We can extend our perception of distances by, ourselves, moving from one place to another, gaining thereby a longer base line, and noting the displacement of projection of the object on a distant background; by that means, distance up to several miles can probably be appreciated. But, when we try to determine the distance of, say, the Moon (240,000 miles away), we are helpless, especially as we have no marked background, except in the case of occultations of the Sun or Stars. But the Astronomer at once comes to our aid; a distance of several miles is carefully measured on a level plane, and, by placing telescopes at the extremities of that known line, we can mark the inclination of those telescopes to each other when focussed upon a particular mountain peak on the moon; by this means we know the angle of parallax (180 deg. less the sum of the two angles of inclination), and, from this and our known length of base line, we can calculate the distance. When however we go a step further and attempt to calculate the distance of the Sun (93,000,000 miles), we find our last base line again absolutely inadequate. But the astronomer helps us again; we now separate our two telescopic eyes by the whole diameter of the earth (7900 miles); this is accomplished by taking from the Equator two simultaneous observations of the Sun, at its rising and setting; for when the Sun is setting, at say the Equinox, it is at that moment rising at exactly the other side of the earth; the inclination of the two telescopes, directed to a certain point on the Sun, will now give the distance approximately, though even this base line is too short for exactitude. When however we attempt to go still further and try to ascertain the distance of stars, which are a million times further off than the Sun, such a base line is quite out of the question. How then can we get a base line for our telescopes longer than the whole width of the earth? The Astronomer again provides the means. The earth takes one year to complete its vast orbit round the sun, and the diameter of that path is 186,000,000 miles. This is made our new base line for separating our telescopes; an observation of a star is taken, say, to-day, and after waiting six months, to enable the earth to reach the other extremity of its vast orbit, another observation is taken, and yet it is found, as we shall see later on, that the distance of the nearest fixed star is so _stupendous_ that even this base line, of 186,000,000 miles, shows absolutely no inclination between the two telescopes except in about a dozen cases, and even in those the angle of parallax, perceivable, is so minute that no reliable distance can be calculated; we can only say that the star is at least as far away as a certain distance, but it may be much farther.