First Mark II Results

Over the weekend, I built a second astrolabe -- the Mark II -- to make it possible to take more accurate sightings. The Sun cooperated on Monday and I was able to take morning, midday, and afternoon sights. If you are interested in working the sights yourself, here is the raw data. Times are in GMT:

11:40:17       21° 30'

15:15:47       61° 30'
21:01:50       33°  0'

Actually, these are the average of sightings taken from both sides, to minimize any lopsidedness in the device.

You can use my actual location for the DR location: 40° 51' N, 73° 24' W.

If you plot the resulting Lines of Position (LOPs), you will get this interesting picture:

Test of Mark II Astrolabe

Like the test of the Mark I astrolabe, the plotted position is south and west of my actual position. In this test, the location was 90 miles off. 'Not bad for such a crude instrument', some might say, but annoyingly, it is much worse than the result obtained with the Mark I instrument.

Test of Mark I Astrolabe

What gives?

I'm not sure, but I have one theory to test. Both plots have similar shapes, and both results are skewed mainly to the south. I suspect that, in a misguided attempt to position the point of the gnomon on the scale, I am turning the astrolabe too far away from the Sun. As the video in my previous post demonstrated, turning the astrolabe away increases the altitude reading, which pushes the result to the south (the sun is higher in the sky as you move south in northern latitudes.)

The next time the Sun cooperates, I will turn the astrolabe away from the Sun just enough to see the gnomon's shadow. This will yield the most accurate result possible with this type of astrolabe, I believe.

I have an idea for the Mark III astrolabe which could completely eliminate this source of error. Unfortunately, I have no idea how to build it! More on that idea next time.

Next Up: As Good As It Gets