Thursday, July 23, 2020

explained comet magnitudes

There have been a few remarks about comet magnitudes. Rich's question today compelled me to try to clarify things. "Why there are... dramatic differences between [different sources]?" I posted the following on the RASC Toronto Centre forum.



ONE - extended

First. Comets are “extended” objects. I.e. they are big objects that take up many arc-seconds or arc-minutes of space on the celestial sphere. Like galaxies. They are unlike stars which are point-source objects. [ed: This is also true for planets—point sources.]

When we measure the magnitude of a star, it’s simple. All the light is concentrated into a small pinpoint. But the light for a comet or a galaxy is spread out. When we measure the magnitude for extended objects, we get a number that is the aggregated or integrated result.

Imagine this. You have grains of sand on some black fabric. All the grains are used to measure the magnitude. Stack the grains into a compact small pile, as tall as possible, and you clearly have a high concentration of dots in a small area, easy to see, as a group. Now swish the pile, run you fingers through the grains, and spread the pile out, over a large area or over much of the fabric. It’s the same number of grains but now widely distributed. Harder to pick off, harder to see, especially from a distance. Just like a galaxy or a comet and its tail.

So that’s the first key point. Comet magnitudes are extended object magnitudes.

A galaxy or comet of magnitude 5 is much more difficult to see compared to a magnitude 5 star. The star is easy, while small. The comet (with tail) or galaxy will need a dark sky, good transparency (as Frank mentioned) excellent vision, averted vision techniques, maybe all four.

Now, here’s a brain-bender. Galaxy size, in square degrees, does not change. So the magnitude value will stay constant (assuming we measured it right to begin with). Comets change in size or area…

TWO - source

Now, the next part is data source. What does Telescopius use for it’s data source and when was it updated? (Razvan noted that: JPL/NASA.) You always have to ask yourself that of any resource you consult. And that’s why I don’t use or relay on just one.

People like Seiichi Yoshida ( and Greg Crinklaw ( consult reliable sources. The official source of small body solar system information is at the Minor Planet Centre ( run by the IAU. I would assume Telescopius uses official sources but off the top of my head, I dunno.

There’s that new (to me) web site COBS ( where observers submit observations on comets. That’s empirical data. That said, when objects are imaged for magnitude, you need to know what filter method is used… That’s another rabbit hole!

THREE - currency

You saw me make a mistake yesterday. I shared the comet ephemeris data from SkyTools (by Crinklaw) but I forgot to update it before posting. It was week-old data?

Astronomical data gets updated over time. With minor bodies in particular, asteroids and comets, they are being perturbed by big objects in the solar system. The Sun dramatically affects comets. Jupiter, though far away, is affecting all asteroids and comets. We saw 27 years ago what happened to comet D/1993 F2 (Shoemaker-Levy 9)…

So orbital data is regularly updated for all these types of objects, hence the existence of the MPC. With more and more observations of an object, we can increase our confidence in the orbital parameters and we can classify orbits as known or certain. There’s a rating scale.

For comet C/2020 F3, we know now that its orbit period has changed with this recent perihelion and it is now calculated to have a 6000 to 7000 year period. Because the period is so long, if I understand the conventions properly, it will NOT receive a P/2020 F3 designation…

That’s why you need to update your planetarium software to get the correct and accurate path for comets and asteroids.

Sidebar: Look up the diameter of Pluto. Go ahead! You’ll find a LOT of different values…

FOUR - cats

And finally there is the special case of comets. They are obviously affected by the Sun in terms of heating, outgassing, tidal, and sublimation processes. Closer to the Sun, the brighter it gets; further out, it dims. They seem to be fragile. So we get disintegration events which further dims the object. And then, mysteriously, unpredictably, comets brighten. I don’t know if scientists fully understand why but imagine there’s a pocket of ice trapped into a hard rock shell and it cracks open due to solar tidal forces and all the ice begins leaking out. I can see that type of thing happened and now more ice is added to the comet tail brightening it.

I think there’s been changes happening at the comet surface and interior that are causing the brightness to shift and change. So, again, we need to update our planetarium or planning apps to show this new data. And we need to corroborate values from our web resources. It can change every day!

News media outlets should not be used as official sources for brightness indicators 'cause they’re trying to “sell newspapers.”

Summary. This is what makes comets fascinating to observe because we never know what’s going to happen exactly. We don’t know exactly where they are from. Probably the Oort cloud. We don’t know if they will survive perihelion. C/2020 F3 did. It is difficult to measure the magnitude. The core is bright. The tail? Doesn’t exist, doesn’t show, is small, is big, has multiple trains (ion, dust), is growing. Ugh. I wouldn’t want to be in charge of measuring and recording this. Comets are often described as dirty balls of ice. They don’t roll off an assembly line with the exact same specifications. So every comet is different and its behaviour will be correspondingly random!

Bottom line is, all our resources like aerith, SkyTools, Telescopius, CalSky, SkySafari, are all dependent on humans updating data according to the latest discoveries and measurements.


Some additional thoughts.

Be aware of the quoted magnitude for comets (or any object). Is it the absolute or apparent value provided?

Periodic comets, one could argue, are getting smaller and smaller with each perihelion pass. So, arguably, dimmer each time.

By extension, any comet will get dimmer over time as material falls away, breaks off, and sublimates.

Found an interesting article from 2006 at Sky & Telescope on methods or techniques for assessing the brightness of a comet. Early on they say, "For centuries the reported magnitudes of naked-eye comets were very ambiguous."

Remember, atmospheric extinction becomes a significant factor for objects close to your horizon. People reporting magnitude estimates should also quote celestial altitude.


On November 2020, I found a post by Pedro of Simulation Curriculum in Cloudy Nights where he told the MPC their data was wrong! They fixed it. But it made me wonder about the cascading effect with the new data. It might not happen immediately across dependent apps...

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