384 
NATURE 
[JUNE 3, 1915 

we now know to be true, that the so-called fixed stars 
are not fixed relatively to one another. It is the first 
positive proof that the distances of the stars are 
sensibly less than infinite. This, then, is the stage at 
which astronomers had arrived less than two hundred 
years ago. The stars are at least 20,000 or 30,000 
times as distant as the sun, but three of the brightest 
of them are perceived to be not infinitely distant. 
The greatest step in the determination of stellar 
distances was made by another Oxford astronomer, 
James Bradley. His unparalleled skill as an astro- 
nomer was early recognised by Halley, who tells how 
“Dr. Pound and his nephew, Mr. Bradley, did, myself 
being present, in the last opposition of the sun and 
Mars this way demonstrate the extreme minuteness 
of the sun’s parallax, and that it was not more than 
12 seconds nor less than g seconds.” ‘Trans- 
lated from astronomical language, the distance 
of the sun is between 95 and 125 millions of 
miles. Actually the distance is 93 million miles. The 
astronomer who so readily measured the distance of 
the sun entered on the great research which had 
baffled his predecessors—the distance of the stars. 
The theory of the determination of stellar parallax 
is very simple: the whole difficulty lies in its execu- 
tion, because the angles are so small that the slightest 
errors vitiate the results completely. Even at the 
present time with large telescopes, and mechanism 
which moves the telescope so that the diurnal move- 
ment of the stars is followed and they appear fixed 
to the observer in the field of the telescope, and with 
the additional help of photography, the determination 
of the parallax of a star requires a good deal of care, 
and is a matter of great delicacy. But in Bradley’s 
time telescopes were imperfect, and the mechanism 
for moving them uniformly to follow the diurnal rota- 
tion of the stars had not been devised. 
This was in some ways very fortunate, as the 
method Bradley was forced to adopt led to two most 
important and unexpected discoveries. Every day, 
owing to the earth’s rotation, the stars appear to 
describe circles in the sky. They reach the highest 
point when they cross the meridian or vertical plane 
running north and south. If we leave out all dis- 
turbing causes and suppose the earth’s axis is quite 
fixed in direction, a star S, if at a great distance from 
the earth, will always cross the meridian at the same 
point S; but, if it is very near, its movement in the 
small parallactic ellipse will at one period of the year 
bring it rather north of its mean position and at the 
opposite period an equal amount south. 
Bradley, therefore, designed an instrument for 
measuring the angular distance from the zenith, at 
which a certain star, y Draconis, crossed the meridian. 
This instrument is called a zenith sector, and is 
shown in the slide. The direction of the vertical 
is given by a plumb-line, and he measured from day 
to day the angular distance of the star from the 
direction of the vertical. From December, 1725, to 
March, 1726, the star gradually moved further south; 
then it remained stationary for a little time; then 
moved northwards until, by the middle of June, it was 
in the same position as in December. It continued 
to move northwards until the beginning of September, 
then turned again and reached its old position in 
December. The movement was very regular and 
evidently not due to any errors in Bradley’s observa- 
tions. But it was most unexpected. The effect of 
parallax—which Bradley was looking for—would have 
brought the star furthest south in December, not in 
March. The times were all three months wrong. 
Bradley examined other stars, thinking first that this 
might be due to a movement of the earth’s pole. 
But this would not explain the phenomena. The true 
explanation, it is said, although I do not know how 
NO. 2379, VOL. 95] 

| truly, occurred to Bradley when he was sailing on 
the Thames, and noticed that the direction of the 
wind, as indicated by a vane on the mast-head, varied 
slightly with the course on which the boat was sail- 
ing. An account of the observations in the form of a 
letter from Bradley to Halley is published in the 
Philosophical Transactions for December, 1728 :— 
“When the Year was compleated, I began to 
examine and compare my Observations, and haying 
pretty well satisfied myself as to the general Laws 
of the Phenomena, I then endeavoured to find out 
the Cause of them. I was already convinced that 
the apparent Motion of the Stars was not owing to a 
Nutation of the Earth’s Axis. The next thing that 
offered itself, was an Alteration in the Direction of 
the Plumb-line, with which the instrument was con- 
stantly rectified; but this upon trial proved insufficient. 
Then I considered what Refraction might do, but here 
also nothing satisfactory occurred. At last I conjec- 
tured that all the Phenomena hitherto mentioned, pro- 
ceeded from the progressive Motion of Light and the 
Earth’s Annual Motion in its Orbit. For I perceived 
that, if Light was propagated in Time, the apparent 
Place of a fixed Object would not be the same when 
the Eye is at Rest, as when it is moving in any other 
Direction, than that of the Line passing through the 
Eye and the Object; and that, when the Eye is moving 
in different Directions, the apparent Place of the 
Object would be different.” 
This wonderful discovery of the Aberration of Light 
is usually elucidated by the very homely illustration 
of how an umbrella is held in a shower of rain. 
Suppose the rain were falling straight down and a 
man walking round a circular track: he always holds 
the umbrella a little in front of him—because when 
he is walking northward the rain appears to come 
a little from the north, when he is going eastward 
it appears to come a little from the east, and so on. 
Although the phenomena Bradley had _ observed 
were almost wholly explained in this way, there were 
still some residual changes, which took nineteen years 
to unravel; and he explained these by a nutation 
or small oscillation of the earth’s axis, which took 
nineteen years to complete its period. I cannot dwell 
on these two great discoveries. For our present pur- 
pose, it should be said that aberration and nutation 
cause far greater changes in the apparent positions 
of the stars than, we now know, are caused by 
parallax. Until they were understood and allowed for 
or eliminated, all search for parallax must have been 
in vain. Further, Bradley’s observations showed that 
in the case of y Draconis, at any rate, parallax did 
not displace the star by so much as 1-0” from its 
mean position, or that the star was 200,000 times as 
distant as the sun. We may say that Bradley reached 
to just about the inside limit of the distances of the 
nearer stars, 
Let me now try to give some idea of what is meant by 
a parallax of 1", which corresponds to a distance 200,000 
times that of the sun. Probably many of you have 
looked at the second star in the tail of the Great Bear, 
Mizar, it is named, and have seen there is a fainter 
star near it, which you can see nicely on a fine night. 
These stars are 600" apart; with a big telescope 
with a magnification of 600 times—and this is 
about as high a magnification as can be generally 
used in England—two stars 1” apart are seen double 
just as clearly as Alcor and Mizar are seen with the 
naked eye. I think this is the most useful way to 
think of 1’—a very small angle, which one needs a 
magnification of 600 times to see easily and clearly. 
Bradley showed that y Draconis did not wander by 
this amount from its mean position among the stars 
in consequence of our changing view-point. 
The next attempt to which I wish to refer is the 


