#unequal hours

[historysurvivalguide]

So, How to Build an Astrolabe: Part Five

Plate to represent the Unequal Hours Arcs

The plate currently shows the various arcs for almucantar and azimuth (see part four). The new section will cover the fun (but sometimes confusing) construction for the unequal hours! They form 12 segments from 11 lines radiating from the below previous sections.

This is a step-by-step guide to the complete construction of an astrolabe that I think might exist. At least, in any accessible form

As always, my goal is for this to be as accessible as possible for any kind of reader, so if there are any questions, please feel free to reach out

So, How to Build an Astrolabe: Tympan Plate – Unequal Hours - History Survival Guide

This week, build an astrolabe’s tympan—latitude—plate to represent the arcs for Unequal Hours   Tympan Plate: Unequal Hours Throughout the y

🏳️‍🌈🚀 Enjoy! Stay safe and keep learning, fellow chrononauts 🚀🏳️‍⚧️

[historysurvivalguide]

Astrolabe: Unequal Hours Update

Spent some time this weekend putting together the finished touches for the unequal hours write-up. The writing itself needed a big re-write for clarity, but it is coming along and assuming I haven’t forgotten something, all the diagrams should be done too. Here’s hoping

[historysurvivalguide]

Astrolabe Unequal Hours

Due to seasons, the amount of daylight for each day of the year can vary. Without the tilt of the Earth causing the seasons, the length of daylight and the length of night will always be 12 hours.

However, on Earth, the shortest day of the year (the Winter Solstice) is about 7 hours long and the longest day of the year (the Summer Solstice) is more than 14 hours

This information is visible on the astrolabe by referencing the arcs for the Unequal Hours. Unequal Hours, sometimes referred to as Seasonal Hours, is a way of predicting the number of daylight (or night) for a given day in the year

Because it is influenced by the tilt of the Earth, the change in daylight through the year becomes more and more pronounced as an observer moves todays the poles. At the equator there is almost no change from day to day and in the polar region, it can alternate between 24 hours of sunlight or darkness

Because unequal hours are influenced by the latitude, they change for each latitude and must be redrawn

Here are two examples to explain how to draw the unequal hour arcs. It is the same problem (more to come when I finish the full write up) but one uses trigonometry and one uses spherical projection

These both describe the same method for finding unequal hours on the astrolabe. The one on the left is from James Morrison’s Astrolabe and the one on the right is from my notes using spherical projection.

Both work, but depending on the audience it can be more or less confusing for a first time reader

Astronomical and astrological treatises (16th century) from the Beinecke Rare Book and Manuscript Library