(1) The displacement of Mars in the sky, as seen from two observatories which differ considerably in latitude, is, of course, greatest when Mars is nearest the earth. Now, it is evident than Mars will be nearer the earth when in opposition than when in any other part of its orbit; and the planet will be least distant from the earth when it is at its perihelion point, and the earth is at its aphelion point, at the time of opposition. This method, then, can be used to the best advantage, when, at the time of opposition, Mars is near its perihelion, and the earth near its aphelion. These favorable oppositions occur about once in fifteen years, and the last one was in 1877. [Illustration: Fig. 157.] Suppose two observers situated at _N'_ and _S'_ (Fig. 157), near the poles of the earth. The one at _N'_ would see Mars in the sky at _N_, and the one at _S'_ would see it at _S_. The displacement would be the angle _NMS_. Each observer measures carefully the distance of Mars from the same fixed star near it. The difference of these distances gives the displacement of the planet, or the angle _NMS_. These observations were made with the greatest care in 1877. (2) Venus is nearest the earth at the time of inferior conjunction; but it can then be seen only in the daytime. It is, therefore, impossible to ascertain the displacement of Venus, as seen from different stations, by comparing her distances from a fixed star. Occasionally, at the time of inferior conjunction, Venus passes directly across the sun's disk. The last of these _transits_ of Venus occurred in 1874, and the next will occur in 1882. It will then be over a hundred years before another will occur. [Illustration: Fig. 158.] Suppose two observers, _A_ and _B_ (Fig. 158), near the poles of the earth at the time of a transit of Venus. The observer at _A_ would see Venus crossing the sun at _V_{2}_, and the one at _B_ would see it crossing at _V_{1}_. Any observation made upon Venus, which would give the distance and direction of Venus from the centre of the sun, as seen from each station, would enable us to calculate the angular distance between the two chords described across the sun. This, of course, would give the displacement of Venus on the sun's disk. This method was first employed at the last transits of Venus which occurred before 1874; namely, those of 1761 and 1769. There are three methods of observation employed to ascertain the apparent direction and distance of Venus from the centre of the sun, called respectively the _contact method_, the _micrometric method_, and the _photographic method_.