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The displacements caused by parallax were thus in a sense doubled, since the star to which the planet seemed approximated in the northern hemisphere, showed as if slightly removed from it in the southern, and vice versâ. As the planet pursued its course, fresh star-couples came into play, during the weeks that the favourable period lasted. In these determinations, only heliometers were employed.
Still, the desire to measure this parallax was only intensified by the practical certainty of its existence, and by repeated failures. The attempts of Bradley failed. The attempts of Piazzi and Brinkley, early in the nineteenth century, also failed. The first successes, afterwards confirmed, were by Bessel and Henderson.
Elkin's parallax of 0.068" for Pollux shows that that star may be a hundredfold more luminous than the sun, while its nearest companion may be a body smaller than our planet Jupiter, but shining, of course, by its own light. Its distance from Pollux, however, exceeds that of Jupiter from the sun in the ratio of about one hundred and thirty to one.
But this angle of parallax or apparent displacement cannot be directly measured cannot even be perceived with the finest instruments. Not from its smallness. The parallactic shift of the nearest of the stars as seen from opposite sides of the earth's orbit, is many times smaller.
From this he is to estimate how far off the other mountain is. To do this, one would have to measure just about the amount of parallax that Bessel found in his star. And yet this star is among the few nearest to our system.
The latter has a mean distance from the sun slightly greater than that of Jupiter, while the mean distance of Eros is less than that of Mars. The orbit of Eros is so eccentric that at times it approaches within 15,000,000 miles of the earth, nearer than any other regular member of the solar system except the moon, thus affording an unrivaled means of measuring the solar parallax.
This celebrated star, first seen by Tycho Brahe in the constellation Cassiopeia, never changed its position, or presented the slightest perceptible parallax. It could not therefore have been a meteor, nor a planet regularly revolving round the sun, nor a comet blazing with fiery nebulous light, nor a satellite of one of the planets, but a fixed star, far beyond our solar system.
But when the first of these two meetings takes place very near the node, giving a nearly central transit, the second falls too far from it, and the planet escapes projection on the sun. The parallax 8·95" derived by Leverrier from the above-described inequality in the earth's motion, was corrected by Stone to 8·91". Month.
Thus things went on until 1837, when Bessel announced that measures with a heliometer the most refined instrument that has ever been used in measurement showed that a certain star in the constellation Cygnus had a parallax of one-third of a second. It may be interesting to give an idea of this quantity.
The absolute depth of spot-cavities at least of their sloping sides was determined by Father Secchi through measurement of the "parallax of profundity" that is, of apparent displacements attendant on the sun's rotation, due to depression below the sun's surface.
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