Comets, and beams to them

Two of the comets that return to us this year come to their perihelia – their closest points to the Sun – a few days apart in April, and they strongly contrast.

Comet Tuttle-Giacobini-Kresák (whose designation 41P means it is 41st among the list of known periodic comets) arrived with almost ideally good timing, and 103P Hartley with almost ideally bad timing.  Timing determines the geometry, which determines the viewing.  Tuttle-Giacobini-Kresák has been making a high arch outside and north of the April part of our orbit.  It’s now descending from that, and you should be able to find it and near it another comet, a magnitude fainter: C/2015 V2 Johnson, whose designation indicates that it is not a periodic comet, but a new one discovered in 2015.

I suggest you read more about these comets in the “Comets” section of “Astronomical Calendar 2017,” to be found through the tab above.  I’ve been working on that section and have it about completed for the year, and you’ll see at the end that I thought of a new diagramming tactic.  I was struggling to think about the antics that one of these comets will make in October and November, and the moments, if any, at which it will be visible and from where.  It’s all very well to have information that you’ve calculated for each date – the magnitude, which is how bright the comet is; the elongation, which is how far away it appear from the Sun; and other numbers that imply whether it is north or south or east or west of the Sun – but those are numbers that your head is trying to turn into a picture.  I realized that drawing a line can show it all simply and simultaneously.

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