Singing Sun

[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.

[original post 2005] To make the difference between tropical and sidereal clearer, as related to the positions of the actual stars, here is a chart.

Zodiac chart

On the outside edge are written the names of the actual constellations. They are shown in their real positions in space related to the time of year as the earth orbits the sun.  The zodiac in green is the tropical zodiac, which starts wherever the vernal equinox points to on March 20th/21st. This is currently a point in the constellation of Pisces. Each astrological zodiac systen uses a “normalized” or regularized set of zodiacs of thirty degrees each, so once you determine the starting point you just count around 30 degrees to find the next “zodiac sign”.

The tropical zodiac doesn’t correspond to the stars. Most tropical zodiac signs are not located in the constellation whose name they borrow; at most they touch by a few degrees.

The sidereal zodiac (which Jyotish uses) is shown in blue. It attempts to make its regularized zodiac signs line up with the real stars as much as is consistent with equal sign sizes. It’s more accurate, tho the need to create 30 degree signs means it’s off a bit in some places from the constellations. As you can see, even in the sidereal zodiac, zero Aries is actually at the end of the constellation Pisces! 

There are a number of different ideas of exactly where sidereal zero degrees Aries should start in order to have the least amount of inaccuracy.

The definition of where zero Aries is placed is called the Ayanamsha (a Sanskrit word). Many ayanamshas take the fixed star Spica as a starting point for zero Libra (since it’s very close anyway). A sidereal zero Aries point never moves; it is in the same place every year. But the ayanamsha is often stated mathematically as the number of degrees from the tropical Aries point on a specific date. Defined this way an ayanamsha changes its value slightly every year, since the tropical zodiac Aries point moves constantly with reference to the stars.

A sidereal ephemeris will give the ayanamsha value for each year, which enables us to easily calculate the offset between the two zodiacs. So it’s useful to have it stated in this form.

The most commonly used ayanamsha is called Lahiri. The offset of the tropical zodiac from the zero Aries point of the Lahiri sidereal zodiac is currently almost 24 degrees. Today is 31st March. Western astrology says the sun is rising in Aries; Jyotish says the sun is rising in Pisces.

[original date 2005-03-24 06:39:39]  The rough chart we constructed in part 1 shows the planets in approximately the positions they were in the sky.

A rough chart

But it doesn’t allow us to state their positions precisely. Astrology is an old subject, it is thought the Sumerians divided the circle into 360 degrees around four and a half thousand years ago. 360 is a useful, divisible number and is close to the 365 days in a year. The invention of degrees will allow us to start pinpointing planetary positions more accurately.

Circular chart

When planets move across the sky, they don’t simply move through degrees, they move against the background of the stars. From earliest times people have grouped the stars they saw into patterns we now call “constellations”. Even though we now know those stars may be at very different distances, constellations are still compelling patterns in the sky. There’s many constellations over the night sky, like Draco, the Dragon, right overhead in the northern hemisphere, but the most familiar ones are those located around the plane of the solar system – the zodiac constellations.

Zodiacal constellations

By marking these constellations around the edge of our chart we can easily divide up our chart into sections.

Chart with zodiac

But there is absolutely no universal consensus on how those positions are numbered or named! Where do we start numbering from? A circle has no beginning or end – so you can start anywhere at all! That’s what the different systems do.

Three starting points

SIDEREAL ZODIAC
Jyotish uses the sidereal zodiac, which means it counts using the real constellations – sort of! Currently we use zero degrees Aries as the beginning of the zodiac; in past times it probably started at zero Taurus, Gemini or Cancer. However, the sidereal zodiac used in astrology is not in fact an exact mapping of the real constellations, although it’s a lot more accurate than the tropical zodiac.

The star maps shown here are not accurate to scale or position, but they do reflect the fact some constellations are bigger than others. Virgo, Scorpio and Pisces are larger than 30 degrees, Libra and Aries are smaller. So astrologers “tidy up” – they even them all out, creating 12 zodiac signs of 30 degrees each.

Because Aries is a small constellation and Pisces is a large one, the start of the sidereal zodiac, nominally 0 degrees Aries, is in fact in the end of the actual constellation Pisces.

There’s a debate about where exactly to start counting; the most common choice is the Lahiri system (“Lahiri ayanamsha”).

GALACTIC CO-ORDINATES
Astronomers often now use “galactic co-ordinates”, which count the degrees from the the galactic center. Radio-astronomy has established the center of our galaxy is in Sagittarius.

TROPICAL ZODIAC
Western astrology uses the “tropical zodiac”, which has a completely different way of calculating where to start counting. It does not start from any actual stars (even tidied-up ones). It uses the first day of spring as the start of the solar year, and projects that position in orbit out against the stars and calls it the zero point. This day is the spring (or “vernal”) equinox, when day and night are equal lengths.

