A birth chart is not going to be accurate without its two major components. An astrologer needs to get the person’s birth date and the exact time he was born. The time of birth is generally the time when you took your first breath and is when the chart is calculated from. There are times that the location is also needed to properly calculate the primary angles of the birth chart with the most accuracy
The conjunctions, oppositions and other astrological bodies are defined in the middler of the chart. On the outer portion of the person’s chart lies the twelve houses more commonly known as the zodiac signs.
A person’s birth chart is not constant and is always changing since the Sun, Moon, planets and the stars are always in different positions relative to the Earth. It is very personal and unique system of astrology that takes into account the exact time the person was born, the birth date and the location at which they were born.
Getting the exact time of your birth is the only complicated thing you may face when compiling a birth chart. It may be found on your birth certificate; still it cannot be accurate at times since it is usually rounded to the nearest quarter hour or half hour which makes the time on your chart approximate but not exact. So it is really necessary for parents to write down the exact time the infant was born. But make sure the location of birth is known as it is needed to compile a birth chart.
Many astrologers based their charts from the sun but it does not necessarily based on Earth. So it is indeed understood that the very important thing needed for a birth chart reading is the exact time as the location would be the sun. Each chart has a different construction and different interpretation. Needless to say, no birth chart is identical and it is needed to be recalculated using points respective of the planet.
Source : http://www.e2etrust.com/article/how-birth-charts-reveal-your-horoscope