FORECAST FOR FORWARD-SCATTERING BRIGHTNESS ENHANCEMENT
AND POSSIBLE DAYLIGHT VISIBILITY OF COMET C/2006 P1 (MCNAUGHT)
Joseph N. Marcus, 19 Arbor Rd., St. Louis, MO 63132, jnm4248@bjc.org and jnmarcus@sbcglobal.net
Comet C/2006 P1 (McNaught) will display a moderate, approximately 10-fold enhancement in
brightness in mid-January as it passes between the earth and the sun, owing to the forwardscattering of sunlight by its dust grains. The first table provides a forecast of the enhancement,
m1, in total visual magnitude as a function of the scattering angle,  (= 180 – phase angle), for
  60. The forecast is based upon my study of the five comets to date which have been
measured in forward-scattering geometry (C/1927 X1, C/1975 V1, C/1980 Y1, 96P, and C/2004
F4), and a novel Henyey-Greenstein function forecast model which I devised to fit the data
(Marcus 2007, submitted to ICQ). In the application below I use a forward-scattering asymmetry
parameter gf = 0.9 and I assume a coma dust-to-gas light ratio of 1 in the visible. The
enhancement is relative to the comet’s baseline brightness as it would be seen outside of the
forward-scattering lobe, taken as  = 90.
Predicted Forward-Scattering Brightness Enhancement of Comet C/2006 P1 (McNaught)
Date
(2007 UT)
Jan. 12.0
Jan. 12.5
Jan. 13.0
Jan. 13.5
Jan. 14.0
Jan. 14.5
Jan. 15.0
Jan. 15.5
Jan. 16.0
Jan. 16.5
Jan. 17.0
Jan. 17.5
Scattering Angle
()
58.2
49.4
41.4
35.1
31.6
31.5
34.3
38.8
44.1
49.5
54.9
59.9
Enhancement
(m1)
-0.7
-1.1
-1.6
-2.0
-2.3
-2.3
-2.1
-1.7
-1.4
-1.1
-0.9
-0.7
The comet reaches a minimum scattering angle min = 31.1 on Jan. 14.3 UT at an elongation  =
5.5 and position angle  = 89.6 from the sun. At that time, the predicted enhancement would
be –2.4 magnitudes.
Despite the fact that C/2006 P1 appears to be “new” in an orbital sense, its initial brightness
increase with decreasing log r has been steep at n  6, but closer to the sun it seems to have
calmed to n  4 in the range 0.2  log r  -0.2. From m1 brightness observations posted on the
ICQ and Astrosite Groningen websites, and from the recent m1 = 4.5: estimate on Dec. 26.65 UT
(at r = 0.606 AU) posted by P. Guzik, Poland, on the comets-m1 website – all of which I
corrected for magnification effects to a 10x standard (by an amount essentially equivalent to that
for “aperture effect” for reflectors: Morris, PASP 85:470, 1973) – I find a least-squares solution
of m0 = 4.9 and n = 4.7 in the brightness formula m1 = m0 + 5 log  + 2.5 n log r over 0.45  log
r  -0.21. An absolute magnitude near m0 = 5 appears to be secure from the observations.
However, the value of n as the comet proceeds to perihelion (q = 0.171 AU on Jan. 12.80 UT),
should be considered uncertain. The next Table gives m1 magnitude forecasts for m0 = 5 and n =
4 (liberal) and n = 3 (conservative), computed classically (ignoring forward-scattering
enhancement) and non-classically (with the forward-scattering term, m1, of the previous Table
added in). In addition to the scattering angle, , the Table also shows elongation, , and position
angle from the sun,  (measured counterclockwise from north), to aid in locating the comet in
daylight.
