Tom Polakis
professor emeritus
Reged: 12/20/04
Posts: 551
Loc: Tempe, Arizona
|
|
I just learned that the bright shell has been expanding at a constant velocity of 575 m/sec since it's outburst on October 24.40. Since the distance to the comet changes so little, the angular size will incease linearly as well. That makes it easy to extrapolate the diameter for upcoming dates.
In my Arizona-centric view, I'll use 10 p.m., which is 5h UT on the next day. For the next two Saturday nights, which are Last Quarter and New Moon, we have.
Sat, Nov. 3 ... 15 arcminutes
Sat, Nov. 10 ... 25 arcminutes
Imagine the sight of a ghostly sphere in Perseus the size of the Full Moon. That's what we have to look forward to!
Tom
--------------------
Tom Polakis
Tempe, AZ
Visual observing, DSLR photography, lunar & planetary imaging
http://www.pbase.com/polakis/
|
Arkalius
professor emeritus
Reged: 08/03/06
Posts: 668
Loc: Orange County, CA, USA
|
|
I'm not sure the whole thing will be naked eye visible at that size... it might be too diffused but then again, maybe it will. That will be wierd and cool.
--------------------
-Arkalius
11" Celestron SCT on Orion Atlas EQ-G
8" Zhumell Dobsonian Reflector
Proud member of the Orange County Astronomers
|
Jeremy Perez
Carpal Tunnel
Reged: 08/12/04
Posts: 1675
Loc: Flagstaff, Arizona, USA
|
|
I've been measuring the diameters of the outer halo, the bright coma, and the off-center condensation each night with my astrometric eyepiece. It's been an interesting observing project. The plot of those results is below (no doubt it would be more valuable if it were averaged with other observers' measurements). I set the upper limit to the average size of the full moon, since that's a great benchmark. Although the outer faint halo looks like it will reach that size by November 1, I can't imagine that it would be visible to the naked eye. My measurements point to November 14th-ish for the coma to reach that point.
Although the growth is linear, my instincts tell me that solar pressure should drag it into equilibrium at some point...whether that will happen before it becomes to faint to see will be interesting. Then again, maybe the particles being ejected are robust enough to resist the solar wind out beyond the orbit of Mars. I'll try to check back in and update the plot after a few more days...
The data table can be found here.
--------------------
Orion SVP 6LT (6" f/8 Newt) || Orion XT8 (8" f/6 Newt) || 15x70 Oberwerk Binoculars
The Belt Of Venus || Astro-Sketch Gallery || Astro-Sketching Resources || Astro-Photo Gallery
|
novbabies
Postmaster
Reged: 06/05/05
Posts: 15678
Loc: Northern Georgia!
|
|
Only time will tell, but those predictions of size are awesome!!
--------------------
Good Seeing!
Mark
Orion 12" XTi f/4.9
VERY old Edmund 6" f/8 reflector
Assorted binoculars
|
Tuugii
professor emeritus
Reged: 03/22/07
Posts: 522
Loc: Ulaanbaatar, Mongolia
|
|
awesome! what is that astrometric EP? 
Tuugii
--------------------
telescope=time machine...
|
thj
sage
Reged: 02/07/06
Posts: 438
Loc: Queen Creek, Arizona
|
|
Cool chart (!) and what a fascinating outlook. Can't wait to see what will happen  .
--------------------
Thomas
(Personal Web Site)
N 33° 09' / W 111° 33'
Orion Atlas 10" EQ-G
Stratus 21/13/8/5 mm EPs
Orion 32mm Q70 EP
Plössls in many shapes and sizes
Celestron NexStar 114GT (for "Grab-n-Go" moments)
|
Dave Mitsky
Postmaster
Reged: 04/08/02
Posts: 6287
Loc: Pennsylvania, USA
|
|
I estimated the size of the coma to be approximately 7 arc minutes on Sunday night and 9 arc minutes last night.
Dave Mitsky
--------------------
Chance favors the prepared mind.
De gustibus non est disputandum.
|
Chopin
Canis Insanus
Reged: 02/03/05
Posts: 3379
Loc: In the doghouse.
|
|
Quote:
I estimated the size of the coma to be approximately 7 arc minutes on Sunday night and 9 arc minutes last night.
Dave Mitsky
That's big! How does one go about estimating the size?
--------------------
Jason®
Phlog
|
Bill Weir
scholastic sledgehammer
Reged: 06/01/04
Posts: 893
Loc: Metchosin (Victoria), Canada
|
|
Quote:
Quote:
I estimated the size of the coma to be approximately 7 arc minutes on Sunday night and 9 arc minutes last night.
Dave Mitsky
That's big! How does one go about estimating the size?
Do you know the FOV of your eyepiece?
Last night a friend and I were looking at it naked eye. It was clearly visible as a small yellowish disc with a bright dot in the center. This was from a reasonably dark site. It looked similar to the telescopic view of a smal planetary nebula where the central star showed.
