The Astronomy Book

97

Earth at 4 + 6, and Mars was at
4 + 12. Jupiter was at 4 + 48 and
Saturn was at 4 + 96. There was no
known planet in the sequence at
4 + 24 = 28, so there appeared to be
a gap in the solar system between
Mars and Jupiter. Titius proposed
that the gap must be occupied by
an unknown body. However, his
findings seemed too good to be
true—and the results for Mars and
Saturn were slightly out, so few
astronomers paid them much heed.
A few years later, in 1772, a
fellow German named Johann
Bode published a slightly modified
version of Titius’s work, which met
with greater acclaim. As a result,
the theory is best remembered
as Bode’s law. When Uranus was
discovered, Bode’s law predicted
that it would be 196 units from
the sun. It was finally shown to
be nearer to 192 units, but that
seemed close enough. Surely, it
meant the 28-unit gap must also
contain a planet.
In 1800, a group of German-based
astronomers led by Franz Xaver von
Zach, Heinrich Olbers, and Johann

Schröter decided to launch a search

of the gap. Their plan was to divide

up the zodiac—the strip of sky in

which all the planets move—and

ask Europe’s top 24 astronomers

to patrol one zone each, searching

for planetlike motion. The team

they put together was dubbed the

Celestial Police. But in the end

it was straightforward luck, not

efficiency, that filled the gap.

Surveying telescope

One of the astronomers among

the Celestial Police was Giuseppe

Piazzi, who was based in Palermo,

Sicily. Like most astronomers at the

time, Piazzi was mainly concerned

with creating accurate star maps.

To that end, he had acquired a

surveying telescope now known

See also: Elliptical orbits 50–55 ■ Observing Uranus 84–85 ■ Asteroids and meteorites 90–91

URANUS TO NEPTUNE

From Mars there follows

a space of 4 + 24 = 28 such

parts, but so far no planet

was sighted there. But should

the Lord Architect have left

that space empty? Not at all.

Johann Titius

Photographed by N A S A’s Dawn

spacecraft in 2015, Ceres is the largest

object in the asteroid belt, and the

only object large enough to have been

made spherical by its own gravity.

as the Palermo Circle. Although

it was not the most powerful

telescope of its day, its altazimuth

mounting could move both vertically

and horizontally, enabling it to

make very accurate measurements

of stellar positions, a feature that

would pay rich dividends.

On the evening of New Year’s

Day 1801, the instructions from

the Celestial Police were still en

route to Piazzi so he spent the

evening surveying stars and

recorded a new, faint object (with

a magnitude of eight) in the ❯❯