Astronomy Software - Professional Recommendations

[Janus]

Why the Earth is in Gemini constelation. Shouldn't be in Cancer ?

I assume that you mean why is the Sun actually in Gemini rather than Cancer as according to the Horoscope?

Due to the precession of the equinoxes, The Sun drifts through the Zodiac at a rate of 1.4° per century. The Horoscope signs where named 2500 years ago when this was not known, and the Astrological dates have not been adjusted to match it.

The precession of the equinoxes is caused because the Earth wobbles slowly like a top as it spins on its axis. This causes the Tropical year(spring equinox to spring equinox) to be a slightly different length than the sidereal year (the time it takes for the Sun to return to the same position relative to the stars). This in turn causes the Seasons(and the calendar date) and the position of the Sun relative to the Stars to slowly drift with respect to each other.

 

[collinsmark]

If you want astronomy software that's quite powerful, and good to look at too, you might research TheSkyX from Software Bisque.
http://www.bisque.com/"

[Edit: If time is important, this software package allows you to "Input any date from 4,712 B.C. to A.D. 10,000 and any time to show a beautiful star chart for your location." (source: http://www.bisque.com/help/theskyx%20pro%20info/welcome.htm" , in the upper left hand corner click on "TheSkyX Professional Edition Website," then "Standard Features and Sample Screens," then on the right panel scroll down to the "Control the Date and Time" section.)]