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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: net.space
Subject: Re: Timing of comet showers
Message-ID: <6282@utzoo.UUCP>
Date: Sat, 11-Jan-86 19:52:05 EST
Article-I.D.: utzoo.6282
Posted: Sat Jan 11 19:52:05 1986
Date-Received: Sat, 11-Jan-86 19:52:05 EST
References: <8601102258.AA02103@s1-b.arpa>
Organization: U of Toronto Zoology
Lines: 48

> ...When the island of Tambora (I think it was) in Indonesia blew up back
> in the late 1800's (?), it triggered a "year without a summer" across
> much of the Northern hemisphere... snow in June, massive crop failures
> and famine, and so forth...

In fairness, it should be pointed out that these effects weren't quite as
universal as they are often presented.  Many areas of the world did not have
particularly unusual weather at that time (I believe it was something like
1816, although I don't have my references handy).  Unfortunately there are
few good quantitative weather records dating from that time -- even
thermometers were rare and crude then -- and it is difficult to get a clear
picture of global effects.  The weather in North America certainly was odd.

> ... probably much smaller than
> the ruckus raised by (for example) the massive erruptions that occurred
> in what is now Yellowstone...

Actually, massive volcanic activity is one of the "dark horse" theories of
massive extinctions.  There is one province of India that mostly sits on
a layer of lava *hundreds of meters* thick; that must have been one #@$%&@
of an eruption, and it has been cited as a possible cause for the Cretaceous-
Tertiary event that wiped out the dinosaurs.

> I suspect that the impact of a 5-mile-diameter comet would have been at
> least as impressive... especially if it happened to strike in an area
> with a substantial body of magma at a shallow depth (on a mid-ocean
> ridge, for example).

There was a suggestion a while ago that Iceland is the remains of the impact
that caused the Cretaceous-Tertiary extinction, in fact.

> I saw a mention somewhere within the past couple of years that
> there is historical evidence that people actually saw the impact
> of a good-sized object on Luna...

I believe the event you're thinking of is the observation by English monks
in the 1100s (?) of a lunar impact that may have been the formation of the
crater Giordano Bruno.  Recently there has been a report that Bruno, the
Tunguska event early in this century, and an impact storm observed by the
Apollo seismometers early in the 70s, are all consistent with a single
swarm of debris in a specific orbit, presumably the remains of an extinct
comet.  It's been tentatively dubbed the Canterbury Swarm, after the monks'
observation.  There is some chance of observing the Swarm at close range;
it will make a near-miss on us within the next few years. (A specific date
was given, but this is all from memory and I don't remember it.)
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry