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From: mangoe@umcp-cs.UUCP (Charley Wingate)
Newsgroups: net.sf-lovers
Subject: Re: visible civilization
Message-ID: <1095@umcp-cs.UUCP>
Date: Sun, 4-Aug-85 22:54:33 EDT
Article-I.D.: umcp-cs.1095
Posted: Sun Aug  4 22:54:33 1985
Date-Received: Wed, 7-Aug-85 02:19:13 EDT
References: <3076@topaz.ARPA> <1356@uwmacc.UUCP>
Organization: U of Maryland, Computer Science Dept., College Park, MD
Lines: 54

In article <1356@uwmacc.UUCP> demillo@uwmacc.UUCP (Rob DeMillo) writes:

>>I'm told that the Earth is the brightest radio source in this region of the
>>galaxy, so finding it shouldn't be too hard if you have radio telescopes.

>>    I'm certainly no expert, but if Earth is brighter than the Sun, in any
>>    band, I'll be very surprised.  You might double check with your source, 
>>    if you can; also about competition with Jupiter, and possibly Saturn.

>>    There is also the problem of the angular distance of Earth from the Sun,
>>    especially from very distant (ie much more than Alpha Centauri) stars.
>>    I've neither the time nor the inclination to go through the math, but I
>>    suspect it would be difficult, perhaps extremely so, to resolve Earth's
>>    signals from the Sun's.

>I'm not expert either, but I am a graduate student in Astronomy, and
>Alastair is right, I'm afraid. Jupiter is much brighter in the 
>radio (or anything!) region than the Earth, and Jupiter would be
>quite difficult to resolve from the Sun unless someone out there
>had VERY good resolution on his/her/its radio scope. Also, if Jupiter
>was resolved seperate from the Sun, that would mean someone out there
>wanted to check us out rather than give us a casual once-over...does
>this mean that we're the subject of someone's research project?

>(Incidently, for anyone who cares at all, one of the reasons that
> Jupiter is brighter than the earth in the radio region is due to
> the internal heat that it generates. A hot, gassious object like
> Jupiter would stand out like a sore thumb next to a cold, lump
> of slag like the Earth...)

As I understand it, the things that make the earth stand out are the 
following:

 1) It's very small and obviously associated with a star.  This makes it clear
    that whatever it is, it's a planet.

 2) In radio frequencies, it is analomously hot, and NOT on spectral lines.

 3) At certain precisely defined frequencies, it is quite bright-- sometimes.
    Certain radio telescopes, when operated as radars, are very bright.

If you look at the solar system from the right directions, there are three
radio sources: two thermal ones, and something substellar which has a really
wierd radio spectrum: it has lines that are not emission lines, and it is
really variable.  If your detectors are sufficiently sophisticated, you should
be able to "see" the earth.  But you have to look at it exactly right.  It
occults over a very long period, and you have to be looking off of emission 
lines.  This makes it difficult to find similar sorts of objects, compounded
by the fact that we have only been doing this for about 20 years, so that only
our very nearest neighbors could have noticed this.  Someone on Sirius,
however, wouldn't have too much trouble noticing that our system had something
really strange in it.

Charley Wingate