Astronomy Principles and Practice Fourth Edition.pdf

Geocentric parallax 121
Figure 10.6. The use of parallax on the terrestrial scale.
Table 10.2. Measurement of distance by parallax.
For a base-line one unit long
Distance to be Order of size of Order of accuracy result
measured (in units) parallactic angle 1 part in
1 60 ◦ 200 000
10 6 ◦ 20 000
100 0·6 ◦ ∼ 30 ′ 2000
1000 0·06 ◦ ∼ 3 ′ 200
Figure 10.7. Geocentric and horizontal parallax.
increasing distance, the length of the base-line remaining constant. Much of the trouble in the use of
this surveying method to measure distances within the Solar System lies in the smallness of available
base-lines with respect to the distances involved.
Consider an observer at O (figure 10.7) on the surface of the Earth, centre C, the zenith being in