From: Peter.Macinnis@p0.f508.n711.z3.fidonet.org (Peter Macinnis)
Newsgroups: k12.ed.science
Subject: SCIENCE HISTORY QUESTION
Date: Sat, 12 Dec 92 11:08:00 PDT
Organization: FidoNet node 3:711/508.0 - The Bay, Neutral Bay NSW

;rep:111,4220
While a quadrant is hard to make, most students should be able to put
together a cross-stave like this:  I have snatched a section out of a
the notes for the manuscript of a book under development (alas, VERY
slowly!).  Simple astrolabes are described in most science texts, but
nobody seems to have thought much about cross-staves, which are far more
fun!

        As you have seen, we cannot measure distances across the sky,
        but we can measure the angles made by the two straight lines
        going from our eye to any two distant objects.  As a rough rule,
        a fist at arm's length blocks out about 10 degrees, and a finger
        covers about 4 degrees.  Greater accuracy means using a
        cross-stave, one of the earliest instruments used to measure the
        angular distances between stars.

        You will need a piece of 41 x 19 DAR ("2 x 1" timber, dressed
        all round) at least 2100 mm long, and a small piece of wood,
        about 6 x 6 x 300 mm. You will also need a pencil, eleven small
        nails, a hammer, a 2 metre tape measure, and a rubber band.  To
        use the cross-stave at night, you will need a small penlight
        torch, and some white paint.

        At a distance of two metres, two nails separated by 35 mm appear
        to be just a degree apart.  If you look at it another way,
        imagine you are in the middle of a circle with a radius of two
        metres.  The circumference of the circle is 1257 cm, so 360
        degrees stretch over 1257 cm, and one degree covers very close
        to 3.5 cm, or 35 mm.  The cross-staff you are going to make
        relies on this simple fact.  The smaller piece of wood will be
        your cross-piece.  Starting 2 cm from one end, carefully mark up
        and draw lines 35 mm apart, across the wood.  Then rule a line
        along the centre of the wood from end to end.

        Tap in a nail at each mark on the cross-piece, keeping each nail
        carefully upright.  Do not drive the nail too deeply, or you
        will split the wood.  Then use the rubber band to attach the
        cross-piece to the stave, and measure to make sure the nail
        heads are exactly 2 metres from the other end.  You can also use
        pins with coloured heads, instead of the nails.

        Now all you have to do is to look along the cross-stave, and
        count the number of nails between any two things, to know how
        many degrees apart they are.  To measure larger angles, pull the
        cross-piece back to one metre, so each pair of nails marks an
        angle of 2 degrees.

        At night, the nails are hard to see.  Tape the penlight torch to
        the long arm, about one metre from your eye.  A dab of white
        paint on each nail will also make it easier to see the nails in
        the torchlight.

Now get out there and map!
                                peter

 # SLMR 2.1 # Let's hear it for the vertically challenged.



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