Southern survey
OnAugust6,1751,Lacaillesettoworkonhismajorproject
ofchartingthestarsofthefarsouthwithhighprecision.
Constant southeast winds made for shaky observing with
largeinstruments.Lacaillefoundthatheobtainedthemost
accuratestarpositionsusingatiny½-inchrefractor,26inches
long,affixedtoaverysolidlymountedmeridianquadrantwith
a1-metre(3-foot)radius.Withthissmallscope,hemeasured
thesiderealtimeswhenstarsnearthemeridiandriftedacross
V-shapedpairsoflinesonaglassreticleinthelittletelescope’s
focalplane.Theaverageofthetimeswhenastarcrossed
thetwolinescouldyielditsrightascension.Thedifference
betweenthetwotimesgaveitsdeclination
with respect to the centre of the reticle. He
workedalonethroughthenights,withno
assistanttowatchtheclockormakenotes.
LacaillehadpromisedtheAcademyan
accurate survey of all southern stars
brighterthan3rdor4thmagnitude,
apopulationofseveralhundred.He
farsurpassedhisownexpectations.
Overthenext11months,during110
observingsessionsof8hourseachand
another16dusk-to-dawnmarathons,
anddespitesufferingheadaches,
rheumatism and fevers, Lacaille
mapped9,766starstoasfaintas
8th magnitude between the Tropic
of Capricorn (declination –23.4°)
andthesouthcelestialpole.Itwas
anastonishingfeatofpersonal
industry and endurance.
In comparison, Edmond
Halley, no slacker
himself, had measured
the positions of just 341 stars from the South
Atlantic island of Saint Helena from 1676 to 1678.
Duringhissurvey,Lacaillealsomeasuredand
catalogued42non-stellarobjects,includingthe
JewelBoxCluster,theEtaCarinaeNebula,andthe
globularclusters47TucanaeandOmegaCentauri.
Lacaille’scatalogueofsoutherndeepskyobjects
predateseventhefirstversionofthemorefamous
cataloguebyhiscountrymanCharlesMessierby
some20years.WithhistinytelescopeLacaille
wasunabletodeterminetheirnature,but
he wrote to the French Academy that
theywere“sovariedthattheirexact
anddetaileddescriptioncanoccupy
astronomers for a long time and give rise
toagreatnumberofcuriousreflections
onthepartofphilosophers”.Lacaille
himself refrained from such reflections,
contenttotendtohismeasurements.
This prodigious celestial bountycreatedanewproblemfor
the French astronomer:
howtoorganiseitall.
Much of the far southern
sky was not yet
demarcated into
constellations. So
Lacaille invented his own.
AsamanoftheEnlightenmentand
unencumbered by classical whimsy, he named most
of his new constellations after, as he put it, “principal figures
of the arts”: The Painter’s Easel; the Sculptor’s Workshop;
Microscopium and Telescopium; Reticulum, the celestial
apotheosis of his little refractor’s measuring reticle; Octans,
the Octant; Fornax, the Chemist’s Furnace; and so on.
Hisoneexceptionwastheconstellation Mons Mensa
(Table Mountain), now simply Mensa, which he named for
thefamouslandmarknearCapeTown.He placed it so the
LargeMagellanicCloudwouldrestuponthe mountain’s
top. Mensa remains the only constellation, of the 88 now
officiallyrecognised,namedafterageographical feature.
WhileattheCape,Lacaillealsoprecisely measured
positionsoftheMoon,VenusandMarswith respect to stars.
Upon his return to France, he compared these measures to
ones made on the same dates by his colleague Joseph Jérôme
deLalandeinGermany.Thetwocollaborators found slight
differences in the angular positions of these objects: parallaxes
caused by the thousands of kilometres between their
observingstations.Thevaluethat they determined
for the Sun’s topocentric parallax (10.2 arcseconds)
compares well with the currently accepted 8.8
arcseconds; the distance to the Sun that they
obtained thereby was just 16% too short.
Lacaille and Lalande also compared
positions of stars low on the horizon in
France and high overhead in South Africa to
estimate the effects of atmospheric refraction
asafunctionoftemperature and pressure.
His celestial survey complete, Lacaille turned
tohisnextmajorgoal:measuring a long arc
of a longitude meridian near the Cape. With
the help of the governor and a translator, he
surveyedahighlyprecisenorth-south arc
135km(84miles)long,apunishing task in
unfamiliar, mountainous countryside and
withoutaworkingknowledge of Dutch.
Lacaille found a surprising result, and it
survived a quick second survey to check it —
degrees of latitude in the Southern
Hemisphere seemed to be 0.2% closer
together than their northern equivalents. In
essence,heconcludedthatEarth was slightly
pear-shaped. But Lacaille had failed to account for
the gravitational influences of Table Mountain and
other mountains near the surveyed line. These causedANTLIASCULPTORRETICULUMNICOLAS-LOUIS DE LACAILLE38 AUSTRALIAN SKY & TELESCOPE November | December 2017