The Moon1.32004/05/11 16:16:19 GMT-52004/05/26 11:50:27.956 GMT-5AlbertVan Heldenhelden@rice.eduAlbertVan Heldenhelden@rice.eduRobertAhlfingerahlfing@rice.eduMoonGalileosunspotsWilliam GilberttelescopeThomas HarriotCollegio RomanoJesuitChristoph Claviuslunar librationslongitudeCopernican theoryA brief discussion of the moon and the scientists who attempted to map it.
The Moon in Sidereus Nuncius
Ignoring the occasional pre-telescopic appearance of
exceptionally large sunspots, the
Moon is the only heavenly body which shows features to the naked
eye--the Man in the Moon. These features are permanent, and it
was therefore obvious that the Moon always keeps its same face
turned to us (although there are minor perturbations that were
not noticed until later). In the philosophy of Aristotle
(384-322 BCE), these features presented somewhat of a
problem. The heavens, starting at the Moon, were the realm of
perfection, the sublunary region was the realm of change and
corruption, and any resemblance between these regions was
strictly ruled out. Aristotle himself suggested that the Moon
partook perhaps of some contamination from the realm of
corruption.
Although Aristotle's natural philosophy was very influential in
the Greek world, it was not without competitors and
skeptics. Thus, in his little book On the Face in the
Moon's Orb, the Greek writer Plutarch (46-120 CE)
expressed rather different views on the relationship between the
Moon and Earth. He suggested that the Moon had deep recesses in
which the light of the Sun did not reach and that the spots are
nothing but the shadows of rivers or deep chasms. He also
entertained the possibility that the Moon was inhabited. In the
following century, the Greek satirist Lucian (120-180 CE) wrote
of an imaginary trip to the Moon, which was inhabited, as were
the Sun and Venus.
The medieval followers of Aristotle, first in the Islamic world
and then in Christian Europe, tried to make sense of the lunar
spots in Aristotelian terms. Various possibilities were
entertained. It had been suggested already in Antiquity that the
Moon was a perfect mirror and that its markings were reflections
of earthly features, but this explanation was easily dismissed
because the face of the Moon never changes as it moves about the
Earth. Perhaps there were vapors between the Sun and the Moon,
so that the images were actually contained in the Sun's incident
light and thus reflected to the Earth. The explanation that
finally became standard was that there were variations of
"density" in the Moon that caused this otherwise perfectly
spherical body to appear the way it does. The perfection of the
Moon, and therefore the heavens, was thus preserved.
It is a curious fact that although many symbolic images of the
Moon survive in medieval and Renaissance works of art (usually a
crescent), virtually no one bothered to represent the Moon with
its spots the way it actually appeared. We only have a few rough
sketches in the notebooks of Leonardo da Vinci (ca. 1500) and a
drawing of the naked-eye
moon by the English physician William Gilbert. None of these drawings
found its way into print until well after the telescope had come into astronomy.
The telescope delivered the coup de grace to attempts to explain
away the Moon's spots and to the perfection of the heavens in
general. With his telescope, Galileo saw not only the "ancient"
spots, but many smaller ones never seen before. In these smaller
spots, he saw that the width of the dark lines defining them
varied with the angle of solar illumination. He watched the dark
lines change and he saw light spots in the unilluminated part of
the Moon that gradually merged with the illuminated part as this
part grew. The conclusion he drew was that the changing dark
lines were shadows and that the lunar surface has mountains and
valleys. The Moon was thus not spherical and hardly perfect.
Galileo's wash
drawings
Galileo was not the only observer of the Moon. Indeed, he was
not the first. Thomas Harriot
drew the first telescopic representation of the Moon and
observed our nearest neighbor for several years. His drawings,
however, remained unpublished.
Those who wished to defend the perfection of the heavens brought
out the old argument about rarity and density. In the letter of
the Collegio Romano
mathematicians to Cardinal
Bellarmine of April 1611, Christoph Clavius (74 years old)
expressed a minority opinion: "But it appears to Father Clavius
more probable that the surface is not uneven, but rather that
the lunar body is not of uniform density and has denser and
rarer parts, as are the ordinary spots seen with the natural
sight." The other three Jesuit mathematicians on the faculty of the college,
however, believed that the lunar surface was indeed uneven. In
this case the opposition faded away over the next few years.
Galileo wrote in a letter, 1610, that he would like to make a
series of representations of the Moon showing its changing
phases. Presumably his purpose was to show how the shadows of
individual features changed with the illumination. It appears
that he abandoned this plan when he saw that there was no need
for such an ambitious and expensive project: even the Jesuit
fathers in Rome were convinced that the Moon's surface was
uneven. Indeed, Galileo never returned to the task of
representing the Moon. (In the 1630s he did, however, observe
lunar librations, which show that the
Moon does not always keep exactly the same face turned toward
the Earth.) Others did little better. Thomas Harriot did make a rough map of
the full Moon but never published it. Representations by Christoph Scheiner, Giuseppe Biancani, and
Charles Malapert were little more than diagrams, useful only for
supporting the verbal argument that the Moon's surface is rough
and uneven. These were, so to speak, generic moons, not
portraits of our nearest neighbor.
