Mechanism to adjudicate Astrological Divination

In the campaign Khareo's Tale, a lot of the plot revolves around astrological divination rituals. Knowing what to provide as the outcome of a divination spell can be difficult to gauge. This is particularly so if you want to consistently scale the usefulness of the information with respect to the margin of success. This post describes how we are doing it on our campaign. 


This is a joint post with Sophie Blouin, who brainstormed with me and had the idea of buying destinies using powerful prophecies (which I find very cool). 

In a nutshell

Astrological divination is concerned with position and timing. The better is the divination roll, the more precise is the prediction in these two scopes. Astrology is a ritual subject to normal ritual modifiers (GURPS Thaumatology) and some specific one about the object of the divination. A star chart prop was designed to simplify the adjudication of divination checks. 

The star chart. The number indicate the MoS cost of each area.
Latin is used for flavor, but the English word is shown in smaller print.

This post is further divided into three sections: Ritual Magic modifiers, Interpreting the results and examples.

Astrological Divination as an effect Shaping ritual

We treat astrological divination as a ritual path. To use astrology, one must have some points into Ritual Magic, and treat divination as a school (GURPS Magic p. 108). Astrology is treated as a technique (hard) of divination. However, other techniques of divination have no default and must be learned in-game. Since each techniques defaults to Divination, there is no point in putting cps into a technique: only Ritual Magic and Divination needs to be developed.  It is thus expensive to improve Divination by a level, but there is only one "spell" to worry about. 

Special bonus and penalties applies as such:

• Birth information
• Time
• Precise to the hour : +2
• Precise to the day: +1
• Precise to the month: +0
• Precise to the year: -1
• Precise to the nearest generation: -2
• If precise but incorrect, downgrade the bonus to what it should be and add -1 to it. For example, if the day is correct and the hour is incorrect, the player would assume +2 while in fact the bonus is +1 -1 = +0
• Location
• Location known within 100m (Locus) : +2
• Location known within 2km (Adspectus): +1
• Location known within a region (provincia): +0
• Location known within a land/kingdom (regnum):-1
• Location known within a world: -2
• If mistaken, apply the same rule as for birth date/time. For example: Character X is assumed to be born in New-York but really was born in Boston. Instead of +1 (Named City), it is true at the regional level (Eastern US) for +0 -1 = -1 
• Other ritual factors apply as per GURPS Thaumatology (optional):
• Ritual preparation
• Astrological equipment + charts and stationaries required or (-5)
• Site preparation as per rules.
• Sympathy of the subject ( -1 vs normal ritual )
• Name of the subject
• Contagion of the subject ( -1 vs normal ritual )
• Sacrifice

Mapping the stars to determine the outcome

There is no distinction between past, present and future in Astrology. Instead, divination is divided into two orthogonal scopes: temporal and positional. 

Temporal Scope

• Aera (era): time precise within 250 years.
• Aevitas (generation): time precise within 20 years.
• Anno (year): time precise within an earth year.
• Mensis (month): time precise within a month.
• Dies (day): time precise within a day.

Positional scope

• Mundus (world) : location precise within a known world/plane.
• Regnum (Kingdom): location precise within a large piece of the world, say 2000 km.
• Provincia (region): location precise within 200 km.
• Adspectum (horizon): location precise within 5km: a named town, city, etc...
• Locus (location): location precise within 100m. 

Star chart as a PNG. Download Google Doc here.

The principle is that the margin of success (MoS) on a divination ritual (astrology) allows the player to choose the precision of the divination on these two scopes. Each remaining MoS can be used to either:

• Further specify the divination with a yes/no question. The astrologer may proposes the question OR the details can be provided by the GM (GM's option). 
• Buy off part of the cost of converting the profecy into a destiny advantage for the character. For example, if a divination reveals a small part in a great feat for a character. The player may decide to use the leftover MoS as a discount to buy off a Minor Destiny advantage. For exmaple, with a leftover MoS of 3, a minor destiny would only cost 2 character points. 

In the case of a critical success, the cost of improving the temporal scope drops to 1 instead of 2. Thus, giving more leeway to provide a specific prediction.

Example 1 : I want to know where my father is right now. He disappeared 6 months ago.

Let's assume that the astrologer managed a MoS of 6. This kind of margin can be achieved with favorable ritual conditions.  

1. The divination can reveal his position that was true within a month of today (Mensis), but no more precise than a named region (Provincia). This combination costs 6.
2. The divination can reveal his location of a names town or key landmark where the father was within a year from today. However, the loose temporal scope makes this precise prediction potentially useless (it may be his location before he disappeared since 6 months ago is within the 1-year precision.
3. The astrologer can elect to remain in the (Anno) scope, but go no better than a name Kingdom/Land (regnum). This costs 4, which leaves room for 2 yes/no questions to be answered: "Your father was/is/will be in Solar Empire. He was/is/will be a prisoner at that time. He wasn't/isn't/will not mistreated at that time."
4. There are other combinations, but I'll show only three.

Example 2 : When will the next typhoon hit this small island ( 2km diameter)

Let's pretend that the real answer is November 22nd 2015. Here are different answers based on different choices by the astrologer.

It will hit the island in two years in the month of November.

Your house will be hit in two years.

Example 3 : When was this temple destroyed?

This divination requires that the positional scope be in the Locus scope. The MoS will then determine how precise the answer is going to be.  In the case of a MoS of 7, the answer has to match one of the following:

• Within an era (250 years), with two yes/no questions to spare.
• Within a generation (20 years), with one yes/no question to spare.
• Within a year.

Depending on the context, one of these option may be preferable. 

Alright, old man, spill the beans!
(Chloe is the main user of this chart)