First of all, we want to examine the previously cited possibility. Because like the Earth every other celestial body also has a gravitational field that extends out indefinitely, losing more and more strength the further out it goes, we aretheoretically, at leastalways under the simultaneous gravitational effect of all heavenly bodies. Of this effect, only the gravitational effect of the Earth and, to some degree, that of our Moon is noticeable to us, however. In the region close to the Earth's surface, in which mankind lives, the force of the Earth's attraction is so predominately overwhelming that the gravitational effect exerted by other celestial bodies for all practical purposes disappears compared to the Earth's attraction.
Something else happens, however, as soon as we distance ourselves from the Earth. Its attractive force continually decreases in its effect, while, on the other hand, the
Figure 4. The curve of the gravitational fields of two neighboring heavenly body G1 and G2 is represented as in Figure 1, with the exception that the gravitational curve of the smaller celestial body G2 was drawn below the line connecting the centers because its attractive force counteracts that of the larger entity G1. The point free of gravitational effects is located where both gravitational fields are opposite and equal to one another and, therefore, offset their effects.
Key: 1. Point free of gravitational effects
effect of the neighboring heavenly bodies increases continually. Since the effect counterbalances the Earth's force of gravity, a point must existseen from the Earth in every directionat which these attractive forces maintain equilibrium concerning their strengths. On this side of that location, the gravitational effect of the Earth starts to dominate, while on the other side, that of the neighboring planet becomes greater. This can be designated as a practical boundary of the gravitational field of the Earth, a concept, however, that may not be interpreted in the strict sense, taking into consideration the large difference and continual changing of the position of the neighboring planets in relation to the Earth.
At individual points on the practical gravitational boundary (in general, on those that are on the straight line connecting the Earth and a neighboring planet), the attractive forces cancel one another according to the direction, such that at those points a completely weightless state exists. A point of this nature in outer space is designated as a socalled "point free of gravitational effects" (Figure 4). However, we would find ourselves at that point in an only insecure, unstable state of weightlessness, because at the slightest movement towards one side or the other, we are threatened with a plunge either onto the Earth or onto the neighboring planet.