Known comets – observation planning tools and methods

 

Updated 2017 June 27

 

Contents

 

Introduction

 

There are numerous sources of information as to currently observable comets and a selection will be covered in the following paragraphs. Magazines such as Astronomy Now and Sky and Telescope make mention of the brighter comets and often carry articles about specific comets of particular interest.

 

This document is in three sections;

What’s up                                            Which part of the sky is available to view depends on your location, time and nearby obstructions.

Finding known comets to observe      Various sources for ascertaining which comets can be seen

How to observe them                          How to point your telescope in the required direction          

 

Some of this may be quite basic but if you have limited experience I hope these guidelines will be useful.

 

What’s up

 

However before you can determine which comets are observable you need to know which part or parts of the night sky will be visible to you at the time you wish to observe. Some alternatives;

- the magazines mentioned above include monthly sky charts

- invest in a planisphere

- go to the Heavens Above website at http://www.heavens-above.com/ - example below

 

 

Finding known comets to observe

 

Moving on to observable comets- comet 260P/McNaught is used as the example so that observers can choose whichever method suits them best.

 

BAA Comet Section

 

‘The Handbook of the British Astronomical Association’ lists comets coming to perihelion for the year of publication plus ephemerides for a selection of the brighter comets.

 

Current comets are listed at http://www.ast.cam.ac.uk/~jds/  (example below). There are also links to ephemerides, finder charts and upcoming comets for current and future years on this same page.

Current comet magnitudes (May 31) and observable region (May 31)

Comet                     Magnitude   Trend    Observable     When visible        Last visual observation

Johnson (2015 V2)              7.5    fade     50 N to 55 S   all night           2017 May

PanSTARRS (2015 ER61)          8      fade     45 N to 65 S   morning             2017 May

41P/Tuttle-Giacobinni-Kresak  10.5    fade     50 N to 55 S   all night           2017 May

71P/Clark                     11      steady   40 N to 90 S   all night           2017 May

Lovejoy (2017 E4)             12      fade     Solar conjunction                  2017 April

PanSTARRS (2016 R2)           13 ?    bright   Poor elongation                    Not yet observed

29P/Schwassmann-Wachmann      13 ?    varies   45 N to 80 S   morning             2017 May

65P/Gunn                      13.5    steady   30 N to 65 S   best evening        Not yet observed

PanSTARRS (2015 O1)           14      steady   50 N to 30 S   all night           Not yet observed

73P/Schwassmann-Wachmann      14      fade     15 S to 30 S   early morning       2017 March

The observable region is an approximate indication of the latitude at which the comet may be seen. Under good conditions comets may be visible outside this range. The period when visible is for the UK if the comet is visible from the UK, otherwise for 40 S or the Equator as appropriate.  The last visual observation is as received by the Section, details are often updated on the basis of observations published elsewhere.   Details are normally fully updated at the beginning of each month, but may be updated more frequently for comets brighter than 10th magnitude; the date of the most recent partial update is given.

 

BAA Computing Section

 

Starting at the Home page, http://www.britastro.org/computing/index.html, selecting Charts then Comets produces a list of comets together with dates when observable. Clicking on a comet e.g. 260P/McNaught produces a finder chart as shown below. Note that these are geocentric charts (seen as if the observer was at the centre of the Earth). Alternatively select Applets/What’s observable/Add object/Comets (from list) or Comets (whole list) to obtain a chart showing all comet positions. For greater accuracy a chart specific to your location can be produced using a planetarium program such as Megastar. Ephemerides for specific locations can be obtained from the Minor Planet Center. Examples of both are shown below.

 

http://www.britastro.org/computing/ch/260PMcNaught2012Nov1Dec1.png

 

SPA Computing Section

 

The Comet Section website at http://www.popastro.com/comet/index.php links to the BAA Comet Section website mentioned above.

 

Heavens Above

 

At http://www.heavens-above.com/ you can obtain charts for comets brighter than magnitude 14. Charts for 260P/McNaught are shown below.

