Comet for Windows Tutorial

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Updated on November 27, 2004

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    Analysis of Comet Tails    

o Calculate apparent length and direction of the tail.

Comet for Windows can calculate the apparent length and direction of the ion tail.

Please launch Comet for Windows, and select "Menu" - "Ephemeris" menu.

First of all, please select "View" - "Setting" menu, and "View Setting" dialog appears. Then, please check "Tail Length and Position Angle" check box, and input the actual length of the tail in astronomical unit.

For example, the ephemeris of Comet LINEAR (C/2002 T7), naked-eye bright comet in the Southern Hemisphere in 2004 spring, becomes as follows.


C/2002 T7 ( LINEAR )
    Date      R.A.      Decl.     r       d     Elong   m1    Tail(') p.a.  
2004- 4-20  23 52.61    4 39.0   0.618   1.218    30    4.3    221.5 255.1  
2004- 4-25  23 55.70    3  3.7   0.616   1.041    34    4.1    298.2 251.2  
2004- 4-30   0  3.61    1  5.3   0.633   0.853    38    3.9    394.0 247.4  
2004- 5- 5   0 20.43   -1 34.2   0.667   0.661    40    3.7    521.9 242.6  
2004- 5-10   0 55.83   -5 40.1   0.716   0.476    40    3.5    709.7 233.9  
2004- 5-15   2 16.52  -12 42.8   0.774   0.323    36    3.2    994.3 209.6  
2004- 5-20   5  2.45  -19 57.4   0.840   0.267    43    3.1   1291.0 154.4  
2004- 5-25   7 33.48  -17 43.8   0.911   0.355    63    3.2    966.6 125.4  
2004- 5-30   8 44.10  -13 50.3   0.985   0.515    72    4.0    621.8 118.8  

The position angle of the tail in the ephemeris shows that the ion tail extends towards west in April, but the direction changes rapidly in mid May, then it comes to extend towards east at the end of May.

The ephemeris also shows that the ion tail becomes longest apparently around May 20 when the comet becomes closest to the earth. The apparent length is so long as 1291 arcmin (21.5 degree) even if the actual length is only 0.1 AU. Therefore, it suggests that we can see the very long tail of several dozens of degree.

o Calculate best time to see the tail.

The dust tail of a comet spreads along the orbital plane of the comet. Therefore, the tail looks more dense and longer when the earth passes cross the orbital plane of the comet, because we come to gaze the flat tail horizontally at that time.

If the orbits of the earth and the comet are almost horizontal, it keeps easy for a long time to see the tail. If the orbits are almost vertical, the tail becomes longer rapidly just before passing cross the orbital plane, and the tail soon becomes hard to see after passing.

Comet for Windows can calculate the best time to see the tail.

Please launch Comet for Windows, and select "Menu" - "Ephemeris" menu.

First of all, please select "View" - "Setting" menu, and "View Setting" dialog appears. Then, please check "Earth Longitude and Latitude from Comet (Comet Orbital Plane Coordinates)" check box.

When the earth latitude from comet becomes 0 degree, the earth passes cross the orbital plane of the comet. The tail is easy to see when the earth latitude is near by 0 degree.

For example, the ephemeris of Comet Bradfield, the long tail was observed in the morning sky in late April in 2004, becomes as follows.


C/2004 F4 ( Bradfield )
    Date      R.A.      Decl.     r       d     Elong   m1   Earth Dir. 
2004- 4-15   1 59.89    1 55.3   0.189   0.922    10    2.3  (248, 16)  
2004- 4-20   1 24.37   14 20.4   0.206   0.835     7    2.4  (143, 13)  
2004- 4-25   1  3.78   25 38.9   0.346   0.934    20    4.3  ( 91,  7)  
2004- 4-30   0 58.59   32 30.4   0.491   1.061    27    5.7  ( 70,  2)  
2004- 5- 5   0 59.34   37  9.3   0.627   1.183    32    6.7  ( 58, -1)  
2004- 5-10   1  2.50   40 39.1   0.754   1.295    35    7.5  ( 50, -4)  
2004- 5-15   1  6.63   43 28.6   0.874   1.397    38    8.8  ( 45, -7)  
2004- 5-20   1 11.04   45 52.7   0.988   1.490    41    9.8  ( 41, -9)  
2004- 5-25   1 15.36   47 59.6   1.097   1.574    43   10.6  ( 38,-12)  
2004- 5-30   1 19.37   49 54.4   1.202   1.650    46   11.4  ( 35,-13)  

The "Earth Dir." shows the earth longitude and latitude from comet. The origin is the direction of the sun from the comet.

