#87 | ast02 – Armillary sphere

Hi guys!

Today, it will be a short post on how to use an armillary sphere and how to read the values on this tool.
An armillary sphere is a spherical object made of rings. This object has a lot of using purposes: finding a star, knowing the path of the sun on the sky, the position of the sun depending on the month and the day, the position of constellations on a given time, etc. This tool was created a long time ago by the ancient Greeks and by the old China. It was upgraded over the era.

An armillary sphere is a measuring tool represented like a model of the objects in the universe. The Earth or the sun is at the core of the armillary sphere. Outside, the celestial sphere is represented by a rotating sphere shows the movements of the sun and its trajectory (known as the ecliptic) on the 12 constellations of the zodiac, symbolized by a band. The larger fixed sphere represents the firmament, the fix stars on the sky. The overall sphere is lying on an equatorial ring representing the horizon and that shows the time division, and a meridian ring is on the top, to show exactly the latitude of the sphere. The downside part of the overall sphere can’t be view in the sky.

How to use an armillary sphere?

Depending on where you are, the latitude will change and it is the first parameter that determines the position of the sphere. Of course, like the telescope, the angle of your latitude must be calculated compared to the polar axis: the angle must face the North (See |ast00 and |ast01). At this point, you have two options: looking for what zodiac constellations you could see from where you are, or knowing the movement of the sun, everything in correlation with the day and month of the year.

For the first option, you need to know what day and month it is: in fact, the zodiac band is separated into 12 parts. That means that each part takes an angle of 30° of the 360° circle. It is not exact, but 1° means approximatively 1 day. So 30° is one month and the entire circle means one year. Each zodiac is shown on one particular month of the year: for example, May is the fifth month of the year and at this time, you can see the Taurus constellation which is really low on the horizon from my latitude. If you rotate the armillary sphere, you can also see, the Gemini, the Cancer, and the Leo constellations. That part of the sky is the beginning of Summer season.

For the second option, the ecliptic is the movement of the sun, and this trajectory follows the path of the Zodiac. It means that for the 12 May, you are at 12° on the Taurus part of the Zodiac band. The sun is at this point, and by rotating the sphere, you can see the entire movement of the sun that day. What you can observe for example is that the sun doesn’t really rise on the East but at the South-East of the Earth. The sunset is at the South for the same day. It means for example that the fact that the sun rises on the East and set on the Ouest is only possible if your localization is on the Equator. So that common knowledge we teach in the elementary school is basically wrong.

fig.01 – An armillary sphere. This one is a pedagogical tool and is not entirely as precise as a real modern scientific one, but it is enough to know exactly how this old system works.

What did I learn?

  • How to read an armillary sphere
  • What is the purpose of the Zodiac band
  • How to “calculate” the position of the sun in the sky at a given time
  • The movement of the zodiac constellations and the sun

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