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I have to plan an excursion to try to find clear evening skies to try to see if we can spot comet C/2021 A1 Leonard about 10 to 15° above the ocean in the Southwest after sunset this weekend.

I'm completely baffled by this plot of magnitude vs time. It looks like something unexpected is happening or already happened with the comets brightness.

Am I too late?

Roughly what visual magnitude can I expect, and what (if anything) did we miss?

Is there an out-of-the-ordinary disparity between visual and CCD brightnesses?

From https://cobs.si/ and this long link

enter image description here

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Comets emit gas and dust at non-uniform rates. Sometimes they have outbursts and appear 1 or 2 magnitudes brighter than usual for a day or more. This is normal but unpredictable. 29P/Schwassmann-Wachmann does this rather frequently.

C/2021 A1 Leonard seems to have had outbursts on December 13 and 20, as Karl Battams explains in a Twitter thread. The model behind the thin red curve in the COBS plot may account for the forward scattering he mentions; there is a little bump when the comet was most nearly between the Sun and Earth. You can reasonably expect the actual visual magnitude to be close to the model prediction.

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  • $\begingroup$ Thanks! The weather is not cooperating so an excursion seems to have two factors against it; no the comet is not likely to be super-bright when we look, and it's not likely to be clear anyway. Have to wait until the next comet it seems. $\endgroup$
    – uhoh
    Commented Dec 23, 2021 at 22:25

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