About Celestial Globes And Armillaries

If you look up in the sky, it appears as if you are at the centre of a vast crystal sphere with the stars fixed on its surface. This sphere is the celestial sphere. It has no particular radius; we record positions of the stars merely by specifying angles. We see only half of the sphere; the remaining half is hidden below the horizon. In this section we describe the several coordinate systems that are used to describe the positions of stars and other bodies on the celestial sphere, and how to convert between one system and another. In particular, we describe altazimuth, equatorial and ecliptic coordinates and the relations between them. The relation between ecliptic and equatorial coordinates varies with time owing to the precession of the equinoxes and nutation, which are also described in this chapter.

Our Place in the Universe, 2017

Using a simple device, the gnomon, ancient astronomers could accurately plot the apparent movement of the Sun. They found that the solar paths varied day by day. The Sun traced a circular path across the sky from east to west, but this pattern shifted north or south from one day to the next. These variations repeat themselves after one year. By exchanging information with observers from other parts of the world, astronomers also knew that the Sun moved differently depending on the observing location. When people in different parts of the world gathered data on the Sun's paths, it became obvious that these daily patterns were not the same everywhere. Observers in the British Isles found that the Sun was very low in the sky in the winter and did not rise high above the horizon. In Greece and Babylon, a vertical stick cast a shadow throughout the year. However, when one traveled south-for example, from Athens to Alexandria-the Sun rose higher in the sky, and the shadow at noon was shorter. In southern Egypt, the Sun could be directly overhead and the shadow could disappear altogether. For example, the Sun could be overhead in Abu Simbel but not in Luxor, just 500 km north. From simple observations using a gnomon, the daily trajectory of the Sun can be measured throughout the year. These solar paths (azimuth and altitude vs. time) can be plotted on the celestial sphere; the results are shown in Fig. .1. For an observer in the mid-northern temperate zone, the Sun appears to move along circular paths which are inclined at an angle with respect to the horizon. As the year progresses, the daily paths of the Sun shift in parallel, while maintaining the same angle with the horizon. If one extrapolates the observed paths of the Sun beyond what can be seen above the horizon, one might hypothesize that the Sun moves in complete

In making observations of the sun and the stars, the surveyor is not interested in the distance of the celestial bodies from the earth but merely in their angular positions. It is convenient to imagine their being attached to the inner surface of a hollow sphere of infinite radius of which the earth is the center. The imaginary sphere is the celestial sphere. The portion of the celestial sphere seen by the observer is the hemisphere above the plane of his own horizon. The reference plane passes through the center of the earth parallel with the observer's horizon, but the radius of the earth is so small in relation to the distances to the stars. Figure 1 represents the celestial sphere. Figure 1. Celestial Sphere Zenith Nadir North North Celestial Pole South Celestial Pole O DEFINITION OF TERMS Celestial Polesare the points on the earth's surface of the celestial sphere pierced by the extension of the earth's polar axis. Celestial Axis-is the prolongation of the earth's polar axis. Zenith-is the point where the plumb line at the place of observation projected above the horizon meets the celestial sphere. It is also defined on the celestial sphere vertically above the observer. Nadir-is that point on the celestial sphere directly beneath the observer, and directly opposite the zenith. Great Circle-a great circle of a sphere is the trace in its surface of the intersection of a plane passing through the center of the sphere. Observer's Horizon-a great circle on the sphere where a plane perpendicular through a plumb line at the place of observation and passing through the center of the earth, cuts the celestial sphere. Observer's Vertical-a vertical line at the location of the observer which coincides with the plumb line and is normal to the observer's horizon. Celestial Equator-a great circle which is perpendicular to the polar axis of the celestial sphere. It is an extension on the plane of the earth's equator outward until it intersects the celestial sphere. Vertical Circle-a great circle passing through the observer's zenith and any celestial body. Such a circle is perpendicular to the horizon, and represents the line of intersection of a vertical plane with the celestial sphere. Hour Circle-a great circle passing through a celestial body and whose plane is perpendicular to the plane of the celestial equator. Meridian-is the great circle of the celestial sphere which passes through the celestial poles and the observer's zenith. This circle is both a vertical and an hour circle. The position of any point on the surface the sphere may be fixed by angular measurements from two planes of reference at right angles to each other passing through the center of the sphere; these measurements are called the spherical coordinates of the point.

European Journal of Physics, 2016

Eratosthenes? teachings with a globe in a school yard Mirjana Bo?i? and Martial Ducloy The Sun lightens and enlightens: high noon shadow measurements Vukota Babovi? and Milo? Babovi? Introduction to solar motion geometry on the basis of a simple model

Tartu Ulikooli Ajaloo Kusimusi, 2013

Der Globusfreund/ Globe Studies, 2025

Three ancient globes have been completely preserved: the Mainz Globe, the Kugel Globe and the Farnese Globe. Their chronological classification is uncertain. This comparative study presents a method of reconstructing the uranographies from the surviving texts (uranologies) of Aratus, (Pseudo-)Eratosthenes, Hipparchus and Ptolemy, and comparing them (a) with each other and (b) with the depictions on the globes. The findings are that the constellation figures cannot be considered individually, tell different stories and have different cultures of origin, and that the Mainz globe (despite its errors) shows the most comprehensive of the three pictorial inventories of the star-heavens.

Ciência e Natura

Spherical Geometry is the basis for what we know as Positional Astronomy. This is one of the oldest approaches to Astronomy as a science, being used by ancient Greeks and possibly by other people before that. Concerning a more formal current mathematical description, there are several types of coordinate systems with respect to the celestial sphere. Each system differs in the choice of its referential plan. Several connections between elements of different types of coordinates are well known once it is a well-developed field. However, the reader will not find in the literature complete mathematical proofs for such formulas. Thus, this article fills that gap. We present in this text several formulas relating the coordinates Zenital Distance, Hour Angle, Azimuth, Declination and Geographic Latitude and their mathematical proofs in detail, explaining all possible steps. Our main contribution is in the form of the presentation and deduction of classical results from Positional Astronomy.

The Physics Teacher, 2014

It is customary to employ a semi-spherical scale model to describe the apparent path of the Sun across the sky, whether it be its diurnal motion or its variation throughout the year. A flat surface and three bent semi-rigid wires (representing the three solar arcs during solstices and equinoxes) will do the job. On the other hand, since very early times, there have been famous armillary spheres built and employed by the most outstanding astronomers for the description of the celestial movements. In those instruments, many of them now considered true works of art, Earth lies in the center of the cosmos and the observer looks at the whole "from the outside." Of course, both devices, the scale model of the sky and the armillary sphere, serve to represent the movement of the Sun, and in this paper we propose to show their equivalence by a simple construction. Knowing the basics underlying the operation of the armillary sphere will give us confidence to use it as a teaching resource in school. From the scale model of the sky to the armillary sphere, A. Gangui, R. Casazza, and C. Paez, The Physics Teacher, 52 (7): 403-405 (2014). http://cms.iafe.uba.ar/gangui/didaastro/