The Caldwell Objects

105 & 108
[degrees to the northwest], distance 5' from centre. Almost
perfectly insulated in a very large space almost entirely devoid
of stars, being the smaller and southern lacuna below the great
"coal sack."
BUZZING AROUND THE FOOT OF THE SOUTHERN
Cross, a tongue's length north of Chamaeleon, is
one of the sky's tiniest constellations: Musca, the
Fly. Originally the constellation was known as
Apis, the Bee, a name Johann Bayer introduced in
1603. Abbe Nicolas Louis de Lacaille substituted
Musca for Apis in or around 1752. But, as Richard
Hinckley Allen points out in his popular book
Star Names: Their Lore and Meaning, Edmond
Halley had called the constellation Musca Apis in
1679, and before him Giovanni Battista Riccioli
cataloged it as Apis seu Musca. Actually, the constellation's
proper full name is Musca Australis
(Southern Fly), because a Musca Borealis also
once buzzed around the northern sky, over the
back of Aries, the Ram. You also may find some
star charts calling Musca Australis by its more
taxonomical name, Musca Indica (Indian Fly).
What a busy history for such a tiny bee (or fly, as
the case may be)! Actually, with an area of 138
square degrees, Musca is deceivingly large, with
slightly more than twice as much celestial real
estate as far more famous Crux.
Shining at magnitudes 2.7 and 3.0, respectively,
Alpha (α) and Beta (β) Muscae skirt the
horizon at my observing site, the 4,200-foot-high
summit of Kilauea (latitude 19° 25' N). Alpha
Muscae is the more southerly of the two stars and
culminates just 1½° above my southern horizon.
Fortunately the sky is very dark and transparent
at the summit and Alpha Muscae is easy to see.
In fact, on paper I should be able to dip down to
declinations as far south as my colatitude of-70°
35'. Now here's the rub. The globular cluster
NGC 4833 (Caldwell 105)
418
GC / NGC: Globular cluster, pretty faint, large, round, stars of
magnitude 12 to 16.
is only 17' farther south; it lies 2½° southeast of
Alpha Muscae at a declination of -70° 52'. But
wait. Atmospheric refraction can lift starlight by
some 30' at the horizon. So, on paper at least, it is
possible to see NGC 4833 from Hawaii, though it
will peek just 13' above the horizon. (Actually it
should climb a bit higher for me because I
observe several thousand feet above sea level.) Of
course, atmospheric conditions at the horizon
would have to be superb — no vog (volcanic
smog), ocean haze, or orographic clouds — to
achieve this feat. Do perfect conditions like these
occur on Kilauea volcano? Yes, occasionally.
Have I succeeded in spotting NGC 4833 from
there? After several tries, yes. NGC 4833, then, is
the southernmost Caldwell object visible from the
United States. Only four Caldwell objects require
a U.S. resident to depart the Fifty States for more
southerly locales.
NGC 4833 is one of two Caldwell globulars
in Musca. The other, NGC 4372 (Caldwell 108),
lies 3° to the southwest of NGC 4833. At a
declination of -72° 39', it is too far south to be
spied from Hawaii. To see NGC 4372 I had to
travel to New Zealand. In fact, I observed both
clusters with the 4½-inch f/7 finderscope on the
20-inch f/13.5 Zeiss reflector at Auckland
Observatory (as well as with the 20-inch itself).
So let's look at both clusters in more detail.
The Abbe Nicolas Louis de Lacaille (1713-62)
discovered NGC 4833 during his two-year
exploration of the southern skies from the Cape
of Good Hope. The cluster is included in his 1755
list of 42 southern nebulae and clusters as a Class
I object (a nebula "without
Deep-Sky Companions: The Caldwell Objects