Now that Ptolemy has described how to construct an astrolabe, he covers its usage. Specifically, Ptolemy begins when
both sun and moon could be observed above the earth at the same time.
As a brief reminder, what Ptolemy is really discussing here is what was necessary for him to make the observations that revealed the moon’s second anomaly. So what is given here is really in that context. As such, neither the sun nor moon is actually required for use of this instrument. In this case, Ptolemy is using the sun to align the instrument for use, and considering the moon to be the target, hence why the sun and moon are required. But, as we’ll see, a star can be used for alignment and there’s no reason one couldn’t be used as a target either.
Before getting started, I’ll repost the image of the instrument here so we can easily reference it:
So to begin,
we set the outer astrolabe ring [$5$] to the graduation [on the ecliptic ring $3$] marking, as nearly as possible, the position of the sun at that moment. Then we rotate the ring through the poles [$4$] until the intersection [of the outer astrolabe ring, $5$, and the ecliptic ring, [$3$] marking the sun’s position was exactly facing the sun.
This set of instructions is actually a bit lacking because it hides a lot in that word “set”. What really needs to be done here is that the position of the sun for the time the observations are to be done must be calculated. Then ring $5$ placed at that position on ring $3$. That’s what he means by set.
Once that’s done, ring $4$ is rotated on peg d until the shadow of the side of rings $3$ and $5$ that are towards the sun fall on the sides further from the sun1.
Essentially what’s being done here is we’re setting up the rings, $2$, $3$, and $5$, all of which relate to the ecliptic coordinate system to their proper position with respect to the equatorial coordinates since these can vary throughout the year.
However, this doesn’t necessarily need to be aligned via the sun.
[I]f we are using a star as sighting [i.e., orienting] object, we set the outer ring to the position assumed for that star on the ecliptic ring, [and then rotated the ring $4$ to such a position] that when we applied one eye to one face of the outer ring, $5$, the star appeared fastened, so to speak, to both [nearer and farther] surfaces of that face.
Oof. This is a rough passage but best I can figure, what Ptolemy is saying here is that we first must know the ecliptic longitude of the star in question. From that, we again set ring $5$ to that position on $3$, and then using the corner where the two intersect to site down, line it up with the star. Or perhaps the star could be lined up on ring $3$ and $5$ independently hovering just above their surface as you look from the side of the ring towards you to the more distant one2.
Regardless, that’s all that needs to be done to align the instrument. From there Ptolemy
rotated the other, inner astrolabe ring [$2$] towards the moon (or any other object we desired) so that the moon (or other object) was sighted through both sighting holes on the innermost ring [$1$] at the same time as the sun (or other sighting star) was being sighted [as described above].
Now that the equatorial ring is configured, the inner ring, $2$ can be aligned such that it points at the object in question. It is free to spin in ecliptic longitude since it is free to rotate around the ecliptic poles at e. Also, since ring $2$ has ring $1$ that rotates within it, this allows for determination of ecliptic longitude.
- Using shadows to determine when things are perfectly aligned with the sun is something we’ve done previously done with the solar angle dial.
- Which is not all that dissimilar from what I often do when taking the altitude and azimuth separately when using the quadrant.