# Quatrain HTML version

This is a drawing of a very crude astrolabe. Picture yourself trying to figure out how
far Venus is above the horizon on a particular night. You see Venus in the night sky, but
you do not know exactly how far above the horizon it is. The Earth‘s horizon is zero
degrees. If you crane your neck back like this, and look directly straight above you into
the sky, that‘s 90 degrees. So any star or planet you look at will be between 0 degrees--
the horizon-- and 90 degrees—straight above you. But how do we know what the exact
altitude is? You look through the bottom end of the straw, and tilt the straw upward at an
angle until you can see Venus through the other end of the straw. Now you can see that
as you tilt the straw upward, the washer on the string is going to swing back towards you,
from 0 degrees to 90 degrees. When you find Venus through the straw, you merely look
at the number of degrees marked by the string and the washer, and you have your answer.
A medieval astrolabe works in much the same way. Morse showed his children the
astrolabe. ―This ring at the top is called the ‗armilla.‘ That is the Latin word for ‗ring.‘
That is what you hold onto when you are measuring with the astrolabe. Screwed into the
center of the astrolabe is this movable lever called the ‗alidade.‘ It looks kind of like a
spinner in a board game. Attached to the ends of the alidade are little raised pieces of
metal with holes in them. You put your eye up to the sighting vane and look through the
holes at the planet or star you are trying to find. The sighting vane acts just like the
straw. You spin the alidade around to the point where you can see the star or the planet
through the holes on the sighting vane, and then the alidade, like the string and the
washer, will point to the correct number of degrees. And then you have your altitude.