An accurate birth time is needed for calculating accurate dates of active and inactive periods during the long lasting cycles, especially when they concern the Ascendant, the Midheaven and the Moon positions of the birth chart. The Ascendant and Midheaven move so quickly that they can be compared to the second hand on a watch. If you have ever watched the moon move across the sky at night, you have noticed that it moves quite a bit in as little as fifteen minutes, especially noticeable when rising or setting. The Rising Sign, or Ascendant, can be off by that distance, in fifteen minutes. The birth pattern is fixed, but the planets keep moving around it day by day at their own rates of speed. For example, it takes Uranus about three to four years to cover the distance represented by a one hour error in the position of the Ascendant and Midheaven; half a sign. Another astronomical phenomenon, called retrograding, can cause a cycle affecting the Ascendant and Midheaven to be miscalculated by a year, even with a four to ten minute error. Cycles that effect natal Mars, out in orbit to Pluto, basically stay accurate even when the time is unknown, as these planets move slowly on a daily basis.

Minor errors up to two hours in birth time are not so serious, except, you still have to forget cycles to the Ascendant and Midheaven, till the birth time is corrected and known. If the error is one or two hours, cycles to the Moon will be off...sometimes an hour.....a week...a month....and in some cases about a year, as it all depends on the speed and movement of the cycling planet. A half hour error will mean that all the cycles except ones effecting the Ascendant and Midheaven will be fairly accurate in most cases.

If the birth time is unknown and the chart is done for noon; A twelve hour error will remove accuracy of all cycles to the Ascendant, Midheaven and the Moon. Cycles to the Sun, Mercury and Venus will be off in some cases and quite close in others.

The same errors in timing will occur if the birth Location (Longitude and Latitude) are incorrect. The difference being that it will only affect the angles; Ascendant and Midheaven.