(Adapted from an Information Special on the subject by David Dukelow of Astro Communications Services, Inc.)
Coalescent charts, which were invented recently by Lawrence Grinnell (Eytan), are similar to composite charts except that they are based on harmonics rather than midpoints. The harmonic used is based on the relative positions of the planets in the individuals’ natal charts. A different harmonic is used for each planet, for the Nodes of the Moon, and for the Midheaven. These charts show unusual sensitivity to transits, and they hold a great deal of information about the dynamics involved in a relationship between two people, or for a person and an event. Here is some basic information about them.
How the Planets’ positions are calculated
Find the shortest arc between the positions of a planet [or function] in the natal charts of two people. Divide this angle into 360 degrees. This gives the harmonic. Next, multiply the position of either planet by the harmonic. Subtract 360 from this number as many times as needed to get a number in the 0 to 360 degree range. This number is the coalescent position, and it is the same regardless of which of the two planets is used.
If, however, the shortest arc [between the two natal positions] passes over 0 degrees Aries, you must add 360 to the position beyond 0 Aries when you determine the angle between the two positions. (If you choose to multiply this planet’s position by the harmonic, you must also add 360 to the position.)
With this method both the angle between the two positions and the positions themselves enter into the calculation of the coalescent positions. There are as many coalescent positions as there are functions in a chart. The coalescent Midheaven is calculated in the same way and the house cusps are forced from this Midheaven position.
NOTE: If the respective positions are less than a degree apart, then the coalescent position is take as the midpoint between them.
EXAMPLE: Sun in one chart at 12 deg Taurus and the Sun in the other chart at 27 deg Gemini. They are exactly 45 deg apart. 360/45 = 8. Convert the Sun’s position at 12 deg Taurus to degrees of longitude: 42 degrees. Multiply this by 8, 42 X 8 = 336, or 6 deg of Pisces. The other Sun position is 27 deg of Gemini or 87 degrees of longitude. 8 X 87 = 696. 696 – 360 = 336, the same figure as for the other position.
If the two planets straddle 0 deg Aries, then 360 must be added to the position on the far side of 0 deg Aries. 15 degrees Pisces to 3 degrees Cancer means that 360 must be added to the longitude of the Cancer position. (93+360) – 345 = 108. 360/108 = 3.333333. 3.333333 X 345 = 1150 – (3 X 360) = 70, or 10 degrees of Gemini as the harmonic position. (Also, (93 + 360) X 3.333333 = 1510. 1510 – (4 X 360) = 70 as for the other position.
It was found that the most logical House System to use with this synastry technique is the Meridian House system. The houses are free of location, which solves the cumbersome problem of how to calculate a location for a synastry chart. In the Meridian system the East and West points are significant and transits to them are felt.
Caution About Cusps
If cusps are less than 12 degrees apart, the accuracy of the birth times for the natal charts becomes significantly important. Where there is a suspicion that one or both charts are not accurate, the house positions are apt to be unreliable.
Declinations: Set to zero.
Nodes: Use like any other factor in the charts.
Zodiac: Use what you like because the principle will still work, but the aspects can change dramatically from one zodiac to another.
Interpretation: Much like any synastry chart except that transits seem to be very active. These charts seem to describe something essential and perhaps spiritual about the relationship.