1 16 Astronomical and Nautical Collections. 



ii. Discordance of the Lunar Observations made at Greenwich, 



and at Paris. In a Letter from Thomas Henderson, Esq. 

 Sir, 



In making some calculations from the observations made at the 

 Royal Observatories of Greenwich and Paris, I have been puzzled 

 by a circumstance, which I do not observe has been yet taken 

 notice of, but which seems to be highly deserving the attention of 

 Astronomers. It is this. From 1800 to 1809, the observations 

 made with the transit instrument at Greenwich show the sun and 

 moon to be further advanced in right ascension, than those made 

 with the transit instrument at Paris, From a mean of a great 

 many observations, the difference appears to be about 3 or 4 se- 

 conds of space. From the Introduction to Burg's Lunar Tables, 

 I observe that the epoch of the moon's mean longitude for 1801 is 

 34 seconds greater by the Greenwich observations, than by those at 

 Paris. 



I find a similar difference, upon comparing the observations of 

 the sun and moon's right ascensions, made with the transit instru- 

 ment and mural circle at Greenwich, in the year 1812, the right 

 ascension of these bodies by the transit instrument being about 4 

 seconds of space less than by the circle. 



This discrepancy appears to be occasioned by some difference in 

 the methods of observing the transits of the sun and moon, and of 

 the stars, by the respective instruments. Should its cause be found 

 out and obviated, the accuracy of the observations of the sun and 

 moon would evidently be much improved. To accomplish this, the 

 fittest course appears to be, to call the attention of astronomers to 

 the fact, through the medium of your valuable Astronomical and 

 Nautical Collections. 



It is owing to this circumstance, that the difference of longitude 

 between Greenwich and Paris, deduced from the moon's meridian 

 right ascensions, is greater than the true difference. See my paper 

 on this subject which I formerly transmitted to you. 

 I am, very respectfully, 

 Sir, 



Edinburgh, Your very obedient humble servant, 



^UtDec, 1824. Thomas Hekpjerson. 



