WHAT IS AN ARMILLARY SPHERE, AND HOW DOES IT WORK?
Updated: Oct 28, 2022
The armillary sphere is an ancient astronomical instrument that demonstrates the movement of the heavenly bodies. It was introduced in China and Europe several centuries before the birth of Christ. If in the old days it was mainly used by astronomers, now they have mostly become museum exhibits and decorative items. But even in this respect, this astronomical instrument can give a good idea of the apparent motion of the heavenly bodies as the Earth revolves around the Sun.
The armillary sphere consists of several rings (armillaries) that represent important celestial circles, forming a cage with the Earth in the center. The spherical framework of rings sits on a base and can rotate freely around its axis. Depending on the observer's location on the globe, this framework can be placed on the lower frame at a precise angle so that it can demonstrate the apparent movement of the Sun across the celestial sphere at any time of the year.
It is important to note that the armillary sphere forms the skeleton of the celestial sphere, but it must be distinguished from the celestial globe, whose main task was to map constellations on the celestial sphere.
Parts of an armillary sphere
The armillary sphere has several rings, each of which has its specific name and each of which represents one important circle on the celestial sphere. The picture below shows one such armillary sphere, where you can see all its most important details with their names. What each circle shows, we will look at in more detail later.
Setting up an armillary sphere
An armillary sphere can show the path of the Sun on the celestial sphere at any point on the globe, at any time of the year. In order to know the celestial events with sufficient accuracy, the armillary sphere must be placed on the lower frame at the right angle, based on the observer's position on the Earth. For this, it is necessary to know the geographic latitude coordinate of the observer. That's why degrees are marked on the meridian circle, as You can see in the upper picture. The meridian is a great circle that passes through the poles of the celestial sphere and also through an imaginary point directly above the observer (zenith).
For example, I live in Estonia which is located about 60 degrees north of the equator. To set up the armillary sphere correctly, according to the geographical latitude, I need to rotate it so that the meridian circle intersects the horizon 30 degrees below the equatorial circle. At this point, 60 degrees is also marked on the meridian circle. The sky above the horizon is always visible to me and makes up half of the entire celestial sphere.
The ecliptic ring is the widest circle on the armillary sphere, on which the constellations of the zodiac are usually inscribed. It intersects the equatorial ring in two places and touches two tropical circles. The ecliptic is the apparent path of the Sun on the celestial sphere. In fact, of course, only the Earth moves around the Sun, and it only seems to us that the Sun moves on the celestial sphere. The ecliptic ring and the equatorial ring are at an angle of 23.5 degrees because the Earth's axis of rotation is not perpendicular to the plane of the orbit but is tilted by 23.5 degrees.
The apparent annual journey of the Sun on the celestial sphere passes through 12 zodiacal constellations. The Sun takes about 30 days to travel through each zodiac sign. While the entire ring of the ecliptic shows the Sun's annual journey on the celestial sphere, you can also find small lines on it that usually mark specific days of the year.
It's important to mention that when I looked at armillary spheres for sale on Amazon and elsewhere online, most of them seem to have more of a decorative value. Many do not have the ecliptic circle attached to the sphere correctly. That means the constellations of the Zodiac are shifted and do not give a correct overview of which Zodiac constellation the Sun is in on a certain date.
For example, at the end of July, the Sun is in the constellation of Cancer. Although the days are not individually marked on the ecliptic ring on the armillary sphere in the lower image, we can still roughly determine a specific date (in our image it is approximately July 25). By rotating the armillary sphere 360 degrees on its axis, we can see the path of the Sun on the celestial sphere on that particular day.
In addition to seeing the maximum height the Sun rises above the horizon that day, when it crosses the meridian circle, we can also see where it rises and sets. At the point where the mark of a given day on the ecliptic ring crosses the horizon in the east, is sunrise on that day, and at the point where it disappears below the horizon is sunset on that particular day.
Equinoctial Colure, vernal, and autumnal equinoxes
Depending on the position of the Earth in its orbit around the Sun, the Sun can be higher or lower than the equatorial plane of the Earth. We can imagine the celestial equator as an imaginary plane if we extend the equatorial plane of the Earth to infinity, to the celestial sphere. Half the year, the Sun is above that plane and half the year below that plane. When the Sun is above the celestial equator, the Sun is higher in the sky in the northern hemisphere, and it is spring or summer. When the Sun is below the celestial equator, the Sun is lower in the sky, and it is autumn or winter in the northern hemisphere. In the Southern Hemisphere, the opposite is true - when the northern hemisphere is warmer, the southern hemisphere is cooler and vice versa.