 

 

Orbital elements

 

Time to take a short break to explain some terminology so that you can better understand what follows. The path followed by a comet orbiting the Sun is defined by its orbital elements plus the epoch (date) for which those numbers are valid as described in the table and diagram below.

 

Name

Symbol

MPC notation

Description

Time of perihelion passage

T

T

Date and time at which the comet passes closest to the Sun

Perihelion distance

q

q

Distance between comet and Sun at time of closest approach (T). q = a(1-e)

Aphelion distance

Q

Q

Distance between comet and Sun when the comet is furthest from the Sun. Q = a(1+e).

Semi-major axis

a

a

(Half ) the length of the long axis of the ellipse.

Eccentricity

e

e

A measure of the deviation of the orbit from a circle, e = c/a. For a circle e = 0, an ellipse e < 1, a parabola e = 1, an hyperbola e > 1

Inclination

i

Incl

The angle between the plane of the orbit of the comet and the ecliptic. If  the inclination is > 90º then the motion of the object is considered to be retrograde.

Longitude of the ascending node

Node

The direction in space of the line where the orbital plane intersects the plane of the ecliptic. It is measured eastwards (increasing RA) from the Vernal Equinox (First Point of Aries).

Argument of perihelion

w

Peri

Defines how the major axis of the orbit is oriented in the orbital plane and is the angle between the ascending node and the perihelion point measured in the direction of motion.

Epoch

 

Epoch

The date for which the values of the above elements are valid.

 

 

 

 

 
Planetarium software

 

All you need is planetarium software into which you can download the latest orbital elements and plot tracks to a similar scale as shown in the Megastar examples below. Using Megastar I plotted comets with a limiting (faintest) magnitude of 16 for 2012 November 11, 00:00 UT.

 

 

Having chosen a comet to observe it is advisable to obtain the latest orbital elements and ephemeris from the Minor Planet Center. A list of comet orbital elements suitable for loading into planetarium programs can be obtained from the MPC at http://www.minorplanetcenter.net/iau/Ephemerides/SoftwareEls.html

and for Megastar in particular from  http://www.minorplanetcenter.net/iau/Ephemerides/Comets/Soft12Cmt.txt

 

A screenshot of a Megastar plot for comet 260P is shown below.

 

 

Alternatively this can be done for individual comets from the MPC Minor Planet and Comet Ephemeris Service at http://www.minorplanetcenter.net/iau/MPEph/MPEph.html One needs to complete the form by entering the comet number, e.g. 260P, Number of dates to output, Ephemeris interval and units and Observatory code or Longitude, latitude and altitude. Specify MPC 9-line under Format for elements output and leave other options as is. Click on ‘Get ephemerides/HTML page’ and the result is;

 

 

 

The orbital elements can be loaded into a planetarium program such as Megastar and the track of the comet plotted as below or the ephemeris used to point your telescope.

 

 

Seiichi Yoshida’s website

 

Three pages which will help to identify current comets suitable for visual observation;

 

Weekly Information about Bright Comets http://aerith.net/comet/weekly/current.html example below;

260P/2012 K2 ( McNaught )

http://aerith.net/pictures/jager/s/260P20120910lrgbwebnew.jpg

First return of a new periodic comet which brightened up to 14 mag in 2005. It brightened very rapidly and became much brighter than originally expected. Now it is very bright as 12.2 mag (Oct. 9, Uwe Pilz). It keeps high for a long time in the Northern Hemisphere. But the comet will be fading after this. It locates very low in the Southern Hemisphere.

Date(TT)  R.A. (2000) Decl.   Delta     r    Elong.  m1   Best Time(A, h)  
Nov. 10   1 31.91   42 40.1   0.695   1.623   148   14.0  22:12 (180, 82)  
Nov. 17   1 30.28   42 39.1   0.740   1.652   145   14.3  21:43 (180, 82)  

 

Visual Comets in the Future (Northern Hemisphere) http://aerith.net/comet/future-n.html - screenshot below.

 

 

Visual Comets in the Future (Southern Hemisphere) http://aerith.net/comet/future-s.html - similar to the above screenshot

 

The tutorial Visual observations of comets will show you how to actually observe comets using telescopes and binoculars

 

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