Sketches of the Moon by Scheiner (1614), Biancani (1620) and Malapert (1619)
If early observations and representations of the Moon were
designed to address the issue of its mountainous nature and
affinity with the Earth, by the 1630s the accent was
shifting. The rough lunar surface was now accepted by
astronomers and they turned their attention to how telescopic
observations could help them solve the problem of longitude. A lunar eclipse is an event
that appears the same to all observers for whom the Moon is
above the horizon (which is, of course, not the case with solar
eclipses). As the Moon enters the Earth's shadow cone, one can
mark the times at which the shadow crosses a particular feature
and later compare this time with the (local) time at which a
distant colleague observed the same event. The difference in
local times translates directly into their difference in
longitude.24 hours=360°. But
a verbal description of the lunar feature under consideration
was not enough. A lunar map was needed on which specific
features could be unambiguously identified. In Aix and Provence,
Nicholas Claude Fabri de Peiresc (still interested in the
problem of longitude) and his friend, the astronomer Pierre
Gassendi, decided to make a moonmap. They engaged the services
of Claude Mellan, one of the foremost artists and engravers of
his age. With Gassendi's sketches and guidance, Mellan engraved
three view of the Moon, first quarter, full Moon, and last
quarter.
Claude Mellan's moon engravings: 1, 2,3.
Mellan's three engravings are surely the finest artistic
renderings of the Moon ever made, but they show an artist's
Moon, not an astronomer's Moon. Mellan wonderfully represented
what he saw through the telescope: at first and last quarter the
details at the edge of the Moon are washed out while the
features near the terminator stand out starkly; conversely, at
full Moon the features in the center are washed out while those
near the edge show prominent relief. Where the solar rays are
perpendicular to the lunar surface they cast no shadows, but
where they rake the surface they throw long shadows. What
astronomers needed was a single map that showed all the features
equally clearly--a composite view that pictured the Moon in a
way it never appeared in reality but was accurate in its
placement of individual features.
The first such map was made by the Belgian cosmographer and
astronomer Michael Florent van Langren in 1645. Two years later
a much more influential effort was published by Johannes
Hevelius. In 1647 Hevelius, a wealthy brewer in the Polish city
of Gdansk, published Selenographia, the first treatise entirely
devoted to the Moon. Hevelius combined all the talents necessary
for his task. He made his own lenses, constructed his own
telescopes, observed the Moon on every clear night for several
years, drew his observations, engraved them himself, and had the
wealth to publish a sumptuous book at his own expense. In
Selenographia he presented engravings of every conceivable phase
of the Moon as well as three large plates of the full Moon: one of the ways the
full Moon actually appeared through the telescope, one the way a
maker of terrestrial maps might represent it (using the
conventions of geographers), and one a composite map of all
lunar features illuminated (impossibly) from the same side. It
is this last map that was to be used by astronomers during lunar
eclipses. Hevelius also suggested a system of nomenclature based
on earthly features.
Hevelius founded the science of selenography (after Selene, the
goddess of the Moon) and showed astronomers how to represent
heavenly bodies. Selenographia was a model for all who came
after him. All lunar maps since his time have used the
convention of single illumination (although while he used
morning illumination modern maps use evening illumination after
van Langren's model). He also instituted the practice of showing
the entire lunar surface visible from the Earth, which, because
of librations, is greater than a hemisphere. Hevelius's
nomenclature, although used in Protestant countries until the
eighteenth century, was replaced by the system published in 1651
by the Jesuit astronomer Giovanni Battista Riccioli, who gave
the large naked-eye spots the names of seas (Sea of
Tranquillity, Sea of Storm, etc.) and the telescopic spots (now
called craters) the name of philosophers and astronomers
(fig. 18). It should be pointed out that although Riccioli wrote
his Almagestum Novum ("New Almagest") in which this map appeared
to combat the Copernican theory,
he was evenhanded in assigning names: Copernicus and Kepler were
assigned prominent craters, and even Galileo received his due.
One last note. As the astronomical telescope with its inverted image came
into use, astronomers quickly adopted the habit of representing the way they saw the
Moon--upside down. This practice was followed until very
recently. Lunar images are now constructed and stored digitally
and can be displayed in any orientation. Astronomers have
therefore reverted to showing the Moon right side up.
Jesuits- The popular name for the
monastic order called the Society of Jesus. The order was
founded by Ignatius de Loyola in 1534, and was recognized by the
pope in 1540. The mission of the Jesuits was in three areas:
teaching, service to the nobility, and missionary work in
foreign lands. Their greatest mark was made in education, and
the Collegio Romano was their primary seminary.lunar librations- The real or apparent
oscillatory motion of the moon.