Reports are coming in of a major outburst by a normally faint comet, P/Holmes (17P). It would normally now be expected to be around magnitude 17 but it appears to have undergone a huge eruption.

Amazingly, the comet has become a million times brighter, and has reached magnitude 2.5. It is starlike in appearance and so an easy object to see with the unaided eye, distorting the shape of the constellation of Perseus.

We don’t know how long the brightening will last, but the comet is well positioned to be seen all night from the UK. At midnight tonight, UK time (24/25 October), it will be at RA 03h 53m Dec +50 08′ in Perseus.

Update: In a special email alert issued by Sky & Telescope magazine, since I wrote the above, senior editor Roger W. Sinnott says that the latest startling outburst may be even stronger than one that occurred 115 years ago, in November 1892, when the comet was first spotted by English amateur Edwin Holmes.

He reports that A. Henriquez Santana, at Tenerife, in the Canary Islands, was first to notice the current outburst shortly after local midnight today (Wednesday, 24 October). The comet was then about 8th magnitude, but within minutes Ramon Naves and colleagues in Barcelona, Spain, caught it at magnitude 7.3. By the time it grew dark in Japan, it was visible from the centres of large cities and had reached magnitude 2.8.

Sinnott says that cometary expert Gary Kronk expects Comet Holmes to remain bright and grow from a starlike point to several arcminutes across over the next few nights as it makes its way slowly westward across Perseus.

Here is a hastily created map showing tonight’s position. For swift alerts to such exciting events, you can sign up to the Society for Popular Astronomy’s free, if quaintly named, Electronic News Bulletins.

Further update: Read the latest on Comet Holmes at the SPA’s website.

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