To determine the sign in which the moon and other planets fall into on any given day, it is necessary to consult an ephemeris or use an astrological computer program which will have a built-in ephemeris; these computer programs make it very easy and quickly calculate the natal chart (also called horoscope) so that the astrologer can spend more time interpreting the chart rather than calculating it.
As the calendar is created by the movement of the Earth around the Sun, and its orbit is very nearly circular, the sign in which the Sun falls on any day of the year is fairly predictable, though it will not vary much even with the effect of the extra day in leap years. These dates are given in the article that deals with each sign separately. Sidereal astrology also has to adjust for the precession of the equinoxes which produces a slow change of the time of year when constellations are visible in the sky at night over a 25,000 year cycle.
Constructing a horoscope
To create a horoscope, an astrologer first has to be certain of the exact time and place of the subject's birth, or the initiation of an event. The local standard time (adjusting for any daylight savings time) is then converted into Greenwich Mean Time or Universal Time at that same instant, which in turn is translated into the sidereal time at Greenwich. The astrologer will next consult a set of tables called an ephemeris, which lists the exact location of the sun, moon and planets for that particular year, date and sidereal time, with respect to the northern hemisphere vernal equinox or the fixed stars (depending on which astrological system is being used). The astrologer can then add or subtract the difference between the longitude of Greenwich and the longitude of the place in question to determine the true local mean time (LMT) at the place of birth to show where planets would be visible above the horizon at that precise moment in time and the place in question.
Horoscopes
The word "horoscope" is derived from Greek words that mean "a look at the hours". Other commonly used names for the horoscope in English include astrological chart, birth chart, astro-chart, celestial map, sky-map, star-chart, cosmogram, vitasphere, radical chart, radix or simply chart, among others. It is used as a method of divination regarding events relating to the point in time it represents. It forms the basis of the horoscopic traditions of astrology.