Environmental Engineering Reference
In-Depth Information
Starlight
u
a
a + d
B
u
A
V
Sun
a
a − d
u
C
D
u
Figure 5.4
Stellar aberration.
Stellar aberration is the phenomenon whereby the positions of the stars move
in ellipses over the course of each year and it is illustrated in Figure 5.4. The
figure shows the motion of an observer on the Earth as they travel around the Sun.
To simplify things, we assume that the observer is looking at a star which lies
in the same plane as the Earth's orbit around the Sun (called the 'ecliptic'). The
heavy arrows show the apparent position of the star when the Earth is at various
points in its orbit. If you are having difficulties with this diagram then it might
help to consider why someone running through the rain might tilt their umbrella
ahead of them even though the rain is falling vertically from an overhead cloud.
However this picture needs to be used carefully, for the light from a distant star
is incident as plane waves on the Earth and if the wavefront normals (i.e. the
vectors perpendicular to the planes of constant amplitude) are perpendicular to the
ether wind then they will be unchanged by it. The situation is analogous to waves
breaking on a beach: the waves may arrive parallel to the beach even if there is a
strong current flowing. However it is not the wavefront normals which matter to
an astronomer on Earth. The telescope they are using to view the star removes a
portion of the incident wavefront. In order to focus that incident portion onto the
eye of the astronomer, the telescope must be tilted to allow for the fact that the light
incident into the telescope is swept by the ether wind. Using the rainfall analogy,
consider a person running through vertically falling rain holding a cylindrical piece
of tubing, then in order for rain entering at the top of the tube to exit at the bottom
of the tube, the tube must be tilted.
Returning to the task in hand, we presume that the Sun is moving through the
ether too, at a speed
V
, and we denote the speed of the Earth relative to the Sun
u
.
