[meteorite-list] Re: Asteroid?

From: Matson, Robert <ROBERT.D.MATSON_at_meteoritecentral.com>
Date: Thu Apr 22 09:53:37 2004
Message-ID: <AF564D2B9D91D411B9FE00508BF1C86901B4E5D5_at_US-Torrance.mail.saic.com>

Hi Tom, Ron, Sterling and List,

I agree with Ron that what you likely saw was the effect
of a plane passing in front of the sun. But you and
Sterling bring up an interesting topic -- that of transits.

> Don't hold your breath waiting for an asteroid to pass
> between you and the Moon or watching for asteroid shadows.
> It's not that we don't have enough passing asteroids, it's
> that the shadow path is a tiny momentary line on the Earth's
> surface. A few thousand feet away and there would be no
> shadow. You have to be in exactly the right spot (and time).

Actually, you will never see an asteroid shadow unless the
asteroid is either very large or very close -- in either
case, it probably means very bad news for you and just about
everyone else on the planet!

Asteroid transits of the sun are quite rare, but they do happen.
Unfortunately, they are basically impossible to observe, even
with a large telescope. It's not because they are fast (they
typically last on the order of an hour or two) or because you
need to be in exactly the right place (since they tend to be
visible from the entire sunlit side of the earth) but because
the asteroids themselves are so tiny and so distant. Their
silouettes are well below the resolving limit imposed by our
atmosphere.

For comparison, transits of the sun by Mercury and Venus *are*
visible (thanks to their much greater sizes) -- in fact there
was a transit of Mercury only a few years back that was visible
in binoculars (filtered of course!). It was a grazing transit
as I recall, but I was able to see it in California.

With a suitable filter, a transit of Venus would be visible
to the naked eye (looking like a very black sunspot moving
across the sun over the course of a few hours). There hasn't
been a Venus transit since the 19th century, but one is coming
up soon. (If people are interested I can post specifics of
this transit tomorrow when I'm at work.)

Lunar transits are of course extremely rare since it requires
the transiting body to not only come closer to the earth than
the Moon's distance, but also relatively close to the line
joining the centers of the earth and Moon. No known asteroids
larger than 500 meters in diameter are expected to do so any
time soon as far as I'm aware (and perhaps none of any size).

However, there *is* a type of transit that you can see with a
little effort: the transit of a manmade satellite. Solar
transits are tough because you need a full aperture solar
filter to observe such an event, and the satellite needs to
be quite large (e.g. the International Space Station). However,
lunar transits by sunlit satellites are quite easy to see.
You just need a series of clear nights when the moon is
favorably positioned an hour or so after sunset (or an hour
or so before sunrise). Lunar transits by satellites brighter
than 5th magnitude occur at least several times a week for
a given location. If you're willing to drive within a 10-mile
radius, you can virtually guarantee such a transit will occur
nightly (when the moon has the right phase). Software packages
can predict these events practically to the second, and can
be used iteratively to determine where you need to be in order
to see a central transit (satellite passes right through the
center of the moon).

Enough rambling ... time for some sleep! Cheers, Rob
Received on Tue 24 Dec 2002 03:47:50 AM PST