Sky & Telescope - May 2020 - 49

A Venusian
Challenge

SE A N WA LK ER / S&T

p TRIPLE TREAT In January, Comet PanSTARRS (C/2017 T2) shared the field of view with the
famed Double Cluster, in Perseus. On May 24th the comet pays a visit to the well-known Ursa
Major galaxy pair, M81 and M82.

galaxies M81 and M82 on May 22nd,
and a 1° brush with Dubhe, the northernmost of the Dipper's two pointer
stars, on June 3rd.
Under a dark, moonless sky the
comet will look like a fuzzy, pale blob
in a pair of 50-mm or larger binoculars. Telescope users can ferret out
delectable details like a tail (which can
change in both length and orientation
over time) and a bright, misty head or
coma - the object's temporary atmosphere of dust and gases released when
solar heating vaporizes its volatile ices.
Within the coma you can often see
what appears to be a starlike center,
called the pseudo-nucleus; the true, icy
nucleus is buried within, obscured by
its own dust and gas.
If you use high magniﬁcation in
your scope to zoom into the coma, you
might also detect hints of cometary
jets - geyserlike dust plumes erupting from the nucleus. A jet looks like
a tiny, low-contrast stub of brighter
material extending sunward from the
pseudo-nucleus. Activity within and
near the true nucleus, especially sudden
bursts of dust or episodes of fragmentation, can alter a comet's appearance
in short order. This is especially true
for "dynamically new" comets like
this one, which is making its ﬁrst trip

near the Sun after several billion years
locked up in the distant deep freeze of
the Oort Cloud.
PanSTARRS is the name given to
the survey's twin 1.8-meter RitcheyChrétien telescopes, named PS1 and
PS2. The PanSTARRS project is operated by the Institute for Astronomy at
the University of Hawai'i and receives
most of its funding from NASA's NearEarth Objects Observation Program.
Located atop Haleakala
¯, on the island
of Maui, PS1 saw ﬁrst light in June
2006. With its 3° ﬁeld of view and 1.4
billion-pixel CCD camera, it can scan
6,000 square degrees of sky every night,
covering the entire sky in 40 hours.
Its primary mission is to detect NearEarth Objects (NEOs) that could pose
potential impact threats. Along the way,
PanSTARRS has swept up numerous
asteroids, comets, variable stars, and
supernovae - basically anything that
ﬂuctuates in brightness or moves!
With more than 200 comets to
its credit (as of late January 2020),
PanSTARRS has practically become a
brand name, making it hard for some of
us to keep them all straight! The dual
instruments are extremely good at what
they do, so expect to see hundreds of
additional comet and asteroid discoveries in the years to come.

THROUGHOUT MAY, binoculars or
a small telescope will easily show
Venus as a miniature version of the
crescent Moon, as the planet thins
dramatically from 24% illuminated
on May 1st, to a ﬁ lament-thin 0.3%
on the 31st. At the same time, its
diameter swells from 40″ to 57″.
This rapid increase in apparent size
presents observers with an opportunity to discern the Venusian crescent
without optical aid.
If you have 20/20 vision, your
eyes have an angular resolution
of roughly 60″. Theoretically, you
should be able to resolve a crescent
Venus when it's near its maximum
apparent size. Is it really possible?
