Astrological charts represent the sky for a particular time, date and place. A natal chart, also called a birth chart or horoscope,is calculated for an individual and is the primary tool used for analyzing personality, comparing with other charts for compatibility,and as a base point for any forecast work. Charts can also be calculated for entities like nations, corporations or organizations. Progressed charts are used in forecasting, horary charts to answer questions. In the past astrologers needed at least three reference books to calculate a chart by hand. An ephemeris is a book of planetary positions covering a particular time period (year, decade,century, etc.). The positions of the planets usually are given once a day for Greenwich, England (the standard reference point)and then are interpolated for the time and place of birth. Tables of logarithms or planetary motion were used to do this. A table of houses was used to produce the houses of the chart (more on these later). Reference books for time zone changes and latitude and longitude were also required for an accurate chart. While it is still possible to use this collection of books to do a chart by hand (requiring about 15 minutes to an hour depending on the skill of the astrologer), most astrologers use computers to do the job today. Advanced programs like Matrix’s Win*Star 4.0 and Day Watch have all of the reference materials built-in so that accurate charts can be calculated in seconds. While it is useful for astrology students to have the knowledge to calculate charts by hand, the artistry of astrology is in the interpretation. The almost instantaneous creation of charts by computer allows astrologers to get on with their analysis more quickly and efficiently. Computer programs also print charts and store them for later retrieval.