Chart Calculations

[original date 2005-03-26 06:22:50]  Recently I was asked:  are the positions of planets in an astrological chart real? Do they correspond to what’s in the sky? How accurate is it?

The short answer is Yes, these days charts are quite accurate with respect to planetary positions.  In my friend’s chart Saturn is located at 27 Capricorn, using the sidereal zodiac. Or approximately 54 degrees from the galactic center. Or on a western astrological chart it’d be labelled as located at 21 Aquarius, using the tropical zodiac. They all mean the same place in the sky…. X degrees measured along the ecliptic plane (the plane of the solar system, that arc in the sky) from their own starting point. 

Usually we don’t measure how far above or below the ecliptic plane a planet is, because they’re never far away from it. But most astrology programs can tell us that too. Saturn’s declination was -15 degrees that day, which means 15 below the ecliptic plane. If we went back in a time machine to the day my friend was born and looked into the sky we would see Saturn located at the far end of the constellation Capricorn, slightly below the ecliptic plane, i.e. in the position the charts give. As long as we knew approximately where the ecliptic plane was in the sky, or what the constellation Capricorn looks like, we could find Saturn just fine using the information from the astrology program. The location the astrology program gives us corresponds to the correct position in the sky.

Astrology programs work out the position of the planets using an ephemeris. I have an ephemeris in book form (more than one actually). It has tables listing the positions of the planets each day at midnight. If you want to know where a planet is any time inbetween you do arithmetic to extrapolate between midnight of one day and the next. The computer software that calculates charts contains similar tables and an arithmetic routine to do the calculation. The arithmetic program is very accurate. (Before computers astrologers had to do the arithmetic by hand and very tedious it was too! Used to take me an hour just to work out the positions of the planets).\r\n

The tables in the ephemeris get their information about where the planets are on each day by starting with known observed positions in the past and projecting future positions from those, using specific mathematical formulae for the orbit of each planet.

The maths of planetary orbits was first worked out by Galileo & Kepler back in the 17th century, so ever since then we have been able to project where the planets are going to be with reasonable accuracy. Planetary orbits are very stable. Since the invention of super-computers we’ve been able to calculate orbits with very good accuracy.  The space program caused NASA to do extremely accurate computation of planetary orbits; this data is publicly available and many astrology programs now use NASA data.

When we cast a chart for someone, the program is projecting where the planets would have been (or will be) but usually doing it with such mathematical accuracy that results are reliable.  As ephemeris data and math formulaes are publicly available there’s no reason not to calculate correctly.

There is a limitation to accuracy. Doing charts for people or events within the last couple of centuries (or projecting them for the next couple) is extremely accurate. But if you start doing charts for people like Jesus or Buddha who lived a couple of thousand years ago, it’s trickier, because even very small errors in the calculations can add up into large errors in planetary positions over a large number of years. Especially for fast-moving bodies like the Moon. Many astrology programs tell you what span of time they’re accurate for. I have a couple of programs that I would trust back or forward at least a few hundred years… eg for Shakespeare’s chart.  

Before computers made it easy to do accurate calculations astrologers used to watch the sky a lot. Now it’s temptation for people not even to know if the moon is full or new today, because they never look at the sky, just at their computers. Myself, I think to do astrology well you need to stay in touch with what the actual planets and stars are doing up there!  Not because the computer calculations are incorrect, but to stay grounded in a basic of astrology: the movements of the heavenly bodies.

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