We can see that the earth passed cross the orbital plane of Comet Bradfield when it appeared in the morning sky, and it was the best time to see the tail at that time.

Another example, the ephemeris of Comet LINEAR (C/2001 K5), the long tail had been observed for a long time, becomes as follows.


C/2001 K5 ( LINEAR )
    Date      R.A.      Decl.     r       d     Elong   m1   Earth Dir. 
2003- 1- 1  17 48.40   21 59.8   5.220   5.841    47   14.8  (355,  6)  
2003- 2- 1  18 17.93   26 19.3   5.253   5.705    58   14.8  (359,  9)  
2003- 3- 1  18 41.47   31 39.8   5.291   5.541    70   14.8  (  3,  9)  
2003- 4- 1  19  0.45   38 43.7   5.342   5.366    83   14.8  (  7,  7)  
2003- 5- 1  19  7.73   45 50.4   5.401   5.251    93   14.9  ( 10,  3)  
2003- 6- 1  19  0.27   52  6.2   5.470   5.214    99   15.0  ( 10, -2)  
2003- 7- 1  18 40.23   55 45.7   5.546   5.260   101   15.1  (  7, -6)  
2003- 8- 1  18 17.84   56 25.9   5.631   5.376    99   15.3  (  2, -9)  
2003- 9- 1  18  8.01   54 40.3   5.724   5.539    95   15.5  (357, -9)  
2003-10- 1  18 15.35   52  1.0   5.821   5.726    90   15.8  (353, -7)  
2003-11- 1  18 37.78   49 30.6   5.928   5.936    84   16.0  (350, -3)  
2003-12- 1  19  9.41   48  3.6   6.037   6.151    78   16.2  (350,  1)  

The earth latitude from comet had been near by 0 degree for a long time. The orbits of this comet and the earth are almost vertical. But actually, it had been easy to see the tail for a long time.

o Calculate the dust trail.

There are many dusts exploded from the comet along the orbit of a short periodic comet. They are called a dust trail. When the dust trail collides with the earth, the dusts fall into the air and become meteors, then we can see the meteor shower.

Comet for Windows can calculate the apparent direction of the dust trail, and calculate the best time to see the dust trail.

Please launch Comet for Windows, and select "Menu" - "Ephemeris" menu.

First of all, please select "View" - "Setting" menu, and "View Setting" dialog appears. Then, please check "Orbit Position Angle and Depression Angle from Earth" check box.

When calculating about the dust trail, we should also calculate the tail. So please also check "Tail Length and Position Angle" and "Earth Longitude and Latitude from Comet (Comet Orbital Plane Coordinates)" check boxes.

For example, the ephemeris of Comet Gunn, bright short periodic comet in 2003 whose dust trail was detected on the ground, becomes as follows.


