My first Magazine Sky & Telescope - 03.2019 | Page 50

MARCH 2019 OBSERVING
Celestial Calendar by S. N. Johnson-Roehr
Charles Messier detected the
open cluster M93 on the night
of the spring equinox, March 20,
1876. The fi eld of view of this im-
age is 30′. Messier estimated the
cluster’s width at 8′ while William
Herschel pegged it as almost
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twice as wide.
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HD 62679
Starfi sh and Stragglers
Celebrate spring by recreating Messier’s historic
discovery of a striking star cluster.
he third week of March 1781
was particularly busy for Charles
Messier, the famed French comet
hunter. Prompted by the discovery of a
faint object in Virgo by his interlocu-
tor Pierre Méchain, Messier spent the
night of March 18th observing eight
new deep-sky objects in what we now
know as the Virgo Cluster. (The number
bumps to nine if you include the len-
ticular galaxy detected by Méchain on
March 4th but drops to six if you dis-
count both Méchain’s discovery and a
galaxy previously logged by Johann Elert
Bode.) The temperatures in Paris were
slightly warmer than typical that week,
so we can imagine Messier was feeling
comfortable and possibly excited by the
potential for further discoveries as he
settled in with his 100-mm refractor at
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M A RCH 2 019 • SK Y & TELESCOPE
the Naval Observatory on Paris’s Left
Bank on March 20th.
Messier discovered only a single
object that night, a modest open star
cluster. He duly entered the cluster into
the fourth edition of his “Catalogue of
Nebulae and Star Clusters” as No. 93,
laconically describing it as “a cluster of
small stars, without nebulosity, between
the Greater Dog [Canis Major] and
the prow of the ship [Puppis, once part
of Argo Navis].” He estimated it to be
about 8′ (arcminutes) across.
Was Messier disappointed that he
logged just one deep-sky object on
March 20th? It must have been a bit
anticlimactic after the long list he’d
written two nights earlier. As it turned
out, M93 was one of his last original
discoveries (excluding comets), and the
fourth edition of his catalog, which was
published in the Connoissance des Temps
for 1784, would be the fi nal version.
Since he didn’t include his feelings
along with the positional data for the
modest open cluster, we can only guess
as to his mindset, but I suspect he was
relieved to have the majority of what he
considered nuisance objects cataloged
so he could get back to the real task at
hand, hunting comets.
Nothing in Messier’s commentary
suggests that M93 offered anything
particularly special to the observer, but
words can be deceptive. In fact, the
open cluster boasts a lengthy obser-
vational history, partly because it’s
visually interesting and partly because
it’s an easy target, hovering at the edge
of naked-eye visibility. Even the most
modest scope can pick up the 6th-
magnitude cluster, and if you’re well
away from light pollution, you should
be able to spot it without optical aid
1½° northwest of the yellow supergiant
Xi (ξ) Puppis.
As astronomers following Messier
soon noted, M93 looks good in the
eyepiece. Caroline Herschel made the
next recorded observation of the cluster
after Messier’s, sweeping it up with her
small scope on February 26, 1783. She
invited her brother, William, to examine
it under more magnifi cation, and they
found it consisted of perhaps 100 or 150
stars that were “very beautiful, noth-
ing nebulous among them.” William
revisited M93 in November 1784 and
described it as “a cluster of scattered
stars, pretty close and nearly of a size,
the densest part of it about 15′ diam.,
but the rest very extensive.”
LTECH
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