WAITING FOR THE SHADOW   SOLAR AND LUNAR ECLIPSE & ASTRONOMICAL OBSERVING - Joe Cali

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Comet (46P) Wirtanen 2018 Apparition
Joseph Cali




In the final months of 2018, Earth will experience what is probably the best apparition of periodic comet (46P) Wirtanen for many many decades to come.  This Jupiter group comet has a period of 5.43 years. The orbit swings very close to the Earths orbit each orbit, however the comet often comes to perihelion  when the  Earth is not in that part of the orbit. The comet's close approach is to that part of the Earth's orbit that the Earth occupies in November-December.  If perihelion occurs at other parts of the year, the comet is quite distant from Earth and therefore faint but this year, perigee occurs just 11.1 million km from the Earth, the closest  approach for many decades.



Visibility of this apparition is pretty equally favourable between north and south hemispheres.  During October and November, the southern hemisphere gets prime view,  at perigee in mid December, the comet is visible to both hemispheres. Mid-southern latitudes will lose sight of the comet around Christmas eve.



Telescope owners may find it's worth observing the  brighterning of the comet from October onwards when it will be an easy telescopic, and faint binocular comet.  In mid-December, expect to possibly see a 3.5 magnitude fuzzy ball,  1-2 degrees diameter, not much of an ion tail (it's pointing away from us) passing between the Hyadies and the Pleiades before disappearing over the north horizon. I have prepared the illustration above to show what the comet might look like on December 17.  Unfortunately there is a gibbous Moon to interfere with observations. 









Want to understand and visualise the orbit and the close passage of Wirtanen to Earth? 

Click here to download a 12MB movie of the orbit I've generated by the NASA Horizons orbit viewer.

Click here to view the 3D orbit viewer on "The sky live."


Finder Charts

These finder charts have been overlaid on Toshimi Taki's excellent widefield sky atlas.
Comet ephemerides  below and used for path plots courtesy NASA Horizons.


Path of comet 46P Wirtanen -  October 1st - December 16th
Click on the map to link to a higher resolution image suitable for printing.



Path of comet 46P Wirtanen - December 2nd - December 24th
Click on the map to link to a higher resolution image suitable for printing.




I have generated the ephemeris below from the NASA Horizons orbital calculator.

The magnitudes in the ephemeris are calculated from standard IAU definitions : -


 T-mag N-mag
   Comet's approximate apparent visual total magnitude ("T-mag") and nuclear
magnitude ("N-mag") by following standard IAU definitions:
    T-mag =  M1 + 5*log10(delta) + k1*log10(r)
    N-mag =  M2 + 5*log10(delta) + k2*log10(r) + phcof*beta
   Units: MAGNITUDES

In the finder charts, I have used magnitude estimates from Seiichi Yoshida's website. 
Seiichi uses different formulae based on previous observations of the comet to predict
visual magnitudes. http://www.aerith.net/comet/catalog/0046P/2018.html

Comet 46P Wirtanen Ephemeris
October 1 - December 30, 2018.

