46P Wirtanen
Closest to Sun   2018 Dec 12 at 1.05AU
Closest to Earth 2018 Dec 17 at 0.0776AU
Maximum Mag 3.5 in late Dec 2018 - A naked eye comet!
Period 5.4 years

Discovered in 1948, Periodic comet 46P Wirtanen will be having an exceptional apparition in 2018 as it passes a mere 11.6 million kms from Earth on 2018 December 17.
From dark, country sky locations, the comet will appear visible to the naked eye as a 3rd magnitude circular fuzzball with a coma diameter about 1.5 degrees.
Observers in light polluted areas will be significantly compromised due to the large, diffuse nature of the comet.
For comparison, the Small Magellanic Cloud located in the southern hemisphere has a total integrated magnitude of 2.8 and diameter of 3 degrees.
The Andromeda galaxy in the northern hemisphere has a total integrated magnitude of 4.3 and diameter of 3 degrees.
 
The ion tail will appear foreshortened as it points away from the Earth, so don't expect to see a lengthy tail, maybe a couple of degrees long.
It will be directed in a north-easterly direction during early December, swinging easterly mid December, through to southerly from late December.

There is a possibility of observing a dust tail or trail around December 14 when the Earth crosses the orbital plane of the comet and we see the enhanced dust tail edge-on.
(update Dec 17 - a faint antitail has been photographed in deep images)
 
Also of note is that the comet crosses the Earth's orbital plane at the same time. This may result in enhanced meteor activity from the Piscid meteor shower.
The comet will cross the orbital plane of the Earth around December 15.
This puts any potential meteors into a focal point. The radiant will be situated in or near Pisces.
However, the comet hasn't been in this orbit for long. An encounter with Jupiter in 1984 reduced the perihelion distance from 1.25AU to its current 1.05AU
(6 apparitions at this distance) so there may not be much dust spread out in its orbit.
The comet appears dust poor, the nucleus is small and dust production appears quite low.
Anyway, it's worth a look.
The encounter velocity would be very slow. Geminid meteor shower peaks at the same time, but should be distinguishable from the Piscids, which will
appear in the early evening.
(update Dec 17 - IMO 2019 meteor shower calendar has prediction according to model calculations by Mikhail Maslov,
the trail of meteoroids ejected from comet 46P/Wirtanen in 1974 passes the Earth at 0.00048 au distance on January 4, 18h26m UT

Light curve data from current and past apparitions can be found at Seiichi Yoshida's website:

http://www.aerith.net/comet/catalog/0046P/index.html

It has shown evidence of an outburst after its perihelion passage in 2002. Hopefully it has a surprise in store for us this time around.

Below is an ephemeris over the observing period at 00UT. (Naked eye visibility in bold)
RA = Right Ascension. Dec = Declination. r = distance to Sun (AU), delta = distance to Earth (AU),
mag = estimated comet magnitude, Elong = elongation from Sun, Phase = ion tail angle Con= Constellation

Predicted coma diameters
are based on Guide 9 software using an equation published at: www.projectpluto.com/update7b.htm#comet_tail
For comparison, my observed visual coma diameter on October 20 was 15', thus we can expect 1.5-2 degrees at closest approach.

P/Wirtanen (46P)
Date        RA        Dec         r      delta  mag  Coma Elong Phase Con
1  Sep 2018 01h32m07s -18 13' 16" 1.6681 0.7810 14.0   3' 137.0 24.4  Cetus
11 Sep 2018 01h41m10s -20 32' 22" 1.5831 0.6667 13.0   4' 141.3 23.4  Cetus
21 Sep 2018 01h48m26s -23 15' 35" 1.4993 0.5663 12.0   5' 143.9 23.2  Cetus
1  Oct 2018 01h53m42s -26 12' 58" 1.4178 0.4793 11.0   7' 144.1 24.4  Fornax
11 Oct 2018 01h56m51s -29 07' 30" 1.3398 0.4041 10.0   9' 141.9 27.4  Fornax
21 Oct 2018 01h58m23s -31 32' 50" 1.2668 0.3386 9.0   12' 137.7 31.9  Fornax
31 Oct 2018 01h59m37s -32 57' 35" 1.2009 0.2799 8.0   16' 132.9 37.3  Fornax
10 Nov 2018 02h02m38s -32 41' 06" 1.1445 0.2255 7.0   22' 128.5 42.6  Fornax
20 Nov 2018 02h11m03s -29 35' 26" 1.0999 0.1736 6.0   30' 126.3 46.3  Fornax
30 Nov 2018 02h30m35s -21 07' 35" 1.0698 0.1250 5.0   45' 129.3 45.5  Cetus
10 Dec 2018 03h11m59s -00 35' 48" 1.0561 0.0869 4.0   70' 143.7 33.6  Cetus
20 Dec 2018 04h35m35s +34 34' 39" 1.0597 0.0803 3.5   80' 160.3 18.3  Perseus
30 Dec 2018 06h39m44s +55 39' 47" 1.0804 0.1131 4.0   55' 147.5 29.3  Lynx
9  Jan 2019 08h15m29s +59 31' 18" 1.1167 0.1647 5.0   35' 141.3 33.4  Lynx
19 Jan 2019 09h01m53s +58 16' 00" 1.1665 0.2249 6.5   25' 140.7 32.3  UMajor

