Astrological Techniques and Uncertain Birth Times

by Hank Friedman

Updated May 9, 2020

Note: This article first appeared as a section of my article, The Optimal Use of Astrology Software, but the topic is so important that I decided to significantly expand it to make it more useful to both beginning and professional astrologers.

You might also read The Importance of an Accurate Birth Time.

Also, please understand that I'm not trying to discourage anyone by writing this article. I just want astrologers to use the astrological methods that can work, given doubtful birth times.


When a client comes to us for a chart reading, we want to give them the most accurate and worthwhile information. In order to do so, we rely upon an array of astrological techniques, many of which are contingent upon the accuracy of the person's birth data.

Our dependence upon an accurate birth time is astrology's Achilles heel. Every time an astrologer is given a bad birth time, the reading is likely to go awry. And when people receive inaccurate readings due to wrong charts, they may no longer trust astrology. This is one of the fundamental reasons why astrology has gotten a bad reputation. It is therefore crucial that we know when a birth time is likely to be inaccurate, and what astrological techniques to use (and not to use) when giving readings with approximate birth charts.

The Bottom Line

I decided to start this article with a brief summary of what to do when the person's birth time is imprecise.

First of all, if you think it has been rounded off (e.g. 11:00 am or 2:30 pm), but are confident that the birth time is correct within a 15 minute leeway or so, then as long as the person's chart doesn't have an Ascendant at the very beginning or end of a sign, reading the natal chart should go well. You can also utilize progressions, returns, and transits (except transits to the Ascendant and MC). Accurate compatibility analysis is also possible. (Do be careful in interpreting house placements of planets near house cusps too.)

If the birth time is even more questionable, stick to interpreting the Solar Chart (i.e. a chart done for either Sunrise or at noon on the birth date, with the Sun rotated to the Ascendant). You can still interpret transits to natal planets except the Moon (and not transits to the Angles).

In cases where the birth time could be 15 minutes off or more, you should avoid all astro-mapping techniques, harmonic charting, Solar Arc's and Profections.

1. Birth Time Recording

If someone gives us a birth time of HH:00, HH:15: HH:30 etc. (where HH indicates the hour, like 11:00), it is reasonable to expect that the birth time may have been rounded up or down. (Of course, statistically, almost seven per cent of people will actually be born at these times, so such birth times might be accurate.)

If a birth time is incorrectly recorded, it is more likely to be earlier than the recorded time, rather than later. There are many possible reasons for recording of inaccurate birth times. The hospital clock itself can easily be off by 5 minutes or more; the birth complicated; the parents excited and focused on the new baby; the time rounded up or down to the nearest quarter or half hour by the nurse, midwife, or doctor, etc. I have also found that mothers do not always remember birth times accurately and sometimes confuse the birth times of their children; or switch AM and PM, especially with a birth time near noon or midnight.

Just this week I had a client tell me that he was born 11:06 pm instead of the previously given 11:30 pm his mother told him. His new chart, with Scorpio rising, fit him much better than the Sagittarius Ascendant of the wrong chart. (That event actually inspired me to revise this article.) This also highlights the problem that when a client comes for a chart reading, they usually expect to be given a reading using the birth time they provided.

We used to have to be very wary of daylight savings time and other time zone issues, too, but today's astrological programs with built-in time change atlases are, in the main, quite accurate. For U.S. births, there are some years when the states of Illinois and Pennsylvania are ambiguous, but most good software will alert you to these issues, so that you can find out exactly what was written on the birth record (e.g. if CST is written in, the hospital adjusted the birth time).

Note: for births after 1970, I used to respect However, I have recently found errors on that website, and also found that birth times and time zones in China are often inaccurate. The best place to check time zone information currently is or a very reliable astrology program like Solar Fire, Shri Jyoti Star, Sirius, or Janus.

Astrologers who want to explore the charts of people born centuries ago will run into additional issues. Early timekeeping was quite imprecise. Sundials originally measured "temporary hours" of varying lengths depending upon the seasons (e.g. in the summer in the Northern Hemisphere, daytime hours were longer than 60 minutes, and at nighttime, shorter than 60 minutes). In fact, standard (60 minute) hours were conceived of in the 13th century, when Abul Hassan introduced the idea of making all hours of equal length, and only in the 15th century were equal hours in general usage.1

Additionally, the issue of the precision of timekeeping devices arises. Sundials could be accurate, if used correctly -- on sunny days, of course -- to about a minute of clock time. Other timekeeping devices -- such as water clocks -- were significantly less precise. Galileo and Huygens made breakthroughs in clock-making in the latter half of the 17th century. Huygens' first clocks lost a only a minute a day, and his later clocks were accurate to about 10 seconds per day.2 Astrologers, therefore, should be very careful about trusting the accuracy of any birth charts calculated prior to the 18th century.