*******************************************************************************
Ephemeris / WWW_USER Fri Sep 28 14:13:26 2018 Pasadena, USA      / Horizons   
*******************************************************************************
Target body name: 46P/Wirtanen                    {source: JPL#K182/7}
Center body name: Earth (399)                     {source: DE431}
Center-site name: Mount Stromlo
*******************************************************************************
Start time      : A.D. 2018-Oct-01 00:00:00.0000 UT     
Stop  time      : A.D. 2018-Dec-30 00:00:00.0000 UT     
Step-size       : 2880 minutes
*******************************************************************************
Target pole/equ : No model available
Target radii    : 0.6 km                                                      
Center geodetic : 149.007700,-35.320485,0.7379299 {E-lon(deg),Lat(deg),Alt(km)}
Center cylindric: 149.007700,5210.55368,-3667.362 {E-lon(deg),Dxy(km),Dz(km)}
Center pole/equ : High-precision EOP model        {East-longitude positive}
Center radii    : 6378.1 x 6378.1 x 6356.8 km     {Equator, meridian, pole}   
Target primary  : Sun
Vis. interferer : MOON (R_eq= 1737.400) km        {source: DE431}
Rel. light bend : Sun, EARTH                      {source: DE431}
Rel. lght bnd GM: 1.3271E+11, 3.9860E+05 km^3/s^2                             
Small-body perts: Yes                             {source: SB431-N16}
Atmos refraction: NO (AIRLESS)
RA format       : HMS
Time format     : CAL
EOP file        : eop.180927.p181219                                          
EOP coverage    : DATA-BASED 1962-JAN-20 TO 2018-SEP-27. PREDICTS-> 2018-DEC-18
Units conversion: 1 au= 149597870.700 km, c= 299792.458 km/s, 1 day= 86400.0 s
Table cut-offs 1: Elevation (-90.0deg=NO ),Airmass (>38.000=NO), Daylight (NO )
Table cut-offs 2: Solar elongation (  0.0,180.0=NO ),Local Hour Angle( 0.0=NO )
Table cut-offs 3: RA/DEC angular rate (     0.0=NO )                          
*******************************************************************************
Initial IAU76/J2000 heliocentric ecliptic osculating elements (au, days, deg.):
  EPOCH=  2458340.5 ! 2018-Aug-10.0000000 (TDB)    RMSW= n.a.                 
   EC= .6585259788502387   QR= 1.055358505144482   TP= 2458465.4292987557     
   OM= 82.16371401356682   W= 356.3305624789888    IN= 11.74555778842221      
  Equivalent ICRF heliocentric equatorial cartesian coordinates (au, au/d):
   X= 1.741904313395499E+00  Y=-3.294591735595458E-01  Z=-5.500008120018625E-01
  VX=-4.566880361446887E-03 VY= 1.245567671525117E-02 VZ= 6.862923545190293E-03
Comet physical (GM= km^3/s^2; RAD= km):                                       
   GM= n.a.                RAD= .600                                          