Through the 25" last night I'd estimate that Jeremy's chart seems about right to me. Also, before the Moon came up, I thought I was noticing an extention off the western edge of the outer halo. It was similar to what I noticed on the night of the 25, in the area where the oter halo formed.This was before the outer halo was clearly visible. Perhapsweare about to start noticing a bit more expansion.
Bill
--------------------
6'' Orion SkyQuest
12.5'' f/5 Custom Truss Dob
William Optics 80mm ZenithStar II ED Doublet
f/5 25" newtonian on a giant GEM, any time I want
Observing sessions grand total for 2007, 171.
So far in 2008, 115
|
Jeremy Perez
Carpal Tunnel
Reged: 08/12/04
Posts: 1675
Loc: Flagstaff, Arizona, USA
|
|
For anyone who is not familiar with it, the eyepiece I've been using is a Meade 12mm Astrometric Eyepiece (click the "alternate view" link at that site to see what the illuminated eyepiece scale looks like). I've been using it to measure double star separations and position angles, but it turned out to be handy for tracking the growth of this excellent comet. I originally calibrated the measurement units in the eyepiece against a set of double stars with well-known, consistent separations. For example, when I have a 2X barlow paired with it on my telescope, each unit corresponds to 8.6 arc seconds. It's fairly straightforward from there to line it up on the different features of the comet and take measurements. Although as it grows, and the boundaries of the different features become fuzzier, it gets more difficult to gauge.
I missed checking it last night due to heavy clouds, but that did give me the opportunity to catch up on the massive sleep deficit this thing has given me 
--------------------
Orion SVP 6LT (6" f/8 Newt) || Orion XT8 (8" f/6 Newt) || 15x70 Oberwerk Binoculars
The Belt Of Venus || Astro-Sketch Gallery || Astro-Sketching Resources || Astro-Photo Gallery
|
David Knisely
Postmaster
Reged: 04/19/04
Posts: 6784
Loc: Beatrice, Nebraska
|
|
Quote:
Quote:
I estimated the size of the coma to be approximately 7 arc minutes on Sunday night and 9 arc minutes last night.
Dave Mitsky
That's big! How does one go about estimating the size?
Well, if the comet is fairly circular (as this one kind of has been), you can use the drift method. With the drive turned off, you put the leading edge of the comet at the "trailing" edge of the field of view (the direction it is going as it drifts out) and with the clock drive turned off, time how long it takes to completely cross the edge of the field and exit the field of view. Using that time and the declination of the comet, you can calculate the approximate size of the coma with the following formula:
d = 15.04*T*Cos(delta), where T is the time to exit the field completely, Cos is the cosine function, and delta is the comet's current declination. For example, last night, the comet had a declination of 50 degrees 24 minutes (50.4 degrees). It took 52.7 seconds for the comet to go from just touching the edge of the field of view to the point where the last bit of coma vanished from view beyond the edge of the field. Thus, the approximate diameter of the comet was 502 arc seconds, or 8.42 arc minutes. Clear skies to you.
--------------------
David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
|
walt r
Carpal Tunnel
Reged: 02/13/07
Posts: 2419
Loc: Doylestown, PA
|
|
I have measured the coma of Holmes on a couple of nights using the drift method David outlined above. I use the same equation except I divide the constant by 60 so that time is in seconds.
Data:
measurements are in the E-W direction
average of three measurements
28Oct2007 0340UT: coma = 6.4 arc min
31Oct2007 0120UT: coma = 8.9 arc min
off-center condensation = 2.4 arc min
I may have glimpsed the outer halo with AV but its not something I could measure.
--------------------
Walt
Obsession 18" f/4.45 #1370 AN/SC
MK67 Deluxe 6" f/12 Mak-Cass, Super Polaris GEM, JMI MicroMax DSC
DIY 60mm f/6 Achromat
Cookbook 245 CCD
|
Mike K
sage
Reged: 04/01/07
Posts: 464
Loc: Central Texas
|
|
Quote:
Using that time and the declination of the comet, you can calculate the approximate size of the coma with the following formula: d = 15.04*T*Cos(delta)
So, I used this method with a stopwatch and crosshair eyepiece averaged over 10 iterations to get 56.545 seconds for T. This yields an approximate size of 11.4 arc-minutes this evening (9:00PM CDT). Is this close to your estimate tonight?
Clear skies,
Mike K.
--------------------
Clear skies,
Mike K.
30°31" N 97°44" W, LP: Red
Observe: Once or twice a week back yard, once a month under dark skies
Favorites: Globulars, planets, face-on spirals
Equipment: CPC925/XT10i/TMB-92SS/Lunt LS60THaDS
Eyepieces: Naglers, Ethoi, UO HDs, Hyperion Zoom
|
Radom
member
Reged: 06/15/04
Posts: 47
Loc: Central Oklahoma
|
|
Hope it's OK to ask a bit further along the same lines: Can someone roughly estimate the current diameter of the (visible) bright coma in miles? How far away is it from Earth? What direction is it moving in relation to us? I've been reading off & on for the past few days. There is so much here & other places, I don't think I'll ever get to read it all. Forgive me if the answers are obvious to everyone but me?
