalent of the Earth's average distance from the sun: 92,955,807 miles (149,565,894 kilometers).  

So, 35 millennia ago, Comet Leonard resided some 325 billion miles (525 billion km) from the sun, enveloped in an almost unimaginably cold environment, hovering just a fraction of a degree above absolute zero, the temperature at which all molecular motion stops. Now, Comet Leonard is in the home stretch of what likely will be its very last visit to the sun, and its conglomeration of icy gases like methane, ammonia and water vapor is reacting to the increasing warmth of the sun.

Comet Leonard's brightness should noticeably ramp up as it approaches the Earth. In the early days of December, observers who are up around the break of dawn (roughly 5:30 a.m. local time), should concentrate on the eastern sky about halfway up from the horizon to the point directly overhead. Now, with binoculars, scan that part of the sky roughly midway between the 3rd-magnitude star Cor Caroli in the constellation Canes Venatici (the Hunting Dogs) and the 3rd-magnitude star Muphrid in the extended foot of Boötes the Herdsman.  

There, you should run across a circular, fuzzy, 6th-magnitude glow perhaps sporting a tail. That will be Comet Leonard.  

For most of us, the second week of December is when Comet Leonard should be most interesting, offering the best compromise between increasing brightness and decreasing altitude at the start of dawn. Moreover, the sky will be free of moonlight, as it will be shortly after the new moon of Dec. 4.  

On the morning of Dec. 6, about two hours before sunrise, look toward the eastern sky. You will immediately notice the brilliant orange-yellow star, Arcturus in the constellation of Boötes the Herdsman. Now, with binoculars, scan that part of the sky about 5 degrees to the left of Arcturus and you should see Comet Leonard. The comet's dust tail, which started to lengthen noticeably during early November, should be pointing almost straight up. 

The comet's magnitude is forecast to be +5.5; bright enough to be glimpsed without any optical aid in a dark sky free of light pollution. The comet will then be 31.9 million miles (51 million km) from Earth and approaching us by about 1.5 million miles (2.4 million km) per day, so it will be brightening noticeably, but it will also drop rapidly lower in the eastern sky with each passing day. 

By the morning of Dec. 12, Comet Leonard will have brightened to magnitude +4.3, but will be only 10 degrees above the eastern horizon at the start of morning twilight. (Your clenched fist at arm's length measures roughly 10 degrees.) According to NASA's Jet Propulsion Laboratory in California, the comet will be making its closest approach to Earth that same day, reaching just 21,687,279 miles (34,902,292 km) away at 10:54 a.m. EST (1354 GMT). 

That day will mark the end of its morning visibility, but Comet Leonard will return for several "cameo appearances" in the evening sky. On Dec. 17, it will be sliding 5° below dazzling Venus. With binoculars, look for it very low toward the southwest horizon about an hour after sunset. 

Forward scatter might improve visibility 

Some skywatchers are hoping that the comet will over-perform in terms of brightness expectations, due to a phenomenon known as "forward scatter." In such a situation, a back-lit comet can appear significantly brighter because the dust and ice crystals emanating from the comet's nucleus scatter light toward the observer.  

During the middle part of December, Comet Leonard will be at a very favorable phase angle relative to the sun and Earth, suggesting that it might get 1 to 2 magnitudes brighter than what current predictions suggest; put another way, the comet might exceed brightness estimates by 2.5 to 6.3 times. Comets that have benefited from forward scattering include Skjellerup-Maristany in 1927, West in 1976, Bradfield in 1980 and McNaught in 2007. All four comets surged in brightness and became much brighter than anticipated. 

Not all comets react favorably to forward-scattering, however. Will Comet Leonard benefit? We can only watch and see what happens.  

The comet will likely be lost to view after Christmas, reaching its closest point to the sun on Jan. 3 at a distance of 57.2 million miles (92 million km). Once it rounds the sun it will be thrown out of the solar system into a slightly hyperbolic orbit, never to be seen again.

Quelle: SC

----

Update: 4.12.2021

.

Leonard is the brightest comet all year. Here's how to see it

Less than a year ago, when Comet Leonard was first discovered, it was still an incredibly dim and obscure chunk of rock traveling out near the orbit of Jupiter.

Now it has reached our neighborhood of the solar system on its journey toward the sun and is being billed as the brightest comet of the year.

Here's how to see it:

"The comet is in the early morning sky right at the moment, and that means getting up very early, probably around 5 a.m. or so and looking more or less to the northeast," Ed Krupp, an astronomer and the director of the Griffith Observatory in Los Angeles, tells NPR.

For people in North America, the best time will probably be Monday morning — weather permitting — when the comet will be near Arcturus, low on the horizon. The star is in the constellation Boötes (the Herdsman). There's an easy way to find it: Follow the curve of the Big Dipper (in Ursa Major) out past the end of the handle. The next bright star you see will be Arcturus. A good memory aid is to remember that from the Dipper handle, you "arc to Arcturus."

"The comet will just be about half the width of a clenched fist to the left" of Arcturus, Krupp says. "You might spot it with the unaided eye, but more likely, you're going to need binoculars [or] a telescope."

Exactly how bright a comet will appear to observers on Earth can be difficult to determine, and often a matter of intense speculation among amateur and professional astronomers alike. Leonard is no different.

But for people who saw the Neowise comet in the summer of 2020, Leonard might be something of a disappointment, says Peter Veres, an astronomer at the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics.

"I wouldn't say this comet will be spectacular if you compare it to Comet Neowise," Veres tells NPR.

He says Leonard might be visible with the naked eye, but "you will need to be in a dark environment, far from the city."

If you miss it in the morning, you'll get another chance in the evening later in the month, Krupp says.

"The optimum time [in the evening] probably is from the Dec. 17 on," he says.

This time, look for the planet Venus to the southwest. The planet is the brightest object in the sky after the sun and moon. The comet will be between Venus and the horizon.

After that, things get a bit more challenging for Northern Hemisphere observers. They will need to book a flight to somewhere in the Southern Hemisphere next month. Australia anyone?

But after Leonard leaves the southern skies behind, you'll be completely out of luck. With an orbital period of 80,000 years, the last people on Earth to have witnessed the comet were our distant ancestors — both Neanderthal and Homo sapiens.

And, we'll be the last humans to ever glimpse it from this vantage point. Because of the shape of Leonard's orbit, the comet is set to be kicked out of the solar system sometime after its Earthly flyby.

Quelle: npr