Tracing the history of medieval astronomy
Today’s project: Finish the scale.
The process on this was very straightforward albeit tedious. At the edge of each line for a degree, I made a slight notch with a razor knife. For each of the 810 transversal dots, I took a small nail and hammered it lightly.
Continue reading “Instrumentation – The Great Quadrant: Day 11”
Today was a much shorter day in part because I’d lost track of time causing a late start and in part because we’re running low on things to do on pieces that don’t currently have paint drying on them.
In particular, getting the scale transferred was one of the larger things remaining. So we started the night by masking off the scale:
Continue reading “Instrumentation – The Great Quadrant: Day 10”
In the last post, we derived equations to demonstrate that the right ascension (α) and declination (δ) of an object can be gotten by knowing four other variables: altitude (a), azimuth (A), sidereal time (ST), and latitude (φ).
In this post, I’ll do an example of using these equations to do just that. For my data, I’ve jumped into Stellarium and selected Altair1 as it’s a nice bright star that I’ll certainly be observing.
Continue reading “Data: Converting Alt-Az to RA-Dec – Example”
Last month, I had a post that briefly introduced the two primary coordinate systems for recording the position of objects on the celestial sphere: the Altitude-Azimuth (Alt-Az) and Right Ascension-Declination (RA (α)-Dec (δ)) systems. There, I noted that Alt-Az is quick and easy to use, but is at the same time nearly useless as objects fixed on the celestial sphere do not have fixed coordinates.
Instead, astronomers2 use the RA-Dec system because fixed objects have fixed positions. My modern telescope does allow for this system to be used rather directly because it has an equatorial mount which tilts the telescope to match the plane of the ecliptic instead of the plane of the horizon. Additionally, it is motorized to allow it to turn with the sky, thereby retaining its orientation in relation a coordinate system that rotates with the celestial sphere. Thus, once it’s set we’re good to go.
However, the quadrants Brahe used were neither inclined to the ecliptic nor motorized. Thus, measurements were necessarily taken in the Alt-Az system and would need to be converted to RA-Dec to be useful. Here, we’ll explore how that conversion works3. Continue reading “Data: Converting Alt-Az to RA-Dec – Derivation”
We’ve finally hit the last chapter in Book I. In this chapter our objective is to “compute the size of an arc of the equator”. At first pass, that doesn’t seem to have much to do with the title. Arcs of equator vs rising times?
However, Earth is a clock, rotating once every 24 hours. Thus, if we know the length of an arc, we know something about when an object following that arc through the sky will rise and set because it’s a certain proportion of 360º per 24h. Notice that if you actually complete that division, it comes out to an even 15º/hr. That’s not a coincidence.
Fortunately, to work on this problem, we won’t even need a new diagram. We can recycle the one from last chapter. Again this time we’ll be wanting to determine all sorts of arc lengths, but we’ll start with the one where $arc \; EH = 30$º.
Continue reading “Almagest Book I: Rising Times at Sphaera Recta”
As this project has progressed, I’ve picked up a number of books related to it. So others can know what I’ve read (and have available should they be interested in reading them themselves), I’ve compiled a list of the books in my medieval astronomy library, as well as some other excellent books on historical astronomy. Continue reading “My Library”
We’re getting very close to the end of construction, so it’s the point where it’s down to a lot of small details. To start, here’s a better picture of the axle assembly:
I’d shown a picture last time, but today, we added a brass fitting to help tighten the portion that goes through the central column, and using a pipe wrench, tightened the portion that will go through the arm so it cannot rotate, forcing the quadrant arm to rotate on it whereas during the test assembly last time, the arm caused the axle itself to turn. The cap on the end still goes on finger tight so it can easily be removed to disassemble the instrument for transport. Continue reading “Instrumentation – The Great Quadrant: Day 9”
It’s said that a project involves blood, sweat, and tears. Today was two of the three. Somehow have managed not to get splinters till today, but got a pretty good one. It was also hot in the workshop:
Continue reading “Instrumentation – The Great Quadrant: Day 8”
In the last post, we derived a method to calculate the angle between the celestial equator and ecliptic and generalized it to be applicable for any angle. Here, I present the table derived from those calculations.
Before going on the several post detour in developing several theorems and making extra sure we understood a few, we stated the goal was to determine the length of an arc at various points between the celestial equator and ecliptic4.
Continue reading “Almagest Book I: The Arcs Between the Equator and Ecliptic”