Comet C/2019 Y4 has broken into pieces that are scattering and fading in brightness despite being closer to the Sun and Earth.  I will include a Hubble Space Telescope photo here:

It will not be a “Great Comet”.

However…

Comet 2020 F8 is now visible in a small telescope or binoculars and will probably be – at the very least – visible without such aids very soon.  Below is a recent image from Universetoday.com

But, you won’t see it now because it is in the Southern sky and is being observed from New Zealand and Australia and other points in the Southern Hemisphere.  It will arc over the Northern sky soon.  In the previous update, I graphed the brightness to be expected due to distance alone.  You can take this as a prediction of 6.3 as the peak magnitude. But, we all know that is almost certainly wrong – because we don’t have enough information. I have added the daily average observed magnitude (orange dots) in Figure 3, below.

The observations are, indeed following the prediction – somewhat.  The differences can be attributed to eruptions of gas and dust, but also to the comet’s rotation.  In college, I learned a method of analyzing sparsely collected observations to detect periodic changes.  It goes something like this:

We cannot monitor objects in telescopes with enough resolution in time to detect a periodic variation along one cycle of rotation or pulsation.  However, since we expect the oscillations to be more-or-less uniform over time, we can collect points from different oscillations, over an extended period and graph them as a single cycle. 

But this requires that we know the period of the cycle.  Astronomers have been historically starved for data and come up with some desperate solutions.  In this case, we can try every possible period of oscillation, put the resulting data on graphs, and pick the one that looks like we think it should for a single cycle.  In the olden days, this laborious amount of calculation could be out-sourced to graduate students or assigned as homework for undergraduates. If you want to imagine doing such calculations “by hand” go ahead.  Me, I don’t have to use imagination because I did it – a few times.

The observations have a time associated with them.  We establish a “zero point” and calculate a time value for each magnitude.  Then each observation time is divided by the trial period.  That leaves a fraction that is the position in the single “combined” oscillation we intend to graph.  Then we change the trial period and repeat – a lot.  Below is a “perfect” theoretical graph to be kept in mind while looking through all the trial graphs.

And before you ask “why didn’t you use a computer at UT in 1976, Steve?”, I should tell you that in the 70’s, a computer with a tiny fraction of your telephone’s capability was a huge machine in a large room attended by several “operators” who scheduled calculations on that hideously expensive device for days in advance. The data and the programs (apps) were read in on punch cards and the output was printed on green-and-white paper.  What “memory“ was available held the simple operating system and your bare-bones program and input data for the time it took to complete the “job”.  Then your data and program were immediately replaced with the next job. No time was available for undergraduate homework.  

Now we have Excel spreadsheets instead of graduate students or IBM 360 “mainframes”.  The needed calculations and a graph on the screen for one trial period is accomplished in a split second with a single click.  The graph below was selected as “plausible” after 291 clicks.  This indicates a period of  four hours, 51 minutes. 

The points plotted come from 74 observations over 14 days.  I should mention that these were not the “raw observations” but were adjusted to remove the distance-related brightening (that green  curve in Figure 3).

This was the “best-looking” result, but there were other “candidates” at six hours, seven minutes and at eight hours, 10 minutes.  Nothing even close to “plausible” was found after that, up to and including a 30-hour trial period.

So, if you hear later that the comet has been determined to be rotating at a period like any of those – remember that you heard it here first!  Otherwise, well I was wrong.  Science has a long history of being wrong, so that’s OK.  The important part is to not insist on your theory in the face of contrary evidence and accept that you were wrong.  (Are you listening, Global Warming Devotees?) 😉

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• astronomy
• comets