2. Astrology Software

With the exception of many of the free and inexpensive astrology programs and apps, and those without time-change atlases, we generally can trust the accuracy of the calculations done by software. If you have any doubts about an app or astrology program's accuracy, you can test it here.

Modern astrology programs are dazzling in their range of features. We astrologers can now create planetary return charts, locational astrology maps, tertiary progressed charts, harmonic charts, and many other charts and listings that we couldn't calculate easily by hand.

We can, however, fall into the trap of trusting the precision of software too much, without realizing that an inaccurate birth time can destroy the validity of many astrological techniques. And astrology software is not a panacea. It cannot compensate for either data entry errors or birth time uncertainties.

I recently was asked by a novice if he could use a well-known professional astrology program to rectify his chart. (He fully expected me to say, "Yes".) The answer is an emphatic "NO" for two reasons:

1. There is no program that automatically rectifies a chart correctly -- I have tested them all. Even the "best" programs (i.e. ones that performed a bit better than the others) only did so after I "tweaked" the settings to conform to my knowledge of what factors should be included (transits -- by Mercury through Pluto -- as the only technique used). And even then, the best programs gave potential ranges of birth times, not any actual birth time. Rectification simply cannot be automated.

2. Only professional astrologers with many years of experience can, with or without software, rectify charts effectively. To do so requires an in-depth knowledge of, and years of experience in using, predictive methods, as well as the ability to isolate the chart factors that are most likely to precipitate life events in each chart.

(I recently wrote again on using software to rectify charts here.)

3. Issues Specific to Vedic Astrology.

There are two confounding factors in the desire to calculate Sidereal charts precisely. The first is that there is no universally agreed upon point in the sky that represents the beginning of the Sidereal Zodiac (i.e. 0 degrees Aries). While the Lahiri commission in India did come up with an ayanamsha that is now generally accepted by the majority of astrologers, many Vedic astrologers, including a few renowned ones, use significantly different starting points.

Second, even if the beginning of the Sidereal Zodiac was agreed upon, astrologers assume that the rate of precession is constant. In fact, however, astronomy tells us that, in all likelihood, it is not. As a result, over time, our Sidereal charts may become more inaccurately calculated.

I believe that Vedic astrology uses Whole Sign Houses (where the whole First House, for example, is the sign of the rising sign) as their house system, at least in part, to minimize the problems of birth time inaccuracy. When using Whole Sign Houses, only when the Ascendant changes sign do planets change their house position, so that for a period from one to two-and-a half hours, the charts stays the same.

If a person is born with the Ascendant near the boundary of a sign (or nakshatra), it is very important to cast a chart for the adjacent sign (or nakshatra) to see if that chart better fits the person's life. (In Western astrology, if the Ascendant is at the beginning or end of a sign, it is also wise to explore whether the adjacent rising sign fits better, but few or no planets will change houses.)

Fortunately, many professional Vedic astrologers already use varga charts and dashas to rectify birth charts (which is much easier to do than in Western astrology). I recently did a reading for a client who had had many Vedic readings. Because her birth time placed the Ascendant near the beginning of a sign, I asked her about her marital history. Her answers immediately showed that her birth time was wrong, and I was able to quickly rectify the chart.

4. The Effect of Birth Time Inaccuracies on Specific Astrological Techniques.

A. Natal Charts. With a 15 minute error in birth time, the degree of the rising sign for a chart with a Pisces or Aries Ascendant (signs of short ascension north of the Equator) can often shift by 7 degrees, and up to 20 degrees for latitudes far from the Equator (e.g. births in Scandinavian countries).

In other words, the more Northern or Southern the birth place, and the faster the Ascendant rises, the more critical it is to have a very accurate birth time. For people born in the Northern latitudes, (Tropical) Aquarius, Pisces, Aries, and Taurus Ascendants rise the fastest, while for people born in Southern latitudes, Leo, Virgo, Libra, and Scorpio Ascendants rise fastest.

If we ignore this issue when working with approximate birth times, we will be delineating charts where the house positions will be off, we won't be able to tell which planets are strongest (based upon their proximity to an angle), and the all-important Ascendant may even be in the wrong sign.

B. Astro-Mapping. If a birth time is just 10 minutes off, the Astro*Carto*Graphy planet rising and culminating lines will be about 135 miles off! A 15 minute birth time error causes Local Space map lines to shift by as much as 300 miles or more, if a city is far away from the birth place. [The Local Space lines radiate from the birth place, and so cities close to the birth place show smaller shifts from changes in birth time than do places further away.]

