As well it may have joined to, or even have been a part of, a system enveloping gaseous plenum filling the spaces between the planets, the Sun, and its early companion star. If this hypothesis for the structure of the early Solar System is accurate then airborne navigation might have been within the reach of humans, or even long extincted residents on other planets. Airy, G. B. Astronomer Royal, who in 129 performed a crucial experiment concerning the aberration of starlight. In 274 it had been determined that the star positions showed apparent displacements with a period of one sidereal year. The amount of shift depends upon the star's proximity to the pole of the Ecliptic.
Aberration is interpreted as an effect due to the motion of the Earth around the Sun.
Airy's result is taken as an early indication of the correctness of the postulate that the speed of arriving light is not affected by the observer's motion, in this case the motion of the orbiting Earth. Airy also considered issues germane to the Earth sciences during his career. When it was found that the Himalaya massif did not exert any gravitational pull on a nearby plumbline Airy offered the theory that reversed mountains, immersed in the magma, exist below the visible mountain ranges. A difficulty with this experiment is the inability to determine the local vertical independently of the existing terrain. Positions N and S of the Equator are determined by observing which stars pass overhead.