   M1=  14.6     M2=  16.9     k1=  12.    k2=  5.      PHCOF=  .030          
**************************************************************************************************************
 Date__(UT)__HR:MN     R.A._(ICRF/J2000.0)_DEC  T-mag  N-mag            delta      deldot    S-O-T /r    S-T-O
**************************************************************************************************************
$$SOE
 2018-Oct-01 00:00 *   01 53 44.22 -26 12 54.1  14.82  16.79 0.47923355883090 -13.7373841 144.1364 /L  24.4417
 2018-Oct-03 00:00 *m  01 54 32.08 -26 48 45.1  14.69  16.71 0.46329963580975 -13.3396175 143.8742 /L  24.8899
 2018-Oct-05 00:00 *m  01 55 14.77 -27 24 20.5  14.56  16.63 0.44782799111086 -12.9538105 143.5122 /L  25.4104
 2018-Oct-07 00:00 *m  01 55 52.42 -27 59 28.0  14.42  16.55 0.43280439473928 -12.5813365 143.0546 /L  26.0025
 2018-Oct-09 00:00 *m  01 56 25.27 -28 33 54.3  14.29  16.47 0.41821260377115 -12.2242425 142.5068 /L  26.6644
 2018-Oct-11 00:00 *m  01 56 53.67 -29 07 25.4  14.16  16.39 0.40403378467662 -11.8848215 141.8755 /L  27.3934
 2018-Oct-13 00:00 *m  01 57 18.08 -29 39 47.0  14.02  16.31 0.39024660223444 -11.5649744 141.1681 /L  28.1859
 2018-Oct-15 00:00 *m  01 57 39.02 -30 10 44.5  13.89  16.24 0.37682789661917 -11.2659006 140.3929 /L  29.0377
 2018-Oct-17 00:00 *   01 57 57.06 -30 40 03.5  13.75  16.16 0.36375345855479 -10.9882027 139.5589 /L  29.9436
 2018-Oct-19 00:00 *   01 58 12.84 -31 07 29.1  13.62  16.09 0.35099859440639 -10.7320906 138.6753 /L  30.8984
 2018-Oct-21 00:00 *   01 58 27.04 -31 32 46.0  13.48  16.02 0.33853851599908 -10.4974648 137.7516 /L  31.8959
 2018-Oct-23 00:00 *   01 58 40.40 -31 55 38.9  13.34  15.95 0.32634871910135 -10.2838616 136.7975 /L  32.9300
 2018-Oct-25 00:00 *   01 58 53.71 -32 15 51.6  13.21  15.87 0.31440546603230 -10.0903495 135.8227 /L  33.9939
 2018-Oct-27 00:00 *   01 59 07.80 -32 33 07.2  13.07  15.80 0.30268634445438  -9.9155288 134.8371 /T  35.0806
 2018-Oct-29 00:00 *   01 59 23.58 -32 47 07.4  12.93  15.73 0.29117070179190  -9.7577541 133.8505 /T  36.1826
 2018-Oct-31 00:00 *m  01 59 42.07 -32 57 32.1  12.79  15.65 0.27983972585223  -9.6154876 132.8734 /T  37.2920
 2018-Nov-02 00:00 *m  02 00 04.45 -33 03 58.6  12.65  15.57 0.26867615988783  -9.4875382 131.9163 /T  38.3999
 2018-Nov-04 00:00 *m  02 00 32.11 -33 06 02.1  12.50  15.49 0.25766387802669  -9.3730603 130.9906 /T  39.4966
 2018-Nov-06 00:00 *m  02 01 06.63 -33 03 14.9  12.36  15.41 0.24678758655801  -9.2713448 130.1081 /T  40.5718
 2018-Nov-08 00:00 *m  02 01 49.83 -32 55 06.7  12.22  15.33 0.23603286443410  -9.1814503 129.2817 /T  41.6138
 2018-Nov-10 00:00 *m  02 02 43.66 -32 41 03.5  12.07  15.24 0.22538666881142  -9.1018018 128.5248 /T  42.6100