Currently this point is at about 6 degrees of sideral Pisces.

The tropical zodiac

On the first day of spring we can look to the east just before sunrise and see the constellation of Pisces, against which the sun will rise. Just to be really confusing, the tropical zodiac calls that the “Aries Point”. Then count off twelve sections of 30 degrees around the circle and call them by the names of the true zodiac constellations. You could call these “virtual constellations” as they don’t correspond to any actual constellations at all. The first 24 degrees of “tropical Aries” are in the constellation of Pisces; only the last 6 degrees are really in Aries.

So the tropical zodiac “signs” and starting point are defined by the earth’s relation to the sun, not it’s relation to the stars.There are two problems with this system:
1. Lots of people confuse tropical zodiac signs with real constellations.
2. The start point moves slowly but constantly.

Due to a very long-term cycle in the earth’s movement, the “Aries Point” moves by about 1 degree every 72 years. Over thousands of years each constellation in turn will be home to the this point.  Around 200 AD the “Aries Point” was at zero degrees sidereal Aries, but it won’t be again for another, oh twenty-six thousand years or so. There is nothing wrong per se with the idea of a tropical zodiac; things would just be a lot simpler if it could be more easily distinguished from the sidereal zodiac.

PLANETARY POSITIONS ARE ALWAYS THE SAME
The position of the planets at any specific time are accurately described in all the systems – as long as you know which system is being used. In my natal chart, the planet Jupiter is at 2 degrees 08 minutes of sidereal Sagittarius. Or 26 degrees 03 minutes of tropical “Sagittarius”. Or approximately -1 degree or 359 degrees from the Galactic Center. They’re all describing the same position – Jupiter was almost conjunct the Galactic Center.

DO THE ZODIAC DIFFERENCES MATTER?
Early astrologers experienced different results as the planets moved through the zodiac constellations. The combination of planets positioned against the zodiac constellations is perhaps the most fundamental feature of astrology.

If you think the constellations themselves, or the powerful fixed stars within them – eg Regulus in Leo – are actually exerting or transmitting an influence on the planets then it matters that we represent these stars adequately. If you think the chief influences are planetary movements within the solar system, then the zodiac is just a picturesque way to describe planetary positions and inaccuracy is not significant. We’ll explore this issue in a future post.

[original post 2005-03-23].  By request, here is an explanation of how horoscope charts are derived from the actual planets and stars, and how there come to be different zodiacs. It’s a complicated subject so there are simplifications here.

THE SOLAR SYSTEM – ORBITS & SPIN
Our solar system contains a number of planets orbiting the sun. The planets are all approximately in the same plane (due to centrifugal force, I believe [check this!]). Our calendar year is derived from the sun, and equals one complete revoltion of the earth around the sun. Other planets take more or less time for one orbit, depending on the speed they are going.

The sun and planets of our solar system

Most planets also spin around, like a top. On earth one spin is one day. The axis of spin on earth is through our north and south poles. A planet’s axis of spin is usually not straight up (right angles) to the plane of the solar system, different planets tilt every which way.

Uranus is spinning on its side!  The earth’s axis is tilted at 23.5 degrees to the (imaginery) straight up-down line.

Earth’s tilt gives us a sideways view

THE VISIBLE ARC
Because the earth spins we’re privileged to see a huge amount of the solar system and visible universe during the course of a day!  By day we see the sun move across the sky – we can’t see the stars because sunlight blots them out (except the moon, which is very close). By night we see any stars and planets currently to earth’s nightside. Because of the 23 degree axis tilt the sun and stars appear to move in an arc across the sky.

We see the sun move along the arc by day

We see planets move along the arc by night

We can’t really see Saturn and Jupiter that well at night without a telescope, but I wanted to make a clear picture. Our view of this arc(“the ecliptic plane”) changes from summer to winter –  when the earth is at different points on its orbit around the sun the arc appears higher or lower. It also looks more up or down depending where on earth you live.

TURNING IT INTO A DIAGRAM
The fact that all the planets move around in approximately the same plane makes it much easier for us to make a 2-dimensional map of where they are. To make a simple chart we can ignore the height of the planets, since they’re all relatively close. Pretend you’re sitting above the solar system, over the sun.

Looking down from above the sun

But we aren’t living on the sun, we’re living on earth – the cosmic forces come towards us from all around the earth. So we need to put down where everything is with respect to earth. To make our chart simple we’ll ignore how far away from us everything is, and draw them all at about the same distance.

Looking down from above the earth

This is the basis of an astrological chart. In real life the planetary orbits are not lined up quite exactly with the solar system plane; some astrologers note the height variations on their charts (“declination”) and treat it as significant. Also, there are reasons why it might be OK to ignore the differences in distance, but that’s a subject for a future post.

In part 2 we’ll look at how those zodiac signs get into the act and why there are different zodiacs.