It is impossible to detect the curvature of a luminous ray travelling from one point to another on the earth’s surface, mainly because weight on the earth is too slight to bend the ray much. A further reason is that our planet is so ridiculously small that we cannot follow the light over a sufficient distance.
But what cannot be done on this little globule of ours, the entire diameter of which light can cover in the twenty-fifth of a second, may possibly be done in the gigantic laboratory of celestial space. We have, almost within our reach—a mere matter of 93,000,000 miles away, that is to say—a star on which weight is twenty-seven times greater than on the earth. We mean the sun. On the sun a body left to itself falls 132 metres in the first second. Its fall is twenty-seven times as rapid as on the earth.
Hence, near the sun, light will be much more bent out of its path by gravitation. The deviation will be all the greater from the fact that the sun is 800,000 miles in diameter, and a luminous ray needs a much longer time to cover this distance than to travel the length of the earth’s diameter. Hence gravitation acts upon the ray of light during a much longer time than upon a ray that reaches the earth, and it will be all the more curved.
Take a luminous ray that comes from a star at a great distance behind the sun. If it reaches us after passing near to the sun, it will behave like a projectile. Its path will no longer be rectilinear. It will be slightly curved toward the sun. In other words, the ray will deviate from a straight line, and the direction it has when our eyes receive it on the earth is a little different from the direction it had when it left the star. It has been diverted.
Calculation shows that this deviation, though very slight, can be measured. It is equal to an angle of a second and three-quarters: an angle which the delicate methods of our astronomers are able to measure.
Certainly such an angle is very far from considerable, for it takes 324,000 angles of one second to make a right angle. In other words, an angle of one second is that at which we should see the two ends of a rod, a metre in length, fixed in the ground, at a distance of 206 kilometres. If our eyes were sharp enough to see a man of normal height standing 200 kilometres away from us, our glance, in passing from his head to his feet, would have a very small angle of deviation. Well, this angle accurately represents the deviation experienced by the light that comes to us from a star when it has passed close to the golden globe of the sun.
Minute as this angle is, the methods of the astronomer are so delicate and precise that he can determine it. The tiny measurement is by no means to be despised. Disdain of the men who devote themselves to such refined subtleties is very much out of place, because our modern science has been revolutionised by this measurement. Einstein is right, and Newton wrong, because we have been able to measure this minute angle and establish the curvature of light.
A great difficulty arose when we wished to verify this. How can we observe in full daylight a ray of light that comes to us from a star and passes close to the sun? It cannot be done. Even if we use the most powerful glasses the stars on the farther side of the sun are completely drowned in its blaze—to speak more correctly, in the light which is diffused by our atmosphere.
To say the truth—if we may venture upon a parenthetic remark at this juncture—night has taught us much more than day about the mysteries of the universe. In literary symbolism, in politics, the light of day is the very symbol of progress and knowledge: night is the symbol of ignorance. What folly! It is a blasphemy against night, the sweetness of which we ought rather to venerate. I do not refer to its romantic charm, but to the mighty progress in knowledge which it has enabled us to make.
Midnight is not merely the hour of crime. It is also the hour of prodigious flight toward remote worlds. During the day we see only one sun: by night we see millions of suns. The blinding veil which the sunlight draws across the heavens may be woven of the most brilliant rays, but it is none the less a veil, for it makes us as blind as the moths which, in a strong light, can see no further than the tips of their wings.