65P/Gunn
    Date      R.A.      Decl.     r       d     Elong   m1    Tail(') p.a.  Earth Dir. Orb(pa,da) 
2003- 4- 1  18 28.84  -24  9.3   2.459   2.174    94   13.2     61.5 267.2  ( 23, -4)  ( 97, 69)  
2003- 4-11  18 40.32  -24 40.9   2.453   2.049   101   13.0     64.3 266.3  ( 23, -3)  ( 96, 68)  
2003- 4-21  18 49.87  -25 16.7   2.449   1.928   109   12.9     65.9 265.9  ( 22, -3)  ( 95, 68)  
2003- 5- 1  18 57.16  -25 58.5   2.447   1.815   117   12.8     65.9 266.0  ( 21, -2)  ( 94, 69)  
2003- 5-11  19  1.86  -26 48.0   2.446   1.712   125   12.6     63.6 266.9  ( 19, -1)  ( 93, 70)  
2003- 5-21  19  3.74  -27 45.4   2.447   1.623   135   12.5     58.3 268.9  ( 16, -0)  ( 92, 72)  
2003- 5-31  19  2.67  -28 49.7   2.449   1.549   144   12.4     49.7 272.6  ( 13,  0)  ( 92, 74)  
2003- 6-10  18 58.76  -29 57.7   2.453   1.495   154   12.3     37.9 279.8  ( 10,  1)  ( 92, 77)  
2003- 6-20  18 52.51  -31  4.4   2.458   1.463   164   12.3     24.1 296.1  (  5,  2)  ( 93, 81)  
2003- 6-30  18 44.75  -32  4.1   2.465   1.456   170   12.3     14.5 344.2  (  1,  3)  ( 94, 85)  
2003- 7-10  18 36.69  -32 52.0   2.473   1.473   166   12.4     21.0  41.2  (356,  4)  ( 95, 89)  
2003- 7-20  18 29.57  -33 25.7   2.483   1.515   157   12.4     33.9  61.9  (352,  5)  ( 96, 92)  
2003- 7-30  18 24.44  -33 45.5   2.494   1.579   147   12.6     45.2  70.9  (348,  5)  ( 97, 95)  
2003- 8- 9  18 22.03  -33 53.2   2.507   1.663   137   12.7     53.3  76.0  (345,  5)  ( 98, 98)  
2003- 8-19  18 22.61  -33 51.6   2.521   1.764   128   12.9     58.2  79.3  (342,  5)  ( 98,100)  
2003- 8-29  18 26.16  -33 42.8   2.536   1.879   119   13.0     60.2  81.3  (340,  5)  ( 97,101)  
2003- 9- 8  18 32.47  -33 28.1   2.553   2.004   111   13.2     60.2  82.6  (339,  5)  ( 97,102)  
2003- 9-18  18 41.20  -33  8.4   2.571   2.137   103   13.4     58.5  83.4  (338,  4)  ( 96,103)  
2003- 9-28  18 51.99  -32 43.6   2.590   2.276    96   13.5     55.8  83.7  (337,  4)  ( 94,102)  

The "Orb(pa,da)" shows the position angle and depression angle of the comet's orbit.

The first "pa" shows the apparent position angle. The dust trail looks extending towards this direction in the photos.

The second "da" shows the depression angle, which is the the angle between the view direction from the earth to the comet and the direction of comets's motion. When the depression angle is 0 degree, the comet is going away from the earth directly. When the depression angle is 180 degree, the comet is coming towards the earth directly.

When the position angle of the dust trail coincides with the position angle of the tail, the dust trail overlaps the tail, and cannot be distinguish them. When the depression angle of the dust trail is about 0 or 180 degree, the dust trail overlaps the coma, and cannot be distinguish them. The dust trails cannot be observed at these times.

The apparent direction of Comet Gunn's dust trail coincides with the tail from April to mid June, so the dust trail cannot be observed at that time. However, the apparent direction of the tail begins to change after mid June, then the dust trail can be distinguished from the tail.

The depression angle of the orbit keeps around 90 degree all though the period. That means we keep to observe the dust trail from the side. It suggests that the dust trail looks long, but faint because the dusts become sparse.

In the case of an extinct comet, it is possible to see the dust trail even when the depression angle is about 0 or 180 degree, because the comet has no coma. When the depression angle is about 0 or 180 degree, the dust trail does not look long, but bright because the dusts become dense. So it may be easier to see the dust trail on the contrary.

The following page introduces the calculation about Phaethon, a source of Geminids meteor shower.

Observation Proposal of Dust Trail of a Dead Comet Phaethon = Source of Geminids Meteor Shower!

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Copyright(C) Seiichi Yoshida (comet@aerith.net). All rights reserved.