The observed position of a meteor's radiant may be explained as follows.

The Earth's velocity Ve is directed along the apex of the Earth's way. A meteor moving with a heliocentric velocity VH from a radiant located at an angular distance b away from the apex will have a resultant geocentric velocity VG given by

VG2 = VH2 + VE2 +2 VH VE cosb

and the radiant will appear to be shifted to an angular distance c from the apex where

sin c =(VH sin b) / VG

The specifics of the Persied meteors, on the 12th August are described in  the following diagram. The plane of the diagram is defined by the observer, the direction of the Earth's way and the radiant.

 Ve represents the Earth's velocity towards the apex (29.78km/s) and VG represents the meteoroid's geocentric velocity (59km/s). The angle between the vectors Ve and VG is the angle between the radiant and the apex of the Earth's way (40). The construction allows us to derive the meteoroids heliocentric velocity, which turns out to be 41.25km/s and derive the angle between the apex and the heliocentric direction of the meteoroids (67.5).

Consequently, if the Earth's motion, about the Sun, was suddenly to be frozen, the Persied meteors would appear from a radiant in Cepheus and would travel considerably slower than normal.