Sky & Telescope - March 2020 - 25

M 93: JOSEF PÖPSEL / STEFA N BINNE WIES / DIE T M A R BÖCK ER /
CA PELL A OBSERVATORY; M46: N. A . SH A RP / NOAO / AUR A / NSF

p SPARKLES IN THE STERN The open star cluster M93 lies in the
constellation that Ptolemy called the Stern Ornament and that today we
know as Puppis. If you know where to look, you might even be able to
spot this compact cluster with the naked eye.

p DOUBLE DELIGHT The rich open star cluster M46 holds a surprise:
the planetary nebula NGC 2348 floats in front of the cluster, but the
two objects are not associated. You should be able to spot the cluster
without optical aid, but you'll need a telescope for the planetary.

the poop deck, the ship's highest horizontal surface. I will
call it the Old Poop Deck to distinguish it from the modern
Puppis, which is much bigger. It's barely above the horizon
as seen from Calais, Maine, or Venice. But as we sail south
toward Florida or Egypt, ever more of Old Argo Navis will rise
above the water each evening.
When we reach latitude 37°N (southernmost Virginia
or Sicily), Old Argo's brightest star appears above the waves,
radically altering the constellation's appearance. This is dazzling Canopus, magnitude -0.7, second only to the Sun and
Sirius in brightness (see page 45 in the February and March
2019 issues). In modern times, we're likely to assume that
anything so bright and low is an oncoming airplane or a
beacon on a distant hill. But in Ptolemy's time, before bright
artiﬁcial lights, Canopus must have been an unmistakable
beacon for an Egyptian sailor heading home to pick up yet
another shipload of wheat to feed Rome's hungry masses.
Almost half the people in the United States live far
enough south to see Canopus. If you're among that lucky
group, ﬁ nd a spot with an unobstructed view of the southern horizon such as a hilltop, beach, or treeless plain. Go
out when Canopus is highest, around 10 p.m. in mid-February or 8 p.m. in mid-March, and the brilliant star should
be instantly obvious unless the sky is extremely hazy. It's an
unforgettable sight.
By the time we've sailed down to Egypt or northern Florida,
we can see all of Old Argo Navis above the horizon - though
just barely. It's best viewed around midnight in mid-February
or 10 p.m. in mid-March, when Canopus is sinking and Old
Argo's second-brightest star, 1.8-magnitude Gamma Velorum,
is near its highest. Gamma forms a prominent zigzag line
together with 2nd-magnitude Delta and Kappa (κ) Velorum

to its lower left. If you extend that line to Gamma's upper
right to include Sigma Puppis, plus a bunch of fainter stars
in between, you end up with the stars Ptolemy described as
forming Argo's keel. I will call it the Old Keel to distinguish
it from the modern constellation Carina, which is a very different grouping, barely overlapping with the Old Keel.
Also in view now is IC 2391, the Omicron (ο) Velorum
Cluster, a prominent hazy patch surrounding 3.6-magnitude
Omicron, just 2° upper right of Delta Velorum. Of all Greater
Argo's many bright naked-eye star clusters, this is the only
one that falls within the region that Ptolemy assigned to Argo
Navis. Binoculars show that it's a very coarse cluster dominated by ﬁve or six stars ranging from magnitude 4 through 6.
One of the stars is a wide double; can you split it in binoculars?

Historical Interlude
We have now visited about half the 45 stars that Ptolemy
assigned to Argo Navis. He described the remainder as scattered around the deck and various other parts of the ship.
It's a little hard to visualize Old Argo Navis; the depiction in
Johannes Bayer's 1603 atlas Uranometria (shown on page 26)
is as good as any and better than most. Note that the ship is
sailing to the left, and the front half is missing, facts that all
ancient authors agreed on.
Conspicuously absent is 2.7-magnitude Mu (μ) Velorum.
Although it's farther north than many other of Ptolemy's stars,
and also fairly bright, he neglected to include it in his catalog.
Granted, measuring the positions of one-thousand-plus
stars using ancient technology was a daunting job. Even
so, Ptolemy's star catalog is pretty sloppy, especially in its
southernmost reaches. He could have done a better job if he
had been highly motivated. Just as Ptolemy didn't care much
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Sky & Telescope - March 2020

Table of Contents for the Digital Edition of Sky & Telescope - March 2020