Parallax

(Annual), the change of the apparent place of a heavenly body, which is caused by being viewed from the earth in different parts of its orbit round the sun. The annual parallax of all the planets is found very considerable, but that of the | fixed stars is imperceptible. te To observe the Moon’s Parallax.—Observe very accurately the moon’s meridian altitude, and note | PA a ACI GL A ex, the moment of time. To this time, equated, compute her true latitude and longitude, and from these find her declination; also’ from her declination, and the elevation of the equator, find her true meridian altitude. Subtract the refraction from the observed altitude: then the difference be- - tween the remainder and. the true altitude will be the\parallax sought. If the observed altitude be not meridional, reduce it to the true altitude for the time of observation. By this means, in 1583, Oct. 12 day 5h. 19m, from the moon’s meridian altitude observed at 13° 38’, Tycho found her parallax to be 54 minutes. To observe the Moon’s Parallax in an Eclipse-—In an eclipse of the moon observe when both horns are in the same vertical circle, and at that moment take the altitudes of both horns; then half their sum will be nearly the apparent altitude of the moon’s centre; from which subtract the refraction, which gives the epparent altitude freed from refraction. But the true altitude is nearly equal to the altitude of the centre of the shadow at that time; now the altitude of the centre of the shadow is known,’ because we know the sun’s place in the ecliptic, and his depre ion,below the horizon, which is equal to the altitude of the opposite point of the ecliptic, in which the centre of the shadow is. Having thus the true and apparent altitudes, _ their difference is the parallax sought. De la Hire makes the greatest horizontal paral- ‘Jax 1° 1 25”, and the least 54/5’. M.le Monnier determined the mean parallax of the moon to be 57 12”. Others have made it 57’ 18”. ‘ In the Philosophical Transactions for 1764, there is given’a very ingenious method by Dr. Murdoch for finding the moon’s parallax and distance, from the received principles of gravitation. He thence finds, the mean horizontal parallax at the, equator 57’ 12-34, supposing the earth immoveable; and 56’ 44°07, supposing the earth _and moon to revolve about their common centre of gravity, To observe the Parallax of Mars.—1. Suppose Mars in the meridian and equator at H; and that the observer, under: the equator in A, observes him culminating with some fixed star. 2. If now the observer were in the centre of the earth, he would see Mars constantly in the samie point of the heavens with the star; and therefore, together _ with it, in the plane of the horizon, or of the 6th horary: but since Mars here has some sensible parallax, and the fixed star has none, Mars will ' be seen in the horizon, when in P, the plane of the sensible horizon; and the star, when in R,the plane of the true horizon: therefore observe the time between the transit of Mars and of the star through the plane of the 6th hour.—3. Convert this time into minutes of the equator, at the rate of 15 degrees to the hour; by which means there will be obtained the arch PM, to which the angle PAM, and consequently the angle AMD, is nearly equal; which is the horizontal parallax of Mars. (Pl. 129; fig. 3). _ if the observer be not under the equator, but in a parallel [Q, that difference will be a le arch QM: wherefore, since the small arches QM and PM are nearly as their sines AD and ID; and since ADG is equal to the distance of the place from the equator, 7. e. to the elevation of the pole, or the latitude; therefore AD to ID, as radius to the cosine of the latitude; say, as the cosine of the latitude ID is to radius, so is the parallax observed in I, to the parallax under the equator. _ Since Mars and the fixed star cauaot be com-