1034 Marvels of the Universe 
discovered a new planet, which received the name of “ Astrea.’’ This discovery clearly suggested 
that there were other small planets in the solar system which might be found if they were looked 
for, and Eros was one of the fruits of this expectation, though very long down the stream, both in 
date and succession. 
It was very quickly seen after the discovery of Eros that there was something altogether un- 
precedented in its movements, for it was passing along the heavens at a most unusual pace ; and so 
soon as it was possible to determine its orbit, astronomers discovered that they had in tow a minor 
planet the orbit of which was wholly unlike that of any yet known. Within a short time of its 
discovery it was retrograding in the heavens at the rate of $° a day, which proved at once that 
its orbit must differ in a marked way from the rest of the family of minor planets, and was 
of a very sensational character, calculated quite to revolutionize some of our ideas of the solar 
system. We had always looked on Mars as our nearest outside neighbour, but the mean distance 
of the new planet from the sun being only 1.46 as compared with Mars’ 1.52 it was evident that the 
new planet (which, by the way, for some time was only known as “ DQ”’) would be under certain 
circumstances nearer to us than Mars, and would have, at any rate, some portion of its orbit within 
that of Mars. The novelty and sensationalism attaching to the matter were intensified by its being 
found that, as the eccentricity of the orbit was as great as 0.23, the perihelion distance would be 
only 1.12 (in radii of the earth’s orbit), and the aphelion distance as much as 1.79 ; whilst the least 
distance from the earth’s orbit is only 0.15, as compared with 0.27, the least distance of Venus when 
in transit, and with 0.38, that of Mars when in perihelion. The perihelion and aphelion distances 
of Mars being 1.38 and 1.67 respectively, it will be seen at once (especially by a study of the annexed 
diagram) how curiously the orbit of Eros compares with that of Mars. It follows from all this 
that Eros will thus on certain occasions be far closer to the earth than any other member of the 
solar system; and thus it becomes available as an excellent means for determining the sun’s 
parallax, which means, in other words, the distance of the earth from the sun, for the parallax 
of Eros when nearest to us will be more than 1’ of arc. It has been estimated that these 
circumstances connected with Eros enable it to be utilized for determining the distance of the earth 
from the sun with an accuracy three times greater than that available by any previous method. 
So soon as Eros became a recognized member of the solar system, the interesting question arose : 
Had it been seen before? This question was the more interesting because, owing to the great 
eccentricity of its orbit, it would vary very much in its visible brightness estimated in star magni- 
tudes. Its magnitude when found in 1898 was no 
more than the 11th, whilst, in January, 1894, when 
it had been at its minimum possible distance from 
the earth, its Opposition briliancy was estimated to 
have been as great as that of a star of the 7th 
magnitude. The strange thing was that it should 
have escaped notice under such circumstances. How- 
A ever, an examination of a large number of photo- 
graphic plates taken in various years beginning with 
1889, and at various observatories, showed that it 
had left its mark on a considerable number of plates. 
This has greatly facilitated the accurate determination 
of its orbit, and its period of revolution round the 
y sun is now known to be 643 days at a mean distance 
of 135,442,000 miles. It has also, unfortunately, 
been ascertained that the planet will not be again in 
its most favourable position for observation until 
Diagram illustrating the method of distinguishing 
a moving star-like point in.a field of fixed stars. January, 1924. 
