Most people, even if they have no interest in astrology, know their Sun sign. All you need for that is an accurate date of birth.
You can determine the astrological sign for any of the planets with just an accurate date of birth, too.
The Moon moves too fast to use date of birth alone. Why? Because in the span of a single day, it can easily cross from one sign into another. We need an accurate time of birth to determine its sign.
Two other important astrological symbols move even faster. The Ascendant (the Rising Sign) and the Midheaven. They zoom across all twelve signs of the zodiac in a single day. That means they change one degree on the zodiac for every four minutes of time. Without an accurate Ascendant and Midheaven, an astrologer cannot accurately determine the signs on the cusps for the twelve astrological houses.
The Moon is incredibly important in forecasting. So are the “angles” of the chart, especially the Ascendant and Midheaven. That means any forecast depends on an accurate birth time.
There are applications of astrology where an accurate time is an absolute “must.” Vocational astrology is one. Relocation astrology is another. Both rely heavily on an accurate Midheaven. A difference of 10 minutes translates into a difference of 2 1/2 degrees at the Midheaven. That can mean an error of an entire state in an Astro*Carto*Graphy map.
Birth times that come from a family member’s memory can be suspect. Especially in families with several children and the birth was decades ago. A birth time recorded in a birth record, a baby book, or a birth announcement in the newspaper is more likely to be accurate.
Even a birth certificate might not accurately capture the birth time. Rounded off times at the hour, half-hour, or even quarter-hour are suspect, compared with a more precise time, like 10:34 AM.
Without a birth time, the astrologer has to make a guess. Some use midnight as a substitute (i.e., the beginning of the day). Some use sunrise. Most use 12:00 noon (making them off, at most, by twelve hours).
The effect of all this, is that the houses in your birth chart will almost certainly be way off when an astrologer substitutes something like 12:00 noon for your time of birth. You’ll likely find that your horoscope just doesn’t fit!
Let’s think through the example of 12:00 noon. Assume you were really born at midnight. Your birth chart will be shifted by as many as six signs.That’s like telling you your rising sign is Libra when it’s really Aries.
Astrologers have a technique for making a more educated guess of birth time. It’s called Birth Chart Rectification. By comparing important dates and facts about your life with the different potential birth charts, an astrologer can infer the probable time of your birth.
It really is worth the time (and expense) to obtain your time of birth from your birth certificate or some other reliable source!