Magnitude Forecasts For Comet C/2006 P1(McNaught), With And Without Forward
Scattering Factored In, For m0 = 5 and Brightness Indices n = 3 and n = 4
Date
(2007 UT)
 ()
 ()
 ()
Jan. 12.0
Jan. 12.5
Jan. 13.0
Jan. 13.5
Jan. 14.0
Jan. 14.5
Jan. 15.0
Jan. 15.5
Jan. 16.0
Jan. 16.5
Jan. 17.0
Jan. 17.5
58.2
49.4
41.4
35.1
31.6
31.5
34.3
38.8
44.1
49.5
54.9
59.9
8.6
7.6
6.6
5.8
5.4
5.6
6.3
7.4
8.7
10.1
11.5
12.9
25.9
33.7
44.5
59.3
78.0
97.7
114.6
127.4
136.6
143.2
148.2
152.0
Magnitude Forecast,
Forward Scattering Ignored
n=3
n=4
-1.0
-2.9
-1.1
-3.0
-1.1
-3.0
-1.1
-3.0
-1.0
-2.9
-0.9
-2.8
-0.8
-2.6
-0.6
-2.4
-0.4
-2.1
-0.3
-1.9
-0.1
-1.6
+0.1
-1.4
Magnitude Forecast,
Forward Scattering Included
n=3
n=4
-1.7
-3.6
-2.2
-4.1
-2.7
-4.6
-3.1
-5.0
-3.3
-5.2
-3.3
-5.1
-2.9
-4.7
-2.4
-4.1
-1.8
-3.5
-1.4
-3.0
-0.9
-2.5
-0.5
-2.0
In the forecast with forward scattering and n = 4, the comet’s brightness would exceed that of
Venus over Jan. 13 and 14. The comet would be visible in daylight by binoculars and probably
to the shielded naked eye in very clear skies. But if n = 3, the comet may be only marginally
visible in daylight binoculars, if at all. If the comet can be located in daylight, visible/infrared
photometry would be possible if the telescope and detectors can be shielded from direct sunlight.
Between Jan. 12.3 UT and Jan. 15.7 UT, the comet will be at 8 deg or less elongation and visible
in the SOHO C3 coronograph field at brightness levels likely to greatly saturate the central pixels
unless precautions for shorter exposures are planned in advance.
At this time C/2006 P1 shares many similarities with C/1927 X1 (Skjellerup-Maristany). The
orbital eccentricities of each comet are nearly exactly 1, the perihelion distances are nearly
identical (C/1927 X1 q = 0.176 AU), the absolute magnitudes are similar (m0  5.5 in my
analysis of C/1927 X1), each were in extremely poor observing circumstances (each far behind
the sun at small elongations during most of their apparitions), and each entered forwardscattering geometry near the time of perihelion. In the case of C/1927 X1, on 1927 Dec. 16.8
UT, at a scatter angle near  = 30, the Slipher brothers at Lowell Observatory, Flagstaff, AZ,
recorded that “the experienced observer saw it readily during the day by merely extending the
hand to shadow the eyes from the sun, which was only about five degrees southwest of the
comet” (Slipher & Slipher, Pop. Astron. 36:300-301, 1928). The comet was still visible in
daylight by naked eye on the next day at   45.
UPDATED BRIGHTNESS FORECAST FOR COMET C/2006 P1 (MCNAUGHT) IN
DAYLIGHT WITH PREDICTED FORWARD-SCATTERING BRIGHTNESS BOOST
Joseph N. Marcus, 19 Arbor Rd., St. Louis, MO 63132, jnm4248@bjc.org and jnmarcus@sbcglobal.net
The updated forecast presented here is based on my analysis of pre-perihelion visual magnitude
observations reported on the ICQ, Astrosite Groningen, and comets-m1 websites. I have
upgraded the baseline forecast because Comet C/2006 P1 has been running slightly brighter than
expected in recent days.
