This is relevant to solar astronomy, defining the edge of Sol's
supremacy as the overwhelming provider of all light -

On any interstellar journey going outward from our solar system, one
eventually reaches a point where the total amount of light received
from our Sun is exactly matched by the total amount of flux coming from
all cosmic sources external to our solar system. The heliocentric
distance of that boundary is dubbed the "Ahad radius", and exists at
the edge of "Ahad's sphere of solar illuminance", some 11,500
astronomical units (roughly a trillion [10^12] miles) from Earth.

On first definition of this heliocentric boundary, Abdul Ahad wrote:-

"Beyond the outer edges of this theoretical sphere, the feeble currents
of light coming from its core (i.e. our own distant Sun) will cease to
make any noticeable ripples in the calm waters of the surrounding
cosmic ocean..." - July, 2004.

In the context of space exploration, no man-made spacecraft has yet
escaped beyond the "Ahad sphere"; Voyager 1 is leading the charge and
will reach the AR in just over 3,000 years from now. Voyager 2,
Pioneers 10 and 11 will all follow after that.

From the paper:
http://uk.geocities.com/aa_spaceagen...erstellar.html

Commentary on Google:
http://groups-beta.google.com/groups?q=ahad+radius

Regards,
Jav