The first literate humans, living in Mesopotamian cities like Ur and Erech more than 5,000 years ago, seemed to believe that comets caused bad things to happen or, at the very least, they foretold that bad things would happen. Either way, they regarded a new comet in the evening sky as a portent of some future disaster.

If they were right, then the time has come for us to don sackcloth and ashes, because a new comet appeared last week in the constellation Perseus.

Well, almost.

The "new" comet turns out to be a familiar one with a period of 6.9 years, now about 100 million miles from Earth. It was discovered by British amateur astronomer Edwin Holmes in 1896 and, according to custom, given his name.

Comet Holmes was, until last week, a tiny ball of dusty ice only a couple of miles in diameter, held together by electrochemical forces as much as by gravity. Its gritty surface reflected just enough sunlight to make it shine at 17th magnitude, far beyond the reach of unaided human eyes. It then brightened abruptly to magnitude 2.5 and became the third-brightest object in Perseus, brighter than all but two of the constellation's stars.

It remains about that bright as I write this.

It is clearly visible even to my feeble eyes and is a rather large and quite unstarlike fuzzy blob when viewed with binoculars.

Try finding it about 7:40 tonight. It will be about halfway down from the "W"-shaped asterism of Cassiopeia to the northeastern horizon and a little to the left of Mirfak, Perseus' brightest star. If you have trouble using this description, try consulting the finder chart at the Web site of Sky and Telescope Magazine,

How did comet Holmes manage to increase its brightness almost a millionfold in only a few days? Some kind of explosion seems to have blown a large quantity of dust and grit from its surface out into the surrounding space.

Moving outward at more than 1,000 miles per hour, this dust now occupies a volume nearly equal to that of the sun.

But what caused the explosion? Not many astronomers seem willing even to guess about that in print right now. So we can be pretty sure that no one knows.

Jon Nance is professor emeritus at Missouri State University.