HISTOBT OF PHYSICAL ASTKOHOMY.
127
he had not "for his data the accurate observations of Bradley and Maske-lyne, nor was the knowledge relative to the reduction of observations, andthe mode of combining them together, sufficiently advanced in his day. Itis not to be wondered at, then, that he erred in his estimation of theplanet’s mass, nearly as much as Lagrange did by a less legitimate process,having made it, in fact, equal only to two-thirds of the Earth ’s mass. De-lambre, under superior advantages, obtained xs’K.'B’iri f° r t ^ le mass of theplanet. By a curious coincidence this result agrees almost exactly withthe most approved evaluation of the Earth ’s mass*. Lindenau concluded,from his researches on the motion of Mercury, that the mass of Venus considerably exceeds the value assigned to it by Delambre. Burchardt,on the other hand, by a comparison of the solar tables with Maskelyne’sobservations, was induced to fix the mass of the planet at T7!T ,Vn t- Mr-Airy, by means of later observations at Greenwich, obtained xtfTjtrr? f°rthe mass of the planet J. M. Le Verrier was conducted to a mass equalto by his researches on the motion of Mercury §. These values
agree very nearly with each other, and also with the value assigned by thediminution of the obliquity of the ecliptic. Mr. Rothman, however, hasconcluded from the motion of Mercury’s nodes, that the mass of Venus isat least equal to The more complete developement of the se-
cular variations of the planetary orbits, can alone lead to desirable pre-cision on this point.
A knowledge of the perturbations occasioned in the motion of the Earth by the action of the other planets, forms an indispensable preliminary tothe construction of accurate solar tables. We have seen that Clairaut first investigated these perturbations by the application of his solution ofthe problem of three bodies. His results were introduced into Lacaille’ssolar tables, which continued in use among astronomers until the close ofthe last century.
One of the first steps taken by the French Board of Longitude, afterits establishment in 1795, was to procure the construction of new tablesof the sun, moon, and planets. In pursuance of this object Delambre in-vestigated the elements of the solar orbit by means of the observations ofBradley and Maskelyne, and calculated tables which were first publishedin 1806. The arguments of the equations depending on perturbation werederived from Laplace’s theory, but the coefficients were determined byobservation, Delambre by this means obtained values of the masses ofVenus, Mars, and the Moon . In 1813, Burchardt, with the view of ob-taining corrections of the solar elements, compared Delambre’s tables withabout 4000 Greenwich observations. He concluded from his researchesthat the epoch, the perigee, and the eccentricity required slight correc-tions. He also found that the mass of Mars should be diminished by-j 1 Tl th, and the mass of Venus by -^thU. In 1827, Mr. Airy compared thesolar tables with 86 observations of Sir James South’s, and concluded thatthe epoch and the perigee, especially the latter, ought to he sensiblyaltered **. In 1828 he discussed 1200 right ascensions of the sun, ob-
* Pontecoulant, in the third volume of his Theorie Analytique da Systenie du Monde,gives 555^53 as the value of the earth’s mass.
f Connaissance des Temps, 1816.
| Phil. Trans., 1828.
§ Connaissance des Temps, 1847.
]| Mem. Astr. Soc., vol. xii.
<j Connaissance des Temps, 1816; see also Memoires de Unstituf, 1812.
** Phil. Trans., 1827.