 2018-Nov-12 00:00 *m  02 03 50.22 -32 20 27.2  11.92  15.14 0.21483821591083  -9.0299746 127.8524 /T  43.5461
 2018-Nov-14 00:00 *m  02 05 11.76 -31 52 33.1  11.77  15.04 0.20437996324761  -8.9627199 127.2807 /T  44.4063
 2018-Nov-16 00:00 *   02 06 50.67 -31 16 28.6  11.61  14.93 0.19400850211831  -8.8960560 126.8281 /T  45.1720
 2018-Nov-18 00:00 *   02 08 49.53 -30 31 10.2  11.45  14.82 0.18372539851046  -8.8252424 126.5158 /T  45.8219
 2018-Nov-20 00:00 *   02 11 11.17 -29 35 20.5  11.29  14.69 0.17353816615420  -8.7445705 126.3683 /T  46.3308
 2018-Nov-22 00:00 *   02 13 58.70 -28 27 24.1  11.13  14.56 0.16346157875351  -8.6469912 126.4147 /T  46.6686
 2018-Nov-24 00:00 *   02 17 15.58 -27 05 22.8  10.96  14.41 0.15351945368316  -8.5236917 126.6897 /T  46.7994
 2018-Nov-26 00:00 *   02 21 05.78 -25 26 48.9  10.79  14.26 0.14374686359679  -8.3637898 127.2350 /T  46.6798
 2018-Nov-28 00:00 *   02 25 33.95 -23 28 38.6  10.61  14.08 0.13419262602755  -8.1541005 128.1010 /T  46.2572
 2018-Nov-30 00:00 *m  02 30 45.64 -21 07 05.6  10.43  13.89 0.12492219488655  -7.8785807 129.3481 /T  45.4688
 2018-Dec-02 00:00 *m  02 36 47.54 -18 17 37.5  10.25  13.69 0.11602148682695  -7.5172656 131.0466 /T  44.2415
 2018-Dec-04 00:00 *m  02 43 47.54 -14 54 58.0  10.07  13.47 0.10760202618574  -7.0451449 133.2739 /T  42.4944
 2018-Dec-06 00:00 *m  02 51 54.93 -10 53 24.1   9.90  13.23 0.09980702470592  -6.4318204 136.1054 /T  40.1475
 2018-Dec-08 00:00 *m  03 01 20.30 -06 07 29.7   9.73  12.97 0.09281683269212  -5.6432183 139.5954 /T  37.1408
 2018-Dec-10 00:00 *m  03 12 15.50 -00 33 35.0   9.58  12.72 0.08685033302931  -4.6473572 143.7369 /T  33.4726
 2018-Dec-12 00:00 *m  03 24 53.23 +05 47 51.4   9.45  12.47 0.08215637104554  -3.4262305 148.3933 /T  29.2662
 2018-Dec-14 00:00 *   03 39 26.20 +12 49 20.5   9.37  12.25 0.07898821728409  -1.9929576 153.1949 /T  24.8688
 2018-Dec-16 00:00 *   03 56 05.60 +20 14 32.6   9.33  12.10 0.07755885732546  -0.4060657 157.4267 /T  20.9559
 2018-Dec-18 00:00 *   04 14 58.66 +27 39 39.5   9.35  12.04 0.07798853919281   1.2336292 160.0625 /T  18.4952
 2018-Dec-20 00:00 *   04 36 05.15 +34 38 43.2   9.42  12.10 0.08027018794632   2.8106108 160.3314 /T  18.2070
 2018-Dec-22 00:00 *   04 59 13.51 +40 50 23.3   9.54  12.25 0.08427438649982   4.2320639 158.5335 /T  19.8033
 2018-Dec-24 00:00 *   05 23 57.93 +46 02 19.6   9.70  12.47 0.08979045197371   5.4500848 155.6958 /T  22.3181
 2018-Dec-26 00:00 *   05 49 38.32 +50 11 32.2   9.88  12.72 0.09657865503110   6.4594726 152.6642 /T  24.9616
 2018-Dec-28 00:00 *m  06 15 24.78 +53 21 57.4  10.07  12.97 0.10441047529878   7.2816809 149.8739 /T  27.3329
 2018-Dec-30 00:00 *m  06 40 25.75 +55 41 24.4  10.27  13.21 0.11308945190670   7.9487627 147.4895 /L  29.2867
$$EOE
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Column meaning:

