D O C U M E N T 2 5 4 O C T O B E R 1 9 2 1 1 6 5

1) The moving light source L with the coordinates can be substituted by

the observer, at time , by a light source with the coordinates at rest

relative to the coordinate system, where r is the distance: light source–observer,

measured with sufficient precision in the inertial system at time , if all body ve-

locities are small against c.

2) The influence of the observer’s motion on the observed locations of the light

source is a reduction[7] in the angle by .[8]

Example:

255. From Eduard Einstein

[Zurich,] 4 October

[1921][1]

[Not selected for translation.]

xr t

t0 L xr t0

r

c

-- –

t0

OL =

c

-- - sin =

L

v O

OL v rel. inertial system

2

-- - =

rel. mov. observer O

2

-- -

v

c

-- – =

OL v rel. inertial system

2

-- - =

rel. mov. observer O

2

-- -

2v

c

------. – =

v

L

v

O

v