POPULAR SCIENCE 



THE STORY OF TRANSITS 



By H. spencer TOY, B.Sc. A.Inst.P.. F.R.A.S. 

 Senior Mathematical and Science Master, Queen's College, Taunton 



One of the most interesting predictions in the history of astro- 

 nomy was that of 1629, when Kepler announced that both 

 Mercury and Venus would cross the sun two years later. It was 

 the first prediction of a transit , and a remarkable achievement 

 for the science of that day. 



The popular ideas of transits are somewhat vague. A man 

 recently made the statement to the writer, " I can always 

 remember the last date that Venus went round, for it occurred 

 a few weeks after I was born, in 1882." He had a hazy notion 

 that something unusual had happened in connection with the 

 planet. 



Another man proposed the question in a different way. 

 The writer was doing the astronomical work for one of the daily 

 newspapers. Those who have had much to do with journalism 

 know that not infrequently the readers of a paper write letters 

 to the editor to air all kinds of views and to ask for all kinds of 

 information. This particular newspaper prints a column or so 

 of answers to correspondents. Some funny questions come 

 sometimes. Venus gives rise to many. One day there came 

 this one : " I have seen it stated in your columns recently that 

 the star now visible in the western sky is the planet Venus. Is 

 this correct, for I seem to remember having seen it stated in a 

 book that this planet would not be seen again until the year 

 2004 ? " 



It is difficult sometimes to find out what your correspondents 

 mean or what they want to know. That would certainly have 

 been the case with this question had not the writer mentioned 

 2004. This date was the clue to the whole matter, for as 

 soon as an astronomer sees 2004 he thinks at once of the next 

 transit of our brilliant sister world ; the confusion in the corre- 

 spondent's mind was due to his ignorance of the nature of this 

 rare phenomenon. Let us, therefore, get a clear idea of what 

 we mean by the transit of a planet. 



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