29 Jan 2019 09h21m26s +55 46' 28" 1.2271 0.2917 7.5   15' 141.7 29.8  UMajor
8  Feb 2019 09h29m29s +52 47' 40" 1.2962 0.3665 9.0   12' 142.4 27.6  UMajor
18 Feb 2019 09h33m47s +49 31' 46" 1.3714 0.4510 10.0   9' 141.9 26.4  UMajor
28 Feb 2019 09h37m48s +46 08' 25" 1.4511 0.5467 11.0   7' 139.7 26.2  UMajor
10 Mar 2019 09h42m53s +42 44' 34" 1.5336 0.6548 12.0   5' 136.1 26.7  UMajor
20 Mar 2019 09h49m32s +39 25' 26" 1.6180 0.7758 13.0   3' 131.6 27.4  LMinor


As can be seen from the above ephemeris, the comet will initially favour viewers in the southern hemisphere, then switching to the northern hemisphere post perihelion.
The comet's light curve is slightly asymmetric, suggesting a peak 12 days post perihelion.
Northerners will spot the comet from the beginning of December as it rises over the North Eastern horizon after sunset.
Southerners will lose the comet from late December as it dips below the North Western evening horizon.

October 2018
As October begins you will find the 11th magnitude comet situated in Fornax, rising in the East after sunset, and transiting overhead at around 1am.
It treks slowly southwards during the month, near the vicinity of Pi and Nu Fornacis.
Moonlight interferes with evening viewing from Oct 11 but the comet is observable all night (best in the morning sky until Oct 20)
From Oct 26, the comet is now conveniently seen in the evening sky and bordering on binocular visibility at 8th magnitude.
It reaches its greatest southerly declination of -33 on Nov 3 before it starts its trek northwards.

November 2018
As November begins, you will find the magnitude 8.0 comet situated in Fornax, in the East after sunset and transiting overhead at midnight.
It reaches its greatest southerly declination of -33 on Nov 3 before it starts its trek northwards, brightening by 0.1 magnitude each night.
Moonlight interferes with evening viewing from Nov 12 but the comet is observable all night (best in the morning sky until Nov 20)
The first chance of naked eye visibility at a dark site occurs on the evening of Nov 25, before moonrise.
The comet will be situated a couple of degrees to the south of magnitude 5.3 star Kappa Fornacis, and appear of similar brightness.
Note that the comets light will be spread out over a 30' coma, thus will appear fainter.
On the evening of Nov 27, the comet is 30' East of Kappa For before crossing into Cetus.
By the start of December, the comet has brightened to magnitude 5.0, with a 45' coma and short tail.

December 2018

December 1 - the comet in Cetus has brightened to magnitude 5.0, with a 45' coma.
December 10 - the comet near Alpha Ceti has brightened to magnitude 4.0, with a coma now > 1 degree across
December 13-14 Earth crosses the orbital plane of the comet. Look out for potential Piscid meteors! 
December 16 - comet is 3 degrees East of Pleiades star cluster. The cluster occupies a 1.5 degree circle and will make a good comparison for measuring coma diameter.
December 17 - comet closest to Earth peaks at magnitude 3.5, with a coma diameter >1.5 degrees across!
December 18 - last chance before moonlight interference.
December 23 - Comet will be situated a degree from Capella in Auriga,  and the show is virtually over for those south of latitude 35S.
The further north you are situated, the better.

The comet will reappear for southerners during February, low over the northern horizon. It will have then faded to magnitude 8.

Future of 46P

It will be 0.76 AU from Earth on 2029 October 10.
An approach to Jupiter in November 2042 will increase perihelion distance from 1.08 AU to 1.22 AU.
Then another approach to Jupiter in November 2054 will increase perihelion distance from 1.22 AU to 1.98 AU. Forget future visual observations!

46p Wirtanen on 2018 December 10.
The naked eye visual magnitude was 4.5 spread out over a 50' coma.

46p Wirtanen on 2018 December 4.
Through 8x40mm binoculars, the total visual magnitude was 5.1 spread out over a 45' coma. DC=4

46p Wirtanen on 2018 November 28.
Through 8x40mm binoculars, the total visual magnitude was 5.6 spread out over a 34' coma. DC=4

46p Wirtanen on 2018 November 14.
Through 8x40mm binoculars, the total visual magnitude was 6.7 spread out over a 27' coma. DC=3/4

46p Wirtanen on 2018 November 3.
Through 8x40mm binoculars, the total magnitude is 7.6 spread out over a 20' coma. DC=3/4

46p Wirtanen on 2018 October 28.
Through 25x100mm binoculars, the total magnitude is 8.6 spread out over a 15' coma. DC=3

 

46p Wirtanen on 2018 October 18.
Through 25x100mm binoculars, the total magnitude is 9.2 spread out over a 15' coma. DC=2.