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9766 stars
CATALOGUE OF 9766 STARS
The weather in Cape Town was much better than in Europe. Lacaille categorised the year’s
weather as follows: one fifth was cloudy (winter), one fifth variable, one fifth calm and clear, and the
remaining two fifths were clear with a strong, south-easterly wind (September to March).
Because of the strong winds, Lacaille viewed through a 10 cm opening in the roof of his
observatory. The proposed catalogue required one hundred nights of uninterrupted viewing, for
six hours at a time, to complete. Each clear night he observed a section of sky covering 6 hours in
Right Ascension and 2.7 degrees in Declination. Two bright stars were observed during each session to
establish accurate positions for the fainter stars in the catalogue. “He used a clock star, usually
Sirius, to check on the performance of his clock. He always observed a reference star for which
the Right Ascension had been determined by observations of corresponding altitudes on the
same or nearby day…. The clock was illuminated by a feeble light provided by a dark lantern
placed opposite.”
“As soon as a star entered or left the plates of the reticle, Lacaille, closing his right eye, which was only used to look in the telescope, and keeping his left eye open, turned a little to present a little paper to the light of the dark lantern at the clock. He recorded his observation on it and quickly returned to the telescope.” To systematically cover the whole area, Lacaille divided the sky into twenty-five zones, between the south celestial pole and declination 23d 18’16”S.
The stars’ positions were determined using a clock and four different copper rhomboid reticles in his telescope. The first was used for zones 1 and 2 near the pole; the second, the small reticle (35 mm in diameter) was used for zones 3 to 7; the third, the large reticle (70 mm in diameter) was used for zones 8 to 21 and the fourth (also 70 mm in diameter but reversed) was used for zones 22 to 25 which were north of the zenith (dec -33.92d). Silk threads formed a cross in the middle of the rhomboid field.
Lacaille began observing in June 1751 but discovered errors in his observations made between
June 1 and August 21, 1751 and these objects were re-observed. He started again on August 23,
1751 and by July 18, 1752 had catalogued all the stars he could easily see, totalling 10,035. This
took 100 sweeps and 76 nights. It can be estimated that Lacaille was observing to approximately
magnitude 7.7 when the total number of stars is compared with the 1997 Hipparcos-Tycho
catalogue which contains 9,409 stars to magnitude 7.6 and 10,353 stars to magnitude 7.7 south
of Declination –23 degrees.
The formula m = 9.1 + 5logD in inches gives Lacaille’s half-inch aperture telescope a limiting magnitude of 7.6 which corresponds well with the estimate from the Hipparcos catalogue.
PUBLICATION OF THE LACAILLE STAR CATALOGUES
Lacaille’s raw observations of 10,035 stars were published in two parts. He reduced the data for
eleven of the twenty-five zones, namely zones 6-16 (Declination 47d to 77d south), and
produced a catalogue of 1942 stars to an estimated magnitude limit of 6.8. (The Hipparcos Catalogue has 1858 stars to magnitude 6.8 in this Declination range.) The first catalogue containing 1942 bright stars was published posthumously in 1763, a year after Lacaille died, as Coelum Australe Stelliferum. It was edited by J D Maraldi.
“The remaining stars were reduced much later in 1844 and published as A Catalogue of 9766 Stars in the Southern Hemisphere for the Beginning of the Year 1750 from observations of Lacaille in 1847, eighty-five years after his death. This was decades after Dunlop and Brisbane completed the Parramatta catalogue in 1827. The British Association for the Advancement of Science paid £200 for Lacaille’s second publication. Mr Wallace did the reductions and John Herschel wrote the preface.
SOUTHERN CONSTELLATIONS DEFINED BY LACAILLE
Lacaille constructed a chart before leaving the Cape that introduced fourteen new constellations.
He placed 1,000 stars in his fourteen new constellations and rejected a constellation called
Robur Carolinum (Charles’ Oak), which Edmund Halley had introduced in 1677 “to pay
homage to the king of England.” This was because nine of its twelve stars were already in
ancient catalogues. French English rivalry also probably paid a part.
The New Constellations were:
1. Apparatus Sculptoris (the Sculptor's Tools)
2. Fornax Chemica (the Chemical Furnace)
3. Horologium (the Clock)
4. Reticulus Rhomboidalis (the Rhomboidal Net)
5. Caela Sculptoris (the Sculptor's Chisel)
6. Equuleus Pictoris (the Painter's Easel)
7. Pyxis Nautica (the Mariner's Compass)
8. Antlia Pneumatica (the Air-Pump)
9. Octans (the Octant)
10. Circinus (the Compasses)
11. Norma, alias Quadrans Euclidus' (Euclid's Square)
12. Telescopium (the Telescope)
13. Microscopium (the Microscope)
14. Mons Mensae (Table Mountain)
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