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 Post subject: Comet Machholz
PostPosted: Sat Nov 04, 2017 8:14 pm 
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Joined: Sun Nov 13, 2011 4:39 pm
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Location: Gilford Park
Return of the comet: 96P spotted by ESA, NASA satellites

https://phys.org/news/2017-11-comet-96p ... lites.html

It's getting fainter, but still visible after midnight by my calculations.

According to: https://theskylive.com/96p-info, its rise time is 6:32 a.m. GMT which is 1:32 a.m. EST.

I await Joe's corrections, as usual.

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 Post subject: Re: Comet Machholz
PostPosted: Sun Nov 05, 2017 12:47 am 
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Chris,

There’s no need to correct the rise time of 96P/Machholz from GMT to EST. Did you similarly correct the rise time of the sun? A rhetorical question of course, because if you did, the sun would rise about 2 am EST and the comet would be in a bright sky.

For events that are visible simultaneously by observers in different time zones, then you would apply the 5-hour differential between GMT/UT and EST. For example, if an observer in London, England, saw the Great Red Spot transit Jupiter’s central meridian at 3 am GMT, a concurrent observer in New Jersey would be seeing it at 10 pm EST.

However sunrise (or comet rise) wouldn’t be visible at the same time in London and NJ. We’d see them rise about 5 hours later. New Jersey, nominally 75°W longitude, is five hours west of London regardless of the civil clock settings since the earth rotates 15° per hour. There will be a slight difference in rise times from exactly 5 hours because of the difference in latitude between London and NJ, and because both the sun and the comet will move slightly during that interval.

For 40°N-75°W on 05-Nov-2017, SkyTools shows sunrise at 6:34 am EST and 96P rising at 6:29 am. The comet is about 12° NNE of the sun and will remain at that nominal distance through the day until sunset. It’s virtually certain that no ground-based observer will be able to see a ninth magnitude comet in broad daylight fairly close to the sun (even if the sky is quite clear). In fact, a ninth magnitude comet might not be all that obvious in the middle of the night depending on the size of the coma and the resultant surface brightness, C/2017 O1 (ASASSN) being a recent example of such “underperformance.”

BTW, the headline at Phys.org, “Return of the comet: 96P spotted by ESA, NASA satellites,” seems a bit misleading, as if ESA/NASA recovered 96P for this apparition. It had already been imaged and observed by amateurs well before this (however, I did not dig into the specifics). The story at Phys.org really is that 96P appeared in the fields-of-view of their solar-observing spacecraft. That’s a clue that it wouldn’t be visible in the middle of the night.

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