AND DISTANCE FROM THE EARTIl 



together with the length of the remaining two sides. 

 In order to find out the distance of any object it is not 

 necessary to advance a single step towards it, but it is 

 essential to step aside to produce what is known as a base- 

 line sufficiently long to make the object appear to shift its 

 relative position, otherwise we cannot form a triangle 

 with its apex at the point to be measured and the other 

 ends resting on either side of the base-line, and the whole 

 success of our undertaking rests upon this. Once these 

 three points are within the meshes of a triangle, the space 

 readily yields up its distance be it ever so remote. 



If the object is near, a very short base-line will suffice 

 to produce a change of position or parallax^ as astronomers 

 call it, and the further the object is removed the longer 

 must we travel in an opposite direction before any alter- 

 ation is apparent. In fact, two lines drawn from opposite 

 sides of the earth and meeting at the centre of the sun 

 would make no appreciable angle and only appear as 

 parallel lines. The extreme difficulty of the problem 

 encountered in finding the sun's distance may be realized 

 from the conditions being identical to a surveyor with a 

 base-line of five feet being required to tell the distance of 

 an object ten miles away. In the case of our nearest 

 celestial neighbor, the moon, its distance is comparatively 

 so close to us that an observer in Hamilton and another in 

 South America will see it at the same moment of time, 

 projected in different parts of the sky, from which cause 

 its real distance of 238,000 miles is easily calculated ; but 

 if the sun is viewed from even the two extreme ends of 

 our globe, its position in the heavens remains absolutely 

 unchanged. Accordingly in attempting to measure the 

 enormous distance of the sun astronomers have to resort 

 to one or other of three different methods which we will 

 briefly consider. 



The principal method in the past has been to determine 

 the distance of Mars and Venus, the two nearest planets 



