Tracing the history of medieval astronomy
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”
Happy Summer Solstice everyone!
Since one of the things Ptolemy was concerned with was the angle between the ecliptic and celestial equator, I built a solar angle dial to help determine this. Given that the measurements needed to be taken on the solstices (or a solstice and equinox), I set up the instrument today to take the measurement.