 
TIME

  Prior to 1962, times are UT1. Dates thereafter are UTC. Any 'b' symbol in
the 1st-column denotes a B.C. date. First-column blank (" ") denotes an A.D.
date. Calendar dates prior to 1582-Oct-15 are in the Julian calendar system.
Later calendar dates are in the Gregorian system.

  Time tags refer to the same instant throughout the solar system, regardless
of where the observer is located. For example, if an observation from the
surface of another body has an output time-tag of 12:31:00 UTC, an Earth-based
time-scale, it refers to the instant on that body simultaneous to 12:31:00 UTC
on Earth.

  The Barycentric Dynamical Time scale (TDB) is used internally as defined by
the planetary equations of motion. Conversion between TDB and the selected
non-uniform UT output time-scale has not been determined for UTC times after
the next July or January 1st. The last known leap-second is used as a constant
over future intervals.

  NOTE: "n.a." in output means quantity "not available" at the print-time.
 
SOLAR PRESENCE (OBSERVING SITE)
  Time tag is followed by a blank, then a solar-presence symbol:

        '*'  Daylight (refracted solar upper-limb on or above apparent horizon)
        'C'  Civil twilight/dawn
        'N'  Nautical twilight/dawn
        'A'  Astronomical twilight/dawn
        ' '  Night OR geocentric ephemeris

LUNAR PRESENCE (OBSERVING SITE)
  The solar-presence symbol is immediately followed by a lunar-presence symbol:

        'm'  Refracted upper-limb of Moon on or above apparent horizon
        ' '  Refracted upper-limb of Moon below apparent horizon OR geocentric
             ephemeris
 
 R.A._(ICRF/J2000.0)_DEC =
   Astrometric right ascension and declination of the TARGET CENTER with respec
to the observing site in the coordinates of the ICRF/J2000 inertial reference
frame. Compensated for down-leg light-time. Units: RA in hours-minutes-seconds
of time (HH MM SS.ff), DEC in angular degrees-minutes-seconds (DD MM SS.f)
 
 T-mag N-mag =
   Comet's approximate apparent visual total magnitude ("T-mag") and nuclear
magnitude ("N-mag") by following standard IAU definitions:
    T-mag =  M1 + 5*log10(delta) + k1*log10(r)
    N-mag =  M2 + 5*log10(delta) + k2*log10(r) + phcof*beta
   Units: MAGNITUDES
 
 delta  deldot =
   Range ("delta") and range-rate ("delta-dot") of target center with respect
to the observer at the instant light seen by the observer at print-time would
have left the target center (print-time minus down-leg light-time); the
distance traveled by a light ray emanating from the center of the target and
recorded by the observer at print-time. "deldot" is a projection of the
velocity vector along this ray, the light-time-corrected line-of-sight from the
coordinate center, and indicates relative motion. A positive "deldot" means the
target center is moving away from the observer (coordinate center). A negative
"deldot" means the target center is moving toward the observer.
Units: AU and KM/S
 
 S-O-T /r =
    Sun-Observer-Target angle; target's apparent SOLAR ELONGATION seen from
the observer location at print-time. Angular units: DEGREES

    The '/r' column indicates the target's apparent position relative to
the Sun in the observer's sky, as described below:

    For an observing location on the surface of a rotating body
(considering its rotational sense):

    /T indicates target TRAILS Sun (evening sky; rises and sets AFTER Sun)
    /L indicates target LEADS Sun  (morning sky; rises and sets BEFORE Sun)

For an observing point NOT on a rotating body (such as a spacecraft), the
"leading" and "trailing" condition is defined by the observer's
heliocentric orbital motion: if continuing in the observer's current
direction of heliocentric motion would encounter the target's apparent
longitude first, followed by the Sun's, the target LEADS the Sun as seen by
the observer. If the Sun's apparent longitude would be encountered first,
followed by the target's, the target TRAILS the Sun.

NOTE: The S-O-T solar elongation angle is numerically the minimum
separation angle of the Sun and target in the sky in any direction. It
does NOT indicate the amount of separation in the leading or trailing
directions, which are defined in the equator of a spherical coordinate
system.
 
 S-T-O =
   "S-T-O" is the Sun->Target->Observer angle; the interior vertex angle at
target center formed by a vector to the apparent center of the Sun at
reflection time on the target and the apparent vector to the observer at
print-time. Slightly different from true PHASE ANGLE (requestable separately)
at the few arcsecond level in that it includes stellar aberration on the
down-leg from target to observer.  Units: DEGREES


 Computations by ...
     Solar System Dynamics Group, Horizons On-Line Ephemeris System
     4800 Oak Grove Drive, Jet Propulsion Laboratory
     Pasadena, CA  91109   USA
     Information: http://ssd.jpl.nasa.gov/
     Connect    : telnet://ssd.jpl.nasa.gov:6775  (via browser)
                  telnet ssd.jpl.nasa.gov 6775    (via command-line)
     Author     : Jon.D.Giorgini@jpl.nasa.gov

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