In the ancient times, it was noticed that some lights moved on the sky while the others remained in the same position in relation to each other. Stars which appeared to be closed together were grouped into constellations. Stars which changed their position were called planets and the seven ones observable directly by eye were considered in order of speed of apparent movement Moon, Mercury, Venus, Sun, Mars, Jupiter and Saturn. Today a lot of ancient mysteries have been solved, but much more are still to be investigated.

This article is an introduction to some of the astronomy issues like absolute and apparent magnitude, relation between magnitude and distance, luminosity of a star, star's flux, spectral classes, colour index and temperature of a star, star distance, parallax etc.

Operating a voice system depends largely on the skills of the involved persons. Proper voice procedure training should provide the operators with the basic knowledge of how to improve their skills in the use of radio communication. Even the best systems can suffer from interference and because of this is possible than the other part cannot hear (understand) the information passed. Therefore it is extremely important that proper voice procedure is used to save time. Briefly, a voice procedure is a set of rules designed to provide security, accuracy and discipline when speaking on the voice loop.

The concept of proper motion in astronomy is defined as the apparent movement of a star on the celestial sphere, usually measured as seconds of arc per year. It is due both to the actual relative motions of the sun and the star through space. The proper motion reflects only transverse motion (the component of motion across the line of sight to the star); it does not include the component of motion toward or away from the sun. The most distant stars show the least proper motion. The average proper motion of the stars that can be seen with the naked eye is 0.1'' per year.

The Hipparcos and Tycho Catalogues are the primary products of the European Space Agency's astrometric mission, Hipparcos. The satellite, which operated for four years, returned high quality scientific data from November 1989 to March 1993. Each of the catalogues contains a large quantity of very high quality astrometric and photometric data.

In addition there are associated annexes featuring variability and double/multiple star data, and solar system astrometric and photometric measurements. Although in general only the final reduced and calibrated astrometric and photometric data are provided, some auxiliary files containing results from intermediate stages of the data processing, of relevance for the more-specialized user, have also been retained for publication. The global data analysis tasks, proceeding from nearly 1000 Gbit of raw satellite data to the final catalogues, was a lengthy and complex process.

The Lagrangian points are locations in space in the vicinity of two orbiting masses where the gravitational forces and the orbital motion balance each other to form a point at which a third body of negligible mass would be stationary relative to the two bodies.

There are a number of web resources describing the Lagrangian points, some of them offering a very good introduction and description of the 5 Lagrangian points, while others insist on the techical aspects of the problem. Nonetheless, in the following article we'll try to add to the non-mathematical and intuitive descriptions available on the web (like, for instance, the ESA page related to the 5 lagrangian points or the Wikipedia article) a detailed-mathematical view of the problem, starting from the technical defintion which states that the Lagrangian points are the stationary solutions of the circular restricted three-body problem.

This article makes reference to the satellite state representations which include:

1. The keplerian elements
Seven (or eight) numbers are required to define a satellite orbit. This set of seven numbers is called the satellite orbital elements, or sometimes "Keplerian" elements. These numbers define an ellipse, orient it about the Earth, and place the satellite on the ellipse at a particular time. In the Keplerian model, satellites orbit is an ellipse of constant shape and orientation. The Earth is at one focus of the ellipse, not the center (unless the orbit ellipse is actually a perfect circle).

2. NASA TLE format
This is the format used by NASA to distribute satellite elements in their NASA Prediction Bulletin.NORAD, or North American Aerospace Defense Command, maintains general perturbation element sets on all resident space objects. These element sets are periodically refined so as to maintain a reasonable prediction capability on all space objects. In turn, these element sets are provided to users, providing them with a means of propagating these element sets in time to obtain a position and velocity of the space object.
The most important point to be noted is that not just any prediction model will suffice. The NORAD element sets are “mean” values obtained by removing periodic variations in a particular way. In order to obtain good predictions, these periodic variations must be reconstructed (by the prediction model) in exactly the same way they were removed by NORAD. Hence, inputting NORAD element sets into a different model (even though the model may be more accurate or even a numerical integrator) will result in degraded predictions. The NORAD element sets must be used with one of the models described here in order to retain maximum prediction accuracy.
All space objects are classified by NORAD as near-Earth (period less than 225 minutes) or deep-space (period greater than or equal 225 minutes). Depending on the period, the NORAD element sets are automatically generated with the near-Earth or deep-space model. The user can then calculate the satellite period and know which prediction model to use.

3. Canonical elements
They have some special characteristics which make them useful for astrodynamics (they simplify the construction of perturbations solutions):

    * Poincare elements
    * Delaunay elements

Propagation means evaluation of the orbital position of the spacecraft through a series of timestamps using a specified orbital model and perturbations. Mainly methods for propagating a spacecraft in its orbit are divided as analytically and respective numerically.
While numerical propagation evaluates the motion of the satellite over many small timestamps, integrating the solution, to find the final spacecraft’s position after a given time span, the analytical method uses a set of equations to evaluate the discrete solution of a satellite’s position at a given time.
The numerical solution reaches the best accuracy, however due to the intensive computations requested (it has to integrate the entire period from the initial time to the current time) it has been successfully replaced in many cases with the analytical solution. This last one has however the disadvantage as the period of the propagation increases from the start condition, the errors accumulated increase as well.
The article presents both the numerical propagation solution (i.e. the differential equations) and the typical analytical solution used in the orbit prediction software branch (SGP/SDP NORAD models).

This chapter treats the issues of the classical orbits-description of the basic orbital concepts and definition of the Keplerian orbital elements (i.e. the perifocus, semimajor axis,eccentricity, perigee, apogee,the line of apsides, the orbit plane and the reference plane, the ascending node, the descending node, right ascension of the ascending node,the argument of perigee,the true anomaly, the mean anomaly, the eccentic anomaly, the escape velocity):
1. Elliptical orbits
2. Parabolic orbits
3. The hyperbolic orbit