Look Out For Naked Eye Comet Neowise - But It's No Harbinger Of Doom

Comet Neowise seen soon after sunset .

A week ago, the buzz started: stargazers, amateur astronomers and comet-hunters have been going crazy about Comet NEOWISE, the brightest comet in seven years.   It is interesting  that now - in the midst of a global pandemic - a comet makes an appearance, and a naked eye -visible comet at that.   

From time immemorial comets have been regarded as portents of doom, or some limited natural catastrophe.  That only began to change when astronomers developed the sophisticated instruments and techniques which revealed them to be just other members of the solar system - but which traveled in much more elliptical orbits then the planets.  But I have no worries this minor comet will incite any hysteria.  It would need to be roughly 3 apparent magnitudes brighter, e.g.

http://brane-space.blogspot.com/2011/07/tackling-simple-astronomy-problems-3.html

And some 5 arc degrees longer. See:  Measuring Angular Distances In Astronomy

What then is a comet? It is basically an immense  "dirty snowball"  as long time science writer Isaac Asimov once described  it.  The ices (snowball part) are of water, ammonia and methane - which are embedded within rocky material . As a comet nears the Sun the icy material evaporates, forming the coma: a cloud of mostly gas and some admixture of dust.  As the ice evaporates the radiation pressure of the solar wind forces the comet to form a 'tail' directed away from the Sun.    Spectroscopy shows the tail comprised of two parts: a dust tail and an ion tail. The former shows a spectrum of sunlight reflected from dust expelled out of the coma.  The latter displays a spectrum with emission lines  - indicating it is an ionized plasma.  The ion tail's spectrum shows gases such as carbon monoxide, carbon dioxide and molecular nitrogen (N2).

The size of comets varies considerably, some with diameters of 180,000 km to ass small as 15,000 km across. The larger the diameter the greater the chance for a remarkable, bright comet.  The length of the tail varies tremendously as well, the longest ever recorded being the comet of 1843  - with a tail 200 million miles long, or more than twice the distance of the Earth from the Sun.

The latest visible comet appeared a few weeks ago in the pre-sunrise eastern sky. Now, however, it has made the  transition to becoming an easy-to-see naked eye object in the far more convenient post-sunset northwestern night sky. 

Officially designated C/2020 F3, Comet NEOWISE (named for the NASA infrared space telescope that discovered it on March 27, 2020) has been falling toward the Sun for more than 3,000 years, and on July 3rd  passed just 27.4 million miles from the Sun—inside the orbit of Mercury.

It’s now fading slightly as it exits the Solar System, though since it’s getting slightly closer to Earth, it ought to remain relatively bright for a while yet. Comet NEOWISE actually gets closest to Earth on July 23, 2020, when it will be 64 million miles distant.

According to Sky & Telescope magazine, you should start looking about 1 hour after sunset, when you’ll find it just over the northwest horizon as the last of twilight fades into darkness. “Look about three fists below the “bowl” of the Big Dipper, which is hanging by its handle high above, and perhaps a little to right,” said the magazine. 

It advises you locate it by first noting the two stars at the bottom of the Big Dipper’s bowl. Then draw an imaginary line through them and toward lower left to a point in the sky a little more than one fist away.  (See again the above link to 'measuring angular distances in astronomy').

Bob King has written an excellent Comet NEOWISE spotter’s guide on Sky & Telescope, which I urge all interested readers  to check it out. I plan to have my binoculars ready just after sundown tonight. (Barring any T-storms!)