OF SOME OF THE STAES AND NEBULAE. 
549 
respect to a star situated at the pole of the ecliptic, the earth’s motion during the whole 
of its annual course will be perpendicular to the direction of the light coming to us 
from it, and will be therefore without influence on its period. 
That part of the earth’s resolved motion which is in the direction of the visual ray, 
and which has alone to be considered in this investigation, may be obtained from the 
following formula : 
Earth’s motion towards star=v . cos X . sin (l— l 1 ), 
where v is earth’s velocity, l the earth’s longitude, l' the star’s longitude, and X the star’s 
latitude. 
At the time when the estimate of the amount of alteration of period of the line in 
Sirius was made, the earth was moving from the star with a velocity of about 12 miles 
per second. 
There remains "unaccounted for a motion of recession from the earth amounting to 29-4 
miles per second , which we appear to he entitled to attribute to Sirius. 
It may be not unnecessary to state that the solar motion in space, if accepted as a 
fact, will not materially aflect this result, since, according to M. Otto Steuve’s calcu- 
lations, the advance of the sun in space takes place with a velocity but little greater than 
one-fourth of the earth’s motion in its orbit. If the apex of the solar motion be situated 
in Hercules, nearly the whole of it will be from Sirius, and will therefore diminish the 
velocity to be ascribed to that star. 
It is interesting, in connexion with the motion of Sirius deduced from these prismatic 
observations, to refer to the remarkable inequalities which occur in the rather large 
proper motion of that star. In 1851 M. Peters showed that the variable part of the 
proper motion of Sirius in right ascension might be represented by supposing that 
Sirius revolves in an elliptic orbit, round some centre of gravity without itself, in a 
period of 50-093 years. This hypothesis has acquired new interest, and seems indeed 
to have received confirmation from direct observation by Alvan Clark’s discovery of a 
small companion to Sirius. 
Professor S afford f and Dr. AuwersJ have investigated the periodical variations of 
the proper motion of Sirius in declination, and they have found that these variations, 
equally with those in right ascension, would be reconcileable with an elliptic orbital 
motion round a centre not in Sirius. The close coincidence of the observed positions of 
the new satellite with those required by theory, seem to show that it may be the hypo- 
thetical body suggested by Peters, though we must then suppose it to have a much 
greater mass relatively to Sirius, than that which its light would indicate. 
At the present time the proper motion of Sirius in declination is less than its average 
* Astron. Nachrichten, No. 748. 
t Proceedings of the American Academy, 'yol. vi. ; also Astron. Notices, Ann. Arbor, No. 28; Monthly 
Notices, vol. xxii. p. 145. 
+ Astron. Nachrichten, No. 1506 ; Monthly Notices, yol. xxii. p. 148, and vol. xxv. p. 39. 
