How do You Calculate an Astrological Chart?

These days, computer software written by someone we barely know, if at all, calculates astrological charts for us. We ought to know how it is done, at least have some idea of it. If we don’t know how a chart is calculated we miss out on very edifying fundamental astrological knowledge. So I will explain the basic overview of how it is done, from an ancient and modern viewpoint.

I am not going to mention Ephemeris and Tables of Houses – which we few remaining souls who have enjoyed calculating charts by hand sometimes will remember fondly – because these are just the output of the calculations put into a handy reference format. What i will describe here is actually the way the epemeris and tables of houses are created, in ancient and modern times.

Ancient Method

Let’s talk about how it was done in the old days (and this is the recorded method in Surya Siddhanta).

It was done by looking up at the sky and practically observing how fast and slow the planets move through the stars – when they become erratic and move backward, and how they slow down and speed up. After making such observations carefully for many years ancient scientists developed math which described the motions of the planets reasonably well.

Using that math, from a given point of time as a reference one could extrapolate where the planets would be in reference to the stars at any other given time previous to or after the point of reference. Here is an example to make it easier to grasp.

If you know a car moves north at 20 miles per hour on average, and you know that 3 hours ago it was at the intersection of 4th street and 5th avenue, you can predict reasonably well that 2 hours ago it was 20 miles north of that point. 1 hour ago it was 40 miles north, currently it is 60 miles north. 4 hours ago it was 20 miles south of the reference point. Etc.

The main reference point for calculations, used by Surya Siddhanta is the planetary alignment at the beginning of Kali Yuga (the current Age of the world). Then, knowing how much a given planet moves each day on average, you can estimate where the planet is on any given day before or after the beginning of Kali Yuga.

As you can tell this is a system of estimates only, it is not precise. If you want to be more precise you have to periodically recheck the cars speed and reset the reference points to less remote times. The Surya Siddhata advises to do this – rechecking the planetary motion and resetting the reference point – and calls the reset-values ‘bijam’ or ‘seeds’ which keep the calculations acceptably accurate. This has not been done for a very long time, though, because we stopped using this estimative method a long time and moved to new methods more based on direct observation.

Modern Method

The modern method benefits from centuries of mathematical refinement and improvement in our observational tools. As such it is far more accurate to the fine details. However, it is essentially the same as the old system – based on observing the planetary motions, creating mathematical descriptions of those motions, and using those formulas to know the position of any planet at any point in the future or past. It’s just that the current formulas are very very accurate and not really as much in need of periodic updating as the more general estimations of the Surya Siddhanta were.

Ancient Reference Point

The old calculations were done in reference to the stars themselves, the nakshatra. New calculations are done in reference to a point that is more stable from our vantage point on earth – the location of the Sun at the vernal equinox.

Why the difference?

I don’t really know, of course. But I suggest that maybe ancient methods use stellar (nakshatra) based math because it is so much simpler and more direct to look up at night and make observations relative to the stars. It is much more abstract and complicated to make observations relative to the point the Sun occupied at the vernal equinox. Therefore only in modern times with the benefit of better formulas and tools does it become viable and practical.

The concept of “ayanamsha” – the difference between the sun’s position at the vernal equinox and a certain fixed star, roughly the same as precession of the equinoxes – is something very old, defined in the Surya Siddhanta. Since the old calculations were made in reference to the sidereal stars (nakshatra), Surya Siddhanta told us how to convert those calculations to positions relative to the sun’s position at the vernal equinox.

So the true, original use of ayanamsha is to convert sidereal calculations to tropical ones, which are then used for creating the chart. Only in relatively modern times, when we let go of the old calculation formulas of Surya Siddhanta and adopted (better) modern formulas did the ayanamsha become misapplied as a way of converting tropical calculations to sidereal – because modern calculations are done in reference to a tropical point – the vernal equinox.

Interestingly, the modern conception of precession of the equinoxes is significantly different from the Surya Siddhanta’s conception of it expressed via “ayanamsha.” The modern theory is that the vernal point CONTINUES to precess (move backwards) through the zodiac and will complete a full lap. The ancient Surya Siddhanta concept is that the vernal point differs from its sidereal references in an arc of +/- about 27 degrees or so, and cycles back and forth within that.

How to Figure Out Nakshatra and Rashi Of A Planet

The way to calculate a planet’s nakshatra and rashi is different depending on if you use the ancient or modern formulas. Using the ancient formulas, your math is relative to a certain fixed star – a nakshatra. Using modern formulas, your math is relative to the position of the sun at the vernal equinox.

So if you are using ancient formulas, the initial math itself would give you the sidereal positions of planets (meaning the positions relative to the stars, the nakshatra). You would then need to utilize ayanamsha to get the tropical positions (relative to the vernal point, in the twelve rashis).

If you are using modern formulas, the initial math itself gives you the tropical position in the twelve rashis, so you would need to utilize ayanamsha to get the sidereal positions relative to the 27/28 stars (nakshatras).

Interestingly, you cannot cast a horoscope without tropical calculations – because you have to know the time of Sunrise on a given day in order to find out the ascendant (the focal point of the whole chart). Sunrise changes relative to the Suns position north or south of the equator – i.e. the Sun’s tropical position.

The ancient way to calculate the ascendant is to first calculate the Sun’s position relative to the stars / nakshtra. Then use ayanamsha to make it a tropical position. Then calculate (using other formulas) the time of Sunrise. And you then know that at that time the ascendant was identical to the tropical position of the Sun. After that you just adjust for the time of day you want the chart cast for, using a proportionate ratio.

Modern calculations are the same, except that since they start with tropical coordinates there is no need for ayanamsha, except if you want the sidereal equivalent of the ascendat, which might perhaps be useful for knowing the nakshatra related to the ascendant, for example.