What are Orbs?
Astrological orbs refer to the orb of influence. They set out the maximum range for an astrological effect to remain significant. As it is a graduated field and depends on the context, astrologers do not follow a fixed set of rules. The Orb for a planetary conjunction can range between 5° and 12°. It's like asking a physician exactly how long is it safe to sunbathe. There will be a wide range of answers.
Jim Lewis Extended his Orbs
Since Jim died, there has been some confusion about orbs for A*C*G lines.
One reason is that it appears that Jim revised his orbs from 300-400 miles [480 - 640 kms]
(which he equated to about 1/4 inch* on a standard A*C*G map) either side of an ACG line
to anything up to 700 miles [1,120 km] (which Jim claimed was about 1/2 inch* either side).
On page 8 of the original brown Astro*Carto*Graphy handbook  and page 5 in the updated [Equinox 1976-2009] version, Jim writes:
"If a place you want to go has no lines near it, what then? First, you should understand the "near" means about 1/4 inch on this map, on either side of the line. ... "
Six years later on page 6-7 of Jim's Cyclo*Carto*Graphy handbook , he wrote:
"The natal lines on the ASTRO*CARTO*GRAPHY map were given an "orb" or area of influence of about 1/4 inch - subsequent experience has suggested this to be conservative. These natal lines seem to affect almost a half inch - up to 700 miles each side of where they pass. ... "
This wider orb of 700 miles equates to about 15 degrees*. I discussed this with Ken Irving (see below) who confirmed that the standard orb was around 300 miles, though in certain situations a maximum orb of 700 miles should be considered.
Astrocartographers have to consider the effects of multiple planetary lines where the strong zones overlap. It requires fine judgement and experience to know when a planetary line will override another more distant line and when the energies combine to create a new joint effect.
Section from George W. Bush's A*C*G map showing how the strength of his Mars ♂ MC line declines as the distance from the peak of the line increases. The whole of Afghanistan is contained in the strong zone.
Orbs are subject to context
My experience is that wider orbs should be considered when there are no other closer lines in the zone. A line within 350 miles will have a predominant effect on the area, even though another line 700 miles away will have a weak, but hard to discern, effect.
The limitation with Jim's rule of thumb (Jim, literally would place his thumb on a line to show an orb) is that it is inconsistent. Due the projection of the globe onto a flat map, distances appear wider in the higher latitudes. With natal charts and transits, astrologers work with orbs based on zodiacal longitude. However, constantly calculating orbs of planets to angles in each location even with sophisticated mapping software is more for the astrologer involved in empirical research or just in love with the technique of astrology than one who seeks to make it of value to a client.
In most astrological techniques (like aspects), the planetary energy fields do not appear to switch on and off, but gradually rise to a peak. So the orb is more subject to the astrologer's style, technique and the objective of the analysis than any hard rules.
Robert Currey, [Cert. A.C.G. Consultant]
Distance & Latitude
Converting a distance into zodiacal degrees or distance on an Astro*Carto*Graphy map varies greatly according to latitude. Due to signs of long and short ascension the zodiacal position will affect the degrees over the distance. Measuring a distance of 700 miles from a Sun MC line at the Autumnal Equinox equates to:
The distances in inches and millimetres are based on an official Astro*Carto*Graphy Map
- 11°01' at the Equator 00°N. 7/16" 11mm on ACG map
- 19°01 at 56°N latitude. 3/4" 18mm on ACG map