--------------------
Steven M Henderson
-----------------
XT12i
Meade 102ED/LDX650
Denkmeier II Power X
Garrett Gemini 12x60 LW
Fujinon SMT-FX 16x70
Universal Astronomics Unimount Light
|
canopus56
sage
Reged: 05/01/05
Posts: 298
|
|
Jeremy,
I took the liberty of using your raw table data to prepare a table of the estimate of the rate of change -
http://www.utahastronomy.com/Current_observing_events#Estimate_of_rate_of_Comet_17P_size_change
The nucleus (central condensation, 0.4 km/s) and the coma (central coma 1.2km/s) appear to have a steady rate of change. The diffuse coma (outer halo) has a variable rate of change which can be probably attributable to the Oct 28th full Moon confounding measurements of the size of the fainter outer halo. - Canopus56
|
novbabies
Postmaster
Reged: 06/05/05
Posts: 15678
Loc: Northern Georgia!
|
|
If the coma is 10' across and the comet is 150 million miles away, trig suggests an approximation (tan(0.167°) * 150000000) of roughly 43.500 miles.
Math gurus, is this correct?
--------------------
Good Seeing!
Mark
Orion 12" XTi f/4.9
VERY old Edmund 6" f/8 reflector
Assorted binoculars
|
RLTYS
Carpal Tunnel
Reged: 12/18/04
Posts: 1741
Loc: New York (Long Island)
|
|
To All
Observed Comet Holmes last night 10/29-30 from 11:45 PM to 12:20 AM under reasonably dark skies. There seem to be some changes to the comets appearance as noted:
12x63 Binoculars: Once again I did a magnitude estimate on Holmes and it seems to be fading. Using stars Alpha Per (1.9) and Delta Per (3.0) it looks like Holmes has faded to about mag 3.0 possibly 3.2. Holmes still shows a pale yellow hue.
10" Refl: Initially I thought that Comet Holmes had not changed any but the more I looked the more I realized that there were some subtle changes.
Outer Coma: At 174x the outer coma was definitely larger, I estimated its diameter between 10" and 12" (arcmin). I also got the impression that it was somewhat rounder then on previous evenings.
Inner Coma: Appears to be not as round or as well defined as on previous observations. It appears almost elongated (?) in shape. The nucleus still appears to look like a badly focused star. Is the nucleus getting larger? Also, no stars were visible within the comet. Holmes continues to move, slowly, in a westerly direction, passing north of and just missing the star SAO 24223. I'm still looking for a reported, subtle, fan shaped tail.
Observed Comet Holmes again on the night of 10/30 from 11:15 to 11:45 PM.
12x63 Binoculars: Using comparison stars 3.0 and 4.1 I estimated Holmes to be mag 3.0.
10" Refl: Outer Coma's diameter has definitely increased to 12" (arcmin). With the Inner Coma being about 4" (arcmin) in dia. The boundary between the outer and inner coma's is becoming less distinct. The nucleus also seems to be getting somewhat larger. Coma Condensation 7.0. If there is a fan shaped tail present it's extremely subtle.
Clear Skies.
Rich (RLTYS)
--------------------
10" F4.8 Refl.
4" F5 Refr.
50mm F12 Refr. (Tasco #6TE-5)
12x63 and 10x50 Binoculars.
"I want to do more then just look."
|
walt r
Carpal Tunnel
Reged: 02/13/07
Posts: 2419
Loc: Doylestown, PA
|
|
Quote:
If the coma is 10' across and the comet is 150 million miles away, trig suggests an approximation (tan(0.167°) * 150000000) of roughly 43.500 miles.
Math gurus, is this correct?
I got 437000 miles across from your equation (which is correct).
--------------------
Walt
Obsession 18" f/4.45 #1370 AN/SC
MK67 Deluxe 6" f/12 Mak-Cass, Super Polaris GEM, JMI MicroMax DSC
DIY 60mm f/6 Achromat
Cookbook 245 CCD
|
canopus56
sage
Reged: 05/01/05
Posts: 298
|
|
To correct this table, I added a note that it estimates the rate of change of the diameter - not the radius. Therefore, divide the rate of diameter change to get the rate of change of the radius. E.g. - nucleus increase rate = 150 meters/sec and the central coma rate = 600 meters /sec. - Canopus56
|
canopus56
sage
Reged: 05/01/05
Posts: 298
|
|
It's the sin rule, not the half angle tanget formula. See
http://www.utahastronomy.com/Current_observing_events#Physical_Size_of_Comet_17P_using_Google_calculator
and
http://www.utahastronomy.com/Current_observing_events#Reference_Table_Physical_Size_of_Comet_17P
- Canopus56
|
Previous
Index
Next
Threaded
Edit
Reply
Quote
[Re: 