Comets are particularly unpredictable phenomena. The current case is C/2019 Y4, which has apparently broken up into at least three pieces – which at last sight were drifting away from each other. It’s visual magnitude has gone from a sudden brightening to 7.8, but then dropped to 8.8 after the break-up and shows no sign of recovering. This, despite the fact that is nearer to the Sun and the Earth than before. It is not visible to the naked eye, even in clear, dark skies. You might find it with a medium amateur telescope.
There is another, more recently discovered comet in the Solar System called C/2020 F8. It, too has undergone a sudden brightening, but is still a bit to dim to see – even in that theoretical dark, clear sky. Since it is in the Southern sky right now, you could not see it anyway.
I have made some diagrams of both comets with the JPL Small-Body Database Browser and added some explanatory text. The planets are all in the same orientations and positions in both.
“So, what next? “, you may ask. Well, as these comets approach the Earth and Sun – at different rates since the Earth and Sun are 1 AU apart – they will brighten. We cam predict the change due to distance alone. Below is a graph of distances predicted over time for C/2019 Y4 (refer to figure 1). The data are from the aforementioned JPL Small-Body Database Browser The graph was generated by your humble narrator in Excel.
The increase of brightness to be expected (if nothing about the comet itself changes) can be predicted by the total distance involved. Keep in mind that light spreads out such that a reduction of ½ the distance will result in 4 times the brightness. Remember that on this stellar Magnitude scale a reduction of 1 magnitude is the equivalent of more than a doubling (about 2.5 times) in brightness. I don’t make these rules, OK?
This needs some calibration since it calculates only differences. That calibration is taken from a recent observation as noted on the graph (also Excel) that follows.
The conclusion is that the peak brightness will be still below naked-eye visibility – around May 28th. Having said that, you will remember that this exercise assumes that the comet itself will not change. But that’s silly! We just saw it increase suddenly in brightness (far in excess of expectation) and then dim again! That was from eruptions of vaporizing ices, that apparently broke this comet into pieces. I told you these things are unpredictable, did I not?
So, why do this calculation of brightness due to proximity? Because it is all we can do! Keep that in mind the next time someone tells you they can predict the climate. 😉
The same sort of calculation can be done for this Johnie-come-lately comet that just showed up. I will skip all the intermediate explanations and go straight to the prediction chart.
You see that the new comet is likely to be brighter than poor old C/2019 Y4. It will probably become magnitude 6.3 – bright enough to see without binoculars or a telescope – out of city lights, in a dark clear sky -but just barely!
And, just now we have news of an observation from the Comet Observation Database . For April 19th (late in the day) the brightness was measured at magnitude 6.8. You can see the red cross on the graph. That is, however, one of four observations on that day – the other three were all magnitude 7.5. To change the whole prediction on a single observation would not be reasonable, so I will wait to do so until a few more observations are made. Did I mention that these things are unpredictable?
You may ask, “Steve, why did you choose such an uncertain occupation?”
I did not choose Astronomy. Astronomy chose me. It is actually a hobby because, while I wanted to be Carl Sagan, I found out they already had one. So, I wound up looking down through the Earth instead, because someone would pay me for that. Now I have nothing better to do. Well, I have other things to do – yes. But, who wants to mow the yard again?