On Sunday, December 9, 2018 at 2:26:42 PM UTC-8, Thomas 'PointedEars' Lahn wrote:
StarDust wrote:
^^^^^^^^
Please post using your real name(, too).

How big is the comet in arc minutes?

Good (and free) astronomy software like Stellarium[1][2] can give you
the numbers (be sure to use the latest version, currently 0.18.2).

For now:

Distance (D) (from Terra) ≈ 0.087 AU ≈ 1.3 × 10⁷ km
(and closing; perigee on 2018-12-16)
Radius of coma (estimated) ≈ 36'000 km (0°09'30")
Tail length (estimated) ≈ 0.00812 × 10⁶ km (0°02'08")
Core diameter (d) = 10.0 km

If you know the diameter d and the distance D in SI units (or any other
non-angular measure), you can calculate the diameter of a celestial object
in degrees (of arc) using the equation

φ = d/D/(2π) × 360°

because sin(φ) ≈ φ for small angles φ (otherwise calculate
φ = arcsin(r/D)/π × 360°):

φ/2 _______ __
\ _.-': |PE
_.-\ .: r
-:'--------:--- d = φ
`-._ :
`-._:______
:-- D ---:

This gives you the diameter in degrees (of arc), so for minutes of arc you
must multiply by 60.

For example, you can get Celestia’s 0°09'30" for the estimated radius of
the coma with

φ ≈ (36'000 km)/(1.3 × 10⁷ km)/(2π) × 360(°) × 60 ≈ 9.09'

[arcsin(18'000 km/(1.3e7 km))/π × 360° × 60 ≈ 9.5' is the exact value
as displayed by Stellarium].

So ignoring the uncertainty of the coma, the comet’s core diameter in
minutes of arc is approximately only

φ ≈ (10 km)/(1.3 × 10⁷ km)/(2π) × 360(°) × 60 ≈ 0.0026'.

See also:
https://www.wolframalpha.com/input/?i=(10+km%2F1.3e7+km)%2F(2pi)*360*60

Thus, in combination with a visual brightness of only 9.05 mag (9.25 mag
with extinction; both according to Stellarium), and light pollution, it
might be very hard to see with an amateur telescope. OTOH, as it should
become brighter when it approaches Sol, observing it might be possible.

However, the simulation with Stellarium indicates to me that even if it is
observable, it might not be easily distinguishable from the fixed stars,
because it is not moving sufficiently fast at this point. And ISTM that an
equatorial mount is highly recommended, otherwise, once found, the comet is
already out of the field of view after ca. 30 s (due to Terra’s rotation).

I used my Celestron Onyx ED f6.2 and Pentax XL40 mm eyepiece, I know this combination gives me a very wide field at low power.

Good luck. (It has been raining here all week, so no luck for me yet.)

[1] https://stellarium.org/
[2] http://dslr-astrophotography.com/add-comets-stellarium/

--
PointedEars

Twitter: @PointedEars2
Please do not cc me. / Bitte keine Kopien per E-Mail.

I use C2A!
http://www.astrosurf.com/c2a/english/

But to remind you, not every one is professor in math!