It is therefore a conic section having its eccentricity equal to unity.
Its mean distance from the sun is 1.46 times i that of the earth; but, besides, the eccentricity of its orbit is large (0.22), so that at the most favourable opportunity it can come within one-seventh of the distance of the sun.
The eccentricity determined in this way is more than a degree in error, owing to the effect of the evection, which was unknown to Hipparchus.
The result of the latter inequality is brought out when it is sought to determine the eccentricity of the orbit from the observations near the time of the first and last quarter.
Therefore the eccentricity found by Hipparchus was only 5°, and was more than a degree less than its true value.