Pre-perihelion Light Curve of C/2006 P1 (McNaught)
Analysis by Joseph N. Marcus, St. Louis, MO, USA
-5
-4
-3
Observations
Solution: m0 = 5.27, n = 4.29
-2
Heliocentric Magnitude
-1
0
1
2
3
4
5
6
7
8
9
10
11
12
0.5 0.4 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8
Log r
The graph plots the comet’s heliocentric magnitude H1 = m1  5 log , against the logarithm of
the comet’s heliocentric distance, r, in AU. m1 is the visual magnitude and  is the comet’s earth
distance in AU. The observations are plotted as circles. Filled circles are observations which
have been corrected for atmospheric extinction. Estimates made at log r  0.4 have been
corrected for magnification artifact to 10x by a method which delivers results similar to Morris’
aperture correction for reflectors (PASP 85:470, 1973). For this analysis, in order to obtain
baseline brightness behavior, I have cut off observations beyond 2007 Jan. 10.7 UT,
corresponding to scattering angles θ 80, in order to eliminate from the baseline any small
forward-scattering effects which should be beginning after then. Least-squares analysis over
0.448  log r  0.717 pre-perihelion yields an absolute magnitude m0 = 5.27  0.09 and a power
index n = 4.29  0.06 in the brightness formula m1 = m0 + 5 log  + 2.5 n log r. To generate a
forecast for the next several days when the comet is in strong forward-scattering geometry, I do
not use these parameters, because the brightness observations show a slight departure from
linearity. Instead, I take the most current points, clustered at an average log r = 0.69 and
average H1 = 2.4 (see graph), and assume n = 4. This leads to an absolute magnitude m0 = 4.5
in the foregoing brightness formula. It gives a baseline forecast 0.5 magnitude brighter than the
m0 = 5, n = 4 forecast used in my earlier communication (which was posted at Astrosite
Groningen). On the other hand, if, after perihelion (Jan 12.80 TT), the brightness of C/2006 P1
falls off at an index of n = 3 instead of n = 4, then the corresponding post-perihelion absolute
magnitude would become m0 = 2.6. The Table below gives magnitude forecasts with these two
parameter sets, with and without forward-scattering enhancement (see first communication)
factored in. As in the first communication, the scattering angle, elongation, and position angle of
the comet from the sun are provided:
Magnitude Forecasts For Comet C/2006 P1(McNaught), With And Without Forward
Scattering Factored In, For Post-Perihelion Parameters (m0 = 4.5, n = 4) or (m0 =2.6, n = 3)
Date
(2007 UT)
 ()
 ()
 ()
Jan. 12.0
Jan. 12.5
Jan. 13.0
Jan. 13.5
Jan. 14.0
Jan. 14.5
Jan. 15.0
Jan. 15.5
Jan. 16.0
Jan. 16.5
Jan. 17.0
Jan. 17.5
58.2
49.4
41.4
35.1
31.6
31.5
34.3
38.8
44.1
49.5
54.9
59.9
8.6
7.6
6.6
5.8
5.4
5.6
6.3
7.4
8.7
10.1
11.5
12.9
25.9
33.7
44.5
59.3
78.0
97.7
114.6
127.4
136.6
143.2
148.2
152.0
Magnitude Forecast,
Forward Scattering Ignored
(2.6, 3)
(4.5, 4)
-3.4
-3.4
-3.5
-3.5
-3.5
-3.5
-3.5
-3.5
-3.4
-3.4
-3.3
-3.3
-3.2
-3.1
-3.0
-2.9
-2.8
-2.6
-2.7
-2.4
-2.5
-2.1
-2.3
-1.9
Magnitude Forecast,
Forward Scattering Included
(2.6, 3)
(4.5, 4)
-4.1
-4.1
-4.6
-4.6
-5.1
-5.1
-5.5
-5.5
-5.7
-5.7
-5.7
-5.6
-5.3
-5.2
-4.8
-4.6
-4.2
-4.0
-3.8
-3.5
-3.3
-3.0
-2.9
-2.5
As we can see, there is very little difference in the magnitudes between the two parameter sets.
However, if the comet behaves as n = 3, then the comet at greater r in later January will be
considerably brighter in southern hemisphere skies than with n = 4 behavior. With forwardscattering enhancement, the comet’s magnitude could reach magnitude 5.7, which would be
over five times the brightness of Venus. In such a circumstance, the comet could be visible in
broad daylight off the limb of the sun to the shielded naked eye.