The following was presented at the Midland Astro Group NOVA course. The topic -- Celestial Spheres.
What is meant by Celestial Spheres? The sphere concept started with the teachings of Ptolemy who described the stars and planets embedded into rotating spheres which hung there likes jewels in the sky. Other great Philosophers extended this work and added other features to the spheres but with the central tenant that the Earth was at the center of the sphere and all
This model -- the so-called geocentric model was accepted by the church as the correct way of thinking about the heavens and that the angles and God were responsible for the movement of the spheres.
During the middle ages though there was much debate over this concept and as the church started to grow in influence it extended the spheres to include regions where each individual religion could call a home or their God. However by the end of the Middle Ages the common philosophies was that the external bodies were moved by external factors.
During the 15th contrary saw the rise of a new concept the heliocentric model did not place the earth in the center of the sphere.
Central to the understanding that this was a real concept was the work of Copernicus. His work in mapping and investigation of the stars and heavenly bodies highlighted that the most likely answer to the numbers he had written down was that the sun must be in the center of the sphere. However, during the 15th Century such an idea was not something you proposed else the church would generally incarcerate you...
So what did they think the spheres were made of? Well this is an interesting debate some say the moon is made of Swiss cheese but.... originally they believed that the spheres were hard and made of real material. While other believed the they were imaginary spheres really did not exist but contained the angles and God which moved objects around. Other observers such as Tycho had determined that the comets moved between the different spheres and therefore they could not be solid whilst eh work of Kepler in the motion of planets showed that the spheres were not spherical not were they solid as determined from the mathematical formulas he devised for planetary motion. However, Kepler did say that the spheres - if they did exist were no wider than the planetary objects that they held -- I have heard of hedging your bets....
Of course we know that the celestial sphere is not composed of these ridged structures but is much more of a dynamic structure in space and time. The term Celestial Sphere is now used to cover the coordinate systems that are used to define the positions of astronomical objects.
The Celestial Sphere in modern terms is essentially a reflection of the latitude and longitudinal co-ordinate system in the sky. It possesses a north and south pole and equators just as our ground based coordinate system does.
Celestial Equator -- this is a direct reflection of the Earths equator in space. as such it is tilted by 24 degrees (the earths tilt) . The location of the equator in the sky is dependent upon the position of the observer and is either at the horizon when viewed from the poles or directly overhead when viewed from the Earths equator.
he ecliptic -- the ecliptic is path that the sun follows through the sky. It is actually the path of the planet during its orbit. The ecliptic is essentially where all major planets orbit and is the area where most planets can be found. All planets deviate slightly from this plane with Mercury have the widest variance ±7 degrees.
The meridian -- this is an imaginary line from the north to South Pole which passes through the Zenith (see below) at the observers location. Its co-ordinate position changes depending upon the local time of day.
The prime meridian -- This is the location of ±0 degrees east/west the current of this passes through the Royal Observatory Greenwich.
The Zenith -- the point directly above the observer. Again the actual coordinate position of this is dependent upon the time and location of the observer.
The Nadir – just like the Zenith but the point that is directly below the observer’s feet (normally soil)
The Celestial Poles – thee are not to be mixed with the Zenith. Essentially they are extensions of the north and south poles into the stars. Items that are close to the poles and are above the horizon all the year from the observer’s location are called circumpolar. This status is dependent upon the latitude of the observer. Generally a good example of a circumpolar star is Arcturus.
Although the location of the Celestial Pole is known they are not static in time. The positions of the poles in the sky change over time. This movement is called precession of the poles. It is caused by the “wobble” of the earth and was originally noted by Hipparcus. The cause of the “wobble” is now understood and was attributed to Newton whom defined a theory to describe the precession. The basis of this is related to the concept that the earth is not a perfect spheroid.
It is known that the earth is slightly wider than it is tall known as an oblique spheroid. This physical nature coupled with the axial tilt of the planet causes a gravitational torque on the rotational axis of the planet. It is this that causes the precession of the poles.
the precession of the poles maps a perfect circle which is similar to the result of looking at a spinning top from the top when it is rotating
More difficult to define is the motion of the stars. A star possesses an absolute velocity and also a direction in space. This means that besides the movement of the pole we also have to account for the motion of the stars in space.
The term hour angle is used on most equatorial mount telescopes in the right accession axis.