428 SCIENCE PROGRESS 



when, in the old famiHar words, " day and night are of equal 

 length." We may imagine that on this most important date, 

 March 22, the earth is at the most important mark on the face 

 of the clock, the XII hour-mark ; then it will be at the IX hour- 

 mark on June 22, at the VI hour-mark on September 22, and at 

 the III hour-mark on December 22, for, as was said just now, 

 the earth goes round the circle in the opposite way to that in 

 which the hands of a clock rotate. The VI hour-mark will now 

 represent the autumn equinox, whilst the IX and III hour-marks 

 will be respectively the summer and winter solstices. We can 

 go a step further and calculate the position of the earth on any 

 day we choose. Thus, for example, it will be on the 57 minute- 

 mark on April 1 1, on the 37 minute-mark on August 10, and so 

 on. 



The path of Venus on the diagram will be a circle of radius 

 a little more than two-thirds that of the clock face, and so 

 will be about one-third of the distance from the circumference 

 to the centre. It will not, however, lie in the plane of the 

 clock face, but will be inclined to it at an angle of about 

 3 1 degrees, and will be placed so that half the circle lies above 

 and half below the surface of the table. It may be helpful 

 to think of this path of Venus as being a ring of wire round 

 which a bead can slide. Since the ring lies partly above and 

 partly below the table, the wire will pass through the table 

 at two points, to each of which we give the name " node." 

 Or, in more scientific language, the nodes are the points where 

 the orbit of a celestial body cuts the plane of the ecliptic, or 

 the orbit of the earth. The point at which the body passes 

 from south to north is called the ascending node, whilst the 

 opposite one is called the descending node. Naturally enough, 

 the line that joins the two is called the line of nodes. In the 

 case we are considering, in which the wire hoop representing 

 the path of Venus is incHned to the table, the line of nodes 

 will coincide with that joining the 17I minute-mark to the 

 47I minute-mark on the face of the clock, and the diagram will 

 become somewhat as shown. 



It will sometimes happen that the sun, Venus, and the earth 

 will be in the same direction, as in the figure, where S is the sun, 

 V Venus, and E the earth. In such a case Venus will not in 

 general be projected against the sun when looked at from the 

 earth, for it will be out of the table altogether, either above it 

 or below. This argument applies to all parts of the wire 

 except to those very near the two points marked M and N, the 

 two nodes. If Venus is at M or N, it will be on the table, and 

 if the earth is at P or Q at this time, Venus will be seen against 

 the sun and a transit will be observed. This will also be true 

 if Venus is very near M or N when the earth is very near P or Q. 



