Picture from: http://spaceplace.nasa.gov/t-shirt/en/

Stellarium is a planetarium software with a 3D simulation of the night sky... and it is free!

There are a lot of planetarium software packages on the market and these have a whole host of functions to enable the astronomer to learn his way around the sky, get to know constellations, where to find planets etc., at any given date and time of the year at any location on the Earth’s surface. With the onset of GoTo systems, they even link up to the telescope so that with the click of a mouse, the telescope will slew to the selected object. However, I have found that, even though the data that is displayed is incredibly accurate, when you look at the computer screen and then look up at the sky, the difference between the graphical representation of the stars and the real twinkling in the sky is immediately apparent.

Stellarium renders 3D photo-realistic skies in real time with OpenGL. It displays stars, constellations, planets, nebulae and others things like ground, landscape, atmosphere

Azimuth - The direction of a celestial object from the observer, expressed as the angular distance from the North or Nouth point of the horizon to the point at which a vertical circle passing through the object intersects the horizon.

One of the most remarkable features of our solar system is that nearly all of the revolutions and rotations are in the same direction.

From a point high above the north pole of the solar system the planets are revolving about the sun and rotating about their axes in a counterclockwise direction. This holds true also for the asteroids. If the planets and asteroids were formed from merely random accretions the would be an even mixture of the directions of revolution and rotation. The sun itself also rotates in a counterclockwise direction.

The satellites of the planets also generally revolve and rotate in a counterclockwise direction.

Of the thirty something satellites only six do not do so; they are said to have retrograde motion.

Venus and possibly Uranus are the exceptions to the counterclockwise rotations of the planets.

Venus travels around the sun once every 225 Earth days but it rotates clockwise once every 243 days. This peculiar combination gives it a day with respect to the sun of 117 Earth days.

Uranus is tilted on its side about 90° so its direction of rotation is ambiguous. Its angle of inclination is usually given as 98° which would mean that its direction of rotation is not retrograde. If its direction of rotation is presumed retrograde then its angle of inclination would be 82°.

If observed from one night to the next, most planets appear to move from West to East against the background stars ... most of the time.

Occasionally, however, the planet's motion will appear to reverse direction, and the planet will, for a short time, move from East to West against the background constellations. This reversal is known as retrograde motion.

Sidereal period, the time required for a celestial body within the solar system to complete one revolution with respect to the fixed stars—i.e., as observed from some fixed point outside the system.

The sidereal period of a planet can be calculated if its synodic period (the time for it to return to the same position relative to the Sun and Earth) is known; the sidereal period of the Moon or an artificial satellite of Earth is the time needed for it to return to the same position against the background of stars.


Retrograde motion, in astronomy, actual or apparent motion of a body in a direction opposite to that of the (direct) motions of most members of the solar system or of other astronomical systems with a preferred direction of motion. As viewed from a position in space north of the solar system (from some great distance above the Earth’s North Pole), all the major planets revolve counterclockwise around the Sun, and all but Venus and Uranus rotate counterclockwise on their own axes; these two, therefore, have retrograde rotation. Of the known satellites of the planets, a minority display retrograde revolution. These include the four outermost moons of Jupiter; Phoebe, the outermost moon of Saturn; and Triton, the largest of Neptune’s moons. The orbital planes of the satellites of Uranus are tilted so greatly that the description of these bodies’ motion as either retrograde or direct has little meaning. The revolutions around the Sun of all known asteroids are direct; of the known periodic comets, only a few, one of which is Halley’s Comet, move in a retrograde orbit.

A separate sense of the term retrograde motion refers to the apparent brief reversal of the motion of a planet as seen from Earth; the effect depends upon the difference in orbital speeds of the planets.