Monday, 25 March 2013

Early Views of Saturn: Galileo and Huygens over Two Saturn Years

I've been busy writing an article concerning the history of astronomical observations of Saturn, spurred in part by the Royal Society's release of the first 'Philosophical Transactions' from 1665 featuring, among other things, drawings of Jupiter and Saturn by the 'Ingenious Mr. Hooke'.  I've just come across a book from the 1960s by Alexander, called 'The Planet Saturn:  A History of Observation, Theory and Discovery.'  It's full of rich detail from original sources, lots of hand-drawn diagrams, and has certainly opened my eyes to how much planetary meteorology was possible even before the use of photographic film in astronomy.  Although known to the ancients as a wandering point of light among the fixed stars, and providing a character for many of our ancient mythologies, observing the details of the Saturnian system required Galileo's invention of the telescope in 1610.

Compilation from Huygen's Systema Saturnium (1659)
showing how Saturn's appearance had changed from
1610 to 1646.
Galileo's first, distorted views of Saturn's disc in July 1610 would have occurred at a heliocentric longitude of 150 degrees (i.e., the position of Saturn in its orbit as observed from the Sun, measured from a longitude of zero at the northern spring equinox, 90 at the northern summer solstice, 180 at the northern autumnal equinox and 270 at the northern winter solstice).  So Galileo was observing in late northern summer. For reference, Cassini arrived at Saturn in 2004 (heliocentric longitude of 293, just after the northern winter solstice), and today we're in northern spring (heliocentric longitude of 43 today).  It's interesting to note that Saturn has been around the Sun 13.5 times between Galileo's observations and those of the Cassini spacecraft 400 year later.  We know the story of Galileo seeing 'strange appendages' that appeared fixed in position and brightness (unlike Jupiter's moons, which moved from night to night), and describing Saturn as a 'triple planet.'  Those accompanying 'servants' vanished two years later at the autumnal equinox, when the ring opening angle had closed to zero as viewed from Earth.  "Has Saturn, perhaps, devoured his own children?", he asked?
Huygens' diagram of a ring cycle from his Systema Saturnium.

Almost half a century passed before Christiaan Huygens (1629-1695), at the age of only 26, solved 'the Saturn problem.'  Alexander's book describes Huygens' discovery of Titan, in March 1655, and how repeated observations between 1656 and 1659 (as Saturn's northern hemisphere was emerging into spring sunlight) confirmed his theory that the two ansae were indeed the broad flat ring that we now know and love.  His Systema Saturnium was published in 1659, and suggested that the rings were inclined by more than 20 degrees to the ecliptic, explaining the variations that observers had witnessed over the past 50 years.

Robert Hooke's observation of Saturn
from June 1666, published in the first
volume of the Philosophical Transactions.
Many observers began to train their telescopes on Saturn in the latter part of the 17th century, including 'the Ingenious Mr. Hooke' in 1666, as reported in the first volume of the Philosophical Transactions.  Robert Hooke's (1635-1703) observations suggested he could actually see the ring itself, crossing the planet's southern hemisphere and disappearing behind the northern hemisphere (observations at a heliocentric longitude of 118 degrees, near northern summer solstice).   Here's what the journal says (rough translation):

A Late Observation about Saturn made by the Same (Mr. Hooke)
June 26 1666 between 11 and 12 at night I observed the body of Saturn through a 60 foot telescope and found it exactly of the shape represented in the figure.  The ring appeared of a somewhat brighter light than the body, and the black lines crossing the ring and crossing the body (whether shadows or not, I dispute not) were plainly visible whence I could manifestly see, that the southernmost part of the ring was on this side of the body, and the northern part behind, or covered by the body.

[Incidentally, Hooke had been the assistant to Robert Boyle of Boyle's Law, and they conducted their experiments in an Oxford house now part of my college on the High Street, University College].

John Flamsteed (1646-1719), the first Astronomer Royal, observed Saturn in 1671 very close to the northern autumnal equinox, and found the rings to be 'very slender, and to one that thought not of them, scarce discernable' (volume 6 of Phil. Trans., heliocentric longitude of 180 degrees, northern autumnal equinox two Saturnian years after Galileo had first viewed the planet's disc).   This brings us full circle in solving the mystery of Saturn's appendages and up to the time of Jean Dominique Cassini, and his discoveries of Saturn's icy satellites and the division in the rings, subject of a later blog post.

Summary of Saturn Years, Measured from Spring Equinox (Heliocentric Longitude of Zero)
Saturn Year One:  1597-1627:  Galileo discovers Saturn's 'strange appendages'.
Saturn Year Two:  1627-1656:  Several theories proposed to explain Saturn's servants.
Saturn Year Three:  1656-1685:  Huygen's solves the Saturn problem and discovers Titan, Hooke and Flamsteed observe Saturn's ring progression.

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