As you can see from the upper image, twice a year, on the autumn and spring equinoxes, the Sun appears to be exactly on the celestial equator. The autumnal equinox is around September 23rd, and the spring equinox is around March 20th. Astronomical autumn and spring begin on these days, and night and day are of the same duration throughout the earth. Although historically the spring equinox occurred in the constellation of Aries and the autumnal equinox when the Sun was in the constellation of Libra, it has now shifted due to the precessional wobbling of the Earth axis. Currently, the spring equinox is when the Sun is in the constellation of Pisces, and the autumn equinox is when the Sun is in the constellation of Virgo.
Equinoctial colure is a great circle that passes through the poles of the celestial sphere and the two equinoxes.
Solstitial Colure, Tropic of Cancer and Tropic of Capricorn
You can see the tropic of the Cancer ring above the ecliptic ring on the armillary sphere. It touches the ecliptic ring at the point where the June solstice occurs. June solstice occurs between June 20 and 22. At this moment, the Sun is at its highest point in the sky of the Northern Hemisphere and at its lowest point in the sky of the Southern Hemisphere, where the winter solstice occurs at this moment.
This day is the longest day of the year in the Northern Hemisphere and the shortest day of the year in the Southern Hemisphere. The Earth's axis of rotation is maximally tilted toward the Sun in the northern hemisphere and maximally tilted away from the Sun in the southern hemisphere. If you rotate the armillary sphere 360 degrees, You can see the path of the Sun on the June solstice, which runs along the Tropic of Cancer.
However, the name can be a bit misleading. In the distant past, the Sun was indeed in the constellation of Cancer when the June solstice occurred, but now it has shifted due to axial precession and occurs at the moment when the Sun is in the constellation of Taurus, near the constellation of Gemini. The image below shows the position of the Sun on the June solstice taken in Stellarium software. It can be seen, that the Sun is barely in the Taurus constellation at this moment and will move into the Gemini constellation in just a few hours.
The December solstice occurs around December 21, when the Earth's axis is maximally tilted away from the Sun in the northern hemisphere. At the same time, however, the Earth's axis is tilted maximally towards the Sun in the southern hemisphere, and there is the summer solstice at that moment. In the southern hemisphere, the Sun is in the highest position in the sky and there is the longest day of the year. In the northern hemisphere, the Sun is at its lowest point in the sky and has the shortest day of the year. On the armillary sphere, the ecliptic ring touches the tropic of the Capricorn ring on the December solstice. It is the southernmost latitude on the celestial sphere to which the Sun moves during the year. Currently, at the moment of the December solstice, the Sun is in the constellation of Sagittarius, not in the constellation of Capricorn, as the name of the circle may suggest.
Solstitial colure is a great circle on the celestial sphere that passes through the poles and the two solstices.
Arctic ring, and Antarctic ring
The two smaller rings near the poles of the armillary sphere are called the Arctic ring and the Antarctic ring. The Arctic Circle is near the North Pole and the Antarctic Circle is near the South Pole. The Arctic circle is located 66.5 degrees north of the equator, and the Antarctic circle is the same distance south of the equator.
The Arctic Circle is the southernmost latitude in the Northern hemisphere where the Sun does not set for a full day on the June solstice. At the same time, south of the Antarctic circle, the Sun does not rise above the horizon, and it is polar night. Understandably, as you move even closer to the North Pole, the polar day lasts longer, and as you move closer to the North Pole, the polar night lasts longer.
Six months later, when it's the December solstice, things are exactly the opposite. On this day, the Arctic circle marks the latitude where the Sun does not rise above the horizon for the whole day. At the same time, south of the Antarctic circle, the Sun does not set for 24 hours.
We briefly got acquainted with the construction of the armillary sphere and saw that all these rings represent some important circles on the celestial sphere. In a way, the armillary sphere is like an analog computer that helps us predict celestial events throughout the year. However, with the development of technology, they have acquired a more decorative value nowadays. Even so, I find that this ancient astronomical instrument can give a fairly good intuitive idea of what is happening in the heavens above and helps to understand the apparent motion of the celestial sphere. If you're planning to buy an armillary sphere that can, for example, accurately determine the position of the Sun in the sky over the course of a year, You should make sure it's adjustable. It must be possible to place it on the frame at the correct angle according to the observer's latitude, and its rings must be correctly positioned in relation to each other.