96
F
or centuries, the number
of known “wandering
stars,” or planets, that
trailed through the night sky was
five. Together with the sun and
moon, that brought the total of
major celestial bodies visible from
Earth to seven—a number imbued
with mystical significance. Then,
in 1781, William Herschel spotted
Uranus out beyond the orbit of
Saturn, which forced astronomers
to rethink this number. However,
when the new planet’s orbit was
placed in an updated plan of the
solar system, it revealed another
number conundrum.Finding a gap
In 1766, a German astronomer
named Johann Titius discovered
a mathematical link between the
orbital distances of the planets.
He divided the orbital distance
of Saturn by 100 to create a unit
to measure all the other orbits.
Mercury’s orbit was 4 units from
the sun, and every other planet’s
position from there was linked
to a doubling of 3, or the number
sequence 0, 3, 6, 12, 24, 48, and 96.
So Mercury was located at 4 + 0
units from the sun, Venus at 4 + 3,THE DISCOVERY OF CERES
The orbits of the
planets appear to follow a
mathematical formula.The formula predicts that
the gap between Mars
and Jupiter should
contain an orbiting body.IN CONTEXT
KEY ASTRONOMER
Giuseppe Piazzi (1746–1826)BEFORE
1596 Johannes Kepler suggests
there are unobserved planets
in the solar system.1766 Johann Titius predicts
the gap between Mars and
Jupiter contains a planet.1781 William Herschel’s
discovery of Uranus confirms
the pattern of orbits proposed
by Johann Bode.1794 Ernst Chladni suggests
meteorites are rocks that were
once in orbit.AFTER
1906 Trojan asteroids are
found in the orbit of Jupiter.1920 Hidalgo, the first “centaur”
asteroid (an asteroid with
an unstable orbit), is found
between Jupiter and Neptune.2006 Ceres is classified
as a dwarf planet.Ceres is a minor planet,
or asteroid—one of
thousands in that
region of space.Ceres, a body seen in
the gap, is too small to be
a planet, but it does not
have the orbit of
a comet.Guiseppe Piazzi As was common for younger^
sons in wealthy Italian families,
Guiseppe Piazzi’s career began
in the Catholic Church. By his
mid-20s, it was obvious that
his abilities lay in academia. In
1781, he was appointed math
professor at a newly founded
academy in Palermo, Sicily, but
soon switched to astronomy.
His first task in this role was to
build a new observatory, which
he equipped with the Palermo
Circle, a telescope built in London
with a 5-ft (1.5-m) wide altitude
scale. It was the most accurate
telescope in the world at thetime. Piazzi was famed for
his diligence, and would take
measurements on at least four
consecutive nights to average
out errors. In 1806, he recorded
the large proper motion of the
star 61 Cygni. This prompted
several astronomers to use the
parallax of that star to measure
the distance between stars.Key works1803 Præcipuarum stellarum
inerrantium (Star catalog)
1806 Royal Observatory of
Palermo (Book 6)