The Birmingham age-herald., 1920-07-04

m tEastern standard time), the earth w.E be at it's greatest distance from the sun. It will then be 3,230.000 miles farther away than when it was nearest the sun on last January 8.
 THE SPHERICAL CLUSTERS OF
 APRIL
 If the observer will look in the con stellation Hercules, now almost over head. he will see, in a straight line be- tween the stars E and F, little misty patch of light. Even very small tele scope will show this to be an almost perfectly round little ball of stars packed closely together, though to show the cluster in all its beauty very large glass is required. When photographs of it are taken with the largest telescopes no less than S0,000 stars can be counted on the plates.
 A similar, perfectly round cluster may be found in the constellation of the Hunting Dogs, in the position c, Of Figure 1, and another is at D, just above the greenish star, G. of the Balance. Al together S6 spherical clusters are now known They are most beautiful and re- markable objects in the telescope, but the discovery of their true nature and of their positions in space and distances from us has only very recently been
 
 known, at least approximately, and 1t is the, at first, almost startling results Of the long investigations that have at tracted the attention and interest of as- tronomers.
 In the first place, it may be said that no spherical cluster is so near us that any of it's stars suffer any measurable d.splacement as we pursue our yearly path about the sun. Our position in space changes 1S6,000,000 during each six months as we go around the sun, put if a heavenly body is so far away that it's light occupies more than 15O years in coming to us, it's displacement owing to this change of position is too small to be measured, even in our largest tele- scopes In fact at this distance away. a star will be displaced in little Par allactic Orbit" the distance across which is almost exactly the same as the thick ness of the finest spider thread.
 Since, therefore, we can detect no dis placement of the stars of spherical cluster directly, we must have recourse to indirect methods of finding their dis tances from us. One of the most re- liable of all methods is by study of the variable stars which many of the clusters contain. No less than 25 years ago 1S2 of these variable stars were found in the cluster at C. 80 were dis covered in the cluster at D. while 122 were found in cluster in Centaurus. This last is the largest and finest clus- ter of the entire heavens, but it is not visible from the United States. Some of the clusters contain but very few or no variable stars at all. the cluster at B, for example, has but seven.
 Many of these variable stars belong to a type known as Delta Cephed Va riables. because the typical variable star of this kind. discovered 2+o years ago. is the fourth brightest star of the con- stellation Cepheus. This star will be found at A. Figure 1. Its light at a per fectly regular interval increases quite suddenly to three times it's usual amount. and then 1t fades away somewhat more slowly. The intervals between the great est brightness of the stars of cluster range from 10 to fl hours, the average period being little more than F hours,