The Caldwell Objects

Mars, became so faint before its occultation, that
it could not be seen by the naked eye, nor with a
3-feet telescope." To Herschel, Cassini's
observation indicated that Mars had "a considerable
but moderate atmosphere" and "its
inhabitants probably enjoy a situation in many
respects similar to ours."
Herschel's observations of Jupiter were
focused mainly on the Galilean satellites. He
noted considerable changes in their brightnesses
and correctly reasoned that they have unequally
tinted regions. As the moons rotated, he
suspected, they would not present us with a
constant quantity of reflected light. He wrote that
"the satellites have a rotatory motion upon their
axes, of the same duration with their periodical
revolutions about the primary planet." He was
correct.
Saturn particularly interested Herschel, and
he was known to follow it continuously for up to
six hours at a time. This dedication led to his
discoveries of the two Saturnian satellites
mentioned previously. He also observed that the
Encke Division upon the A ring is not in the
middle of that ring's breadth, and noted:
That this black belt is not the shadow of a chain of
mountains [which some astronomers believed] may
be gathered from its being visible all around on the
ring. It is evident that this black zone is contained
between two concentric circles.
When Saturn's rings turned edgewise,
Herschel correctly noted that they were thinner
than the disks of its satellites. He surmised that
the planet's poles were flattened, like those of
Jupiter, and that its atmosphere was dense. He
also determined the ringed planet's rotation rate
and noted that its rings rotated as well.
On January 11, 1787, Herschel continued his
observations of "Georgium Sidus," noting the
position of some very faint stars near the
460
planet. On the next night, two of those "stars"
were missing. By February 5th, after a half dozen
more observations, he knew he had discovered a
new satellite of the planet. Before making the
announcement, however, he wanted to see it
move. So, on the evening of February 7th he kept
the satellite in view for a total of nine hours,
watching "this planet faithfully attend its primary
planet." Herschel called this new satellite Oberon.
During these studies he also discovered a second
satellite, which he called Titania.
Herschel was a great believer in using high
magnification or power:
When you want to practice seeing, apply a power
something higher than what you can see well with
and go on increasing it after you have used it some
time. These practices I have acquired and I can now
see with powers that I used to reject for a long time.
To search for satellites around Uranus, he
would use magnifications up to 7,200x and a
"Field Bar" to hide the planet. Both his satellite
discoveries, however, were made at the relatively
low magnification of 157x, the satellites being
"very nearly the dimmest objects that can be
seen." From observations made in 1787 and 1792
Herschel discovered that very "small" (faint) stars
dimmed when they approached Uranus, and that
the satellites became regularly invisible when
they arrived at certain distances from the planet.
He did not realize he was detecting evidence of
Uranus's ring system, which would not be
discovered for another 200 years.
TO THE STARS
Herschel was fascinated with variable stars and
spent much time trying to understand them. On
February 2, 1781, he wrote " Memorandum A of
the Uncommon Lustre of the Periodical
Deep-Sky Companions: The Caldwell Objects