Astro-mapping methods are virtually useless unless the birth time is very accurate. For astro-mapping to be reliable, the birth time cannot be more than 5 minutes off. In other words, astrologers shouldn't use Astro*Carto*Graphy or other astro-mapping techniques for a person with a birth time that is likely to have been rounded off (e.g. 2:00 AM).

C. Transits. Most astrologers consider the transits of Uranus, Neptune, and Pluto to the angles -- the Ascendant and Midheaven -- to be among the most important transits in a person's life. What they don't realize is that an error in the birth time of just 15 minutes can make the timing of these transits to angles off by a minimum of two months (when the outer planet is moving quickly and the ascendant sign is of long ascension -- like Leo, Virgo, Libra and Scorpio in Northern latitudes) to over one year if the outer planet is moving slowly (i.e. near its station), to a maximum of seven years for someone with Pisces or Aries -- short ascension signs in the Northern Hemisphere -- rising! In other words, unless the birth time is very accurate, the timing of transits to angles must be considered impossible to determine with any precision.

D. Vedic methods. As mentioned in the previous section, most Vedic astrologers use whole sign houses. When a 15 minute error in the birth time changes the rising sign, the chart changes drastically because every planet changes house, Ashtakavarga values are completely altered, and the entire reading becomes invalid. Even when a 15 minute shift does not change the rising sign, the onset of Vimshottari Dasha and Bhukti periods can shift from 3 weeks (if the Moon is moving slowly) to 3 months (if the Moon is moving quickly). Also, most of the Varga charts change (and therefore cannot be relied upon), e.g. the Navamsa chart will usually change.

I recently did a birth chart reading for twins. Because they were born only 10 minutes apart, both their Western and Vedic birth charts were identical with each other's. However, their Bhava charts, Navamsha charts, and a large number of their divisional charts were very different. And their life courses, as a result, as read by Vedic methods, will be very different. It again showed me how great a difference of 10 minutes in the birth time can make in a person's life. Every professional Vedic astrologer knows that there are birth times where even a couple of minutes change will radically alter all of the charts, and the person's life represented by them.

E. The Parallax Moon. Most astrologers don't realize that they base their readings upon the position of the Moon as calculated from the center of the Earth, not at the location where the person was born. While it remains to be seen which Moon position should be used for which astrological techniques, the difference between the two of up to one degree (when the Moon squares the MC) creates a significant ambiguity in the Moon's position.

I was recently contacted by a person who swore her Vedic program was computing the Dashas incorrectly. It turned out that the program's author had preset the software to use the Parallax-corrected Moon position (i.e. measured from the surface of the Earth, not its center) and this resulted in large differences in the dates when each Dasha started. Thorough research needs to be done to determine which astrological approaches should be done with the Moon reckoned from the center of the Earth, and which from the actual place of birth.

F. Solar Returns (aka Varshaphal). If a birth time is 15 minutes off, the Ascendant and Midheaven of the Solar Return can be about 3 degrees off, occasionally rendering some of the planet positions by house questionable. Solar Returns are therefore not as sensitive as most of the other techniques mentioned here, and a good predictive method to use in these cases.

G. Progressions and Directions. A 15 minute difference in birth time causes only a 3 degree difference in the Secondary, Minor, and Tertiary Progressed Ascendant and Midheaven, and very little difference in the positions of the progressed planets. Therefore, progressions are a good technique to use with approximate birth times.

Solar Arc directed angles, on the other hand, shift like natal chart angles do, up to 10 degrees, so for extreme latitudes (e.g Alaska), Solar Arc angles cannot be trusted (but Solar Arc planetary positions can certainly be used).

H. Compatibility Analysis. The analysis of the inter-aspects between charts, which is the most accurate method of assessing compatibility, is not very birth time sensitive (with the exception of aspects to the Angles in Short Ascension Ascendant charts, and of course, aspects by the Moon if the birth time is completely uncertain), but the placement of one person's planets into the other person's houses is again quite sensitive to birth time inaccuracies.

While Composite and Davidson charts are not very sensitive to birth time inaccuracies, Coalescent charts are quite sensitive for charts with Short Ascension Ascendants.

However, if you do not know the birth time at all of either person, all methods of compatibility analysis will prove unreliable, because you will not know the planets in or ruling or aspecting the 7th house, and therefore will not be able to assess relationship patterns.

[I remember a time when a client gave me her partner's birth time and the comparison looked very good. Then she called back with a different birth time (on the same day) and the comparison didn't work well at all.]

Sensitivity of Astrological Techniques
to 15 Minute Birth Time Error

Astrological Technique

Result of 15 minute birth time error


Rising sign in natal chart

For Short Ascension Ascendants 7 -20 degrees shift in Rising Sign

very significant

outer planet transits to the Ascendant

events at least two months off if planet moving swiftly for Long Ascension Ascendants,
or up to seven years off if planet is near a station for Short Ascension Ascendants

very significant

Solar Return Ascendant

3 degrees off of the Solar Return Ascendant


Progressed Ascendant

3 degrees off for Progressed Ascendant


Solar Arc Directed Ascendant

8 degrees off for Short Ascension Ascendants


Profected Ascendant

7 degrees off for Short Ascension Ascendants



map lines will be 135 miles off

very significant

Harmonic Chart Ascendant

birth time error shifts Ascendant greatly

very significant

Antiscia chart's Ascendant

7 degrees off for Short Ascension Ascendants


Local Space chart

focal planets off for Short Ascension Ascendants


Vimshottari Dasa

3 weeks to 3 months off, depending upon the speed of the natal Moon


Varga charts

~50% chance the D-3 & D-4 will change,
all other vargas Ascendants change

very significant

In the table above, the term Short Ascension Ascendants refers to Aquarius, Pisces, Aries, and Taurus Ascendants for births above the equator. Note: if a person has a Long Ascension Ascendant (e.g. Leo, Virgo, Libra, and Scorpio for births above the equator), the techniques mentioning Short Ascension Ascendants in the table can be employed with somewhat imprecise birth times.

5. Can we ignore inaccuracies?

Geoffrey Cornelius, in his book The Moment of Astrology, suggests that chart readings can be accurate even when the birth time is completely wrong. In my experience, I have found that when the birth time is wrong, it becomes glaringly obvious very quickly. In order to practice our art effectively, we must be reading the correct charts.

Once I had a client who waffled between an AM and PM birth time. She settled on PM, and I prepared the work up. Shortly into the reading, it became obvious to me that the PM chart didn't fit, and I switched to talking about factors that were true for both an AM or PM birth, and the reading ended up going well.

6. Solutions: How to give accurate readings with Approximate Birth Times

As astrologers, we always prefer to work with charts with accurate birth times. And Thank God that, at this point in history, we usually can. But what can we do to serve clients without precise times of birth? We need to, first, stay aware of the varying sensitivities of our astrological techniques to birth time inaccuracies, so that we can keep our readings accurate, and our understanding sound.

If the birth time is at all approximate, we cannot and should not use astro-mapping techniques, harmonic charts, several Vedic approaches (such as Ashtakavarga, divisional chart analysis, and exact Dasha/Bhukti timings, etc.), or outer transits to angles. In some cases, even the natal house positions become unreliable.

A. No Birth Time. If the birth date is correct, but the birth time is completely uncertain, the best strategy is to calculate charts for midnight, noon, and midnight the next day to see which factors remain constant throughout the day, and only focus on those factors during the reading. For example, most planets will stay in the same sign all day and therefore can be delineated by sign, and most of the aspects between the natal planets will be present throughout the day, etc.

B. Approximate Birth Time. If the birth time is probably not more than 15 minutes off (e.g. the chart has a rounded off birth time), then calculate the charts for 15 minutes before the given time, the given time, and 15 minutes after the given time, to see if the Ascendant or Midheaven changes, any planets change house (or even sign), etc. Again, only delineate the factors that stay the same during this 30 minute time range.

You will be able to address planets in signs and houses and aspects (except for of those that change within the time range of uncertainty), as well as house rulerships (unless the Ascendant or house cusps change sign), as well as progressions, Solar Returns, transits to planets (but not to angles), and -- for Vedic astrology -- delineate dashas and yogas, too.

It is wonderful when our chart readings click, and we need to do everything we can to keep it so. By being aware of the effects of errors in birth time on astrological techniques, we can practice our art more effectively. May we all do astrology honor by staying conscious of these principles.


1. History of the Sundial at

2. Classical time, by J J O'Connor and E F Robertson at derived from: A Elzinga, Christian Huygens and the elimination of time, Acad. Roy. Belg. Bull. Cl. Sci. (5) 73 (10) (1987), 394-404.

Click to Return to Return to Home Page

Send e-mail to Hank Friedman  by clicking here

If the above email link doesn't work, please send me an email to:

Copyright © 2020 Hank Friedman --- ALL RIGHTS RESERVED