A weak point of this method of measuring stellar distances is that it always gives what is called a relative parallax--i. e., the difference between the parallaxes of _A_ and _B_; and while it is customary to select for _B_ a star or stars supposed to be much farther off than _A_, it may happen, and sometimes does happen, that these comparison stars as they are called are as near or nearer than _A_, and give a negative parallax--i. e., the difference between the angles at _A_ and _B_ proves to be negative, as it must whenever the star _B_ is nearer than _A_. The first really successful determinations of stellar parallax were made by Struve and Bessel a little prior to 1840, and since that time the distances of perhaps 100 stars have been measured with some degree of reliability, although the parallaxes themselves are so small--never as great as 1''--that it is extremely difficult to avoid falling into error, since even for the nearest star the problem of its distance is equivalent to finding the distance of an object more than 5 miles away by looking at it first with one eye and then with the other. Too short a base line. 189. THE SUN AND HIS NEIGHBORS.--The distances of the sun's nearer neighbors among the stars are shown in Fig. 122, where the two circles having the sun at their center represent distances from it equal respectively to 1,000,000 and 2,000,000 times the distance between earth and sun. In the figure the direction of each star from the sun corresponds to its right ascension, as shown by the Roman numerals about the outer circle; the true direction of the star from the sun can not, of course, be shown upon the flat surface of the paper, but it may be found by elevating or depressing the star from the surface of the paper through an angle, as seen from the sun, equal to its declination, as shown in the fifth column of the following table, _The Sun's Nearest Neighbors_