|MadSci Network: Astronomy|
I will try to give you all the information that you need for your calculations. First, let us look at the structure of our Galaxy relative to the Earth and the Galactic center. The Milky Way Galaxy is a spiral type and our solar system is located in the Orion arm. The next spiral arm in toward the center of the Galaxy is the Sagittarius arm. (The infrared instrument on IRAS produced a view of the Milky Way Galaxy [see the image at the top of this document] and people have tried to produce maps showing our general location in the Galaxy.)
You asked about Sagittarius A, which is the brightest radio source in the constellation. This is generally thought to be the direction to the Galactic center, and is located to the right of the star gamma Sagittarius. With any good star chart of the Sagittarius region, you can locate gamma Sagittarius.
Currently, the Sun is in Sagittarius from mid-December to mid-January. At this time the Sun is between the Earth and the stars in the constellation Sagittarius. We are closest to the Galactic center six months after that.
What I did was to go to a star map at this location and place myself at the prime meridian at midnight to find the date at which gamma Sagittarius was directly overhead. I determined this to be about 2000 July 6 at midnight. This year that is the date we would be closest to the Galactic center.
Because the Earth wobbles a bit, its orientation to the stars precesses about once each 26000 years. This means that the day we will be closest to Sagittarius A will migrate approximately one month for each 2000 years.
I hope that this helps you with your calculations.
Try the links in the MadSci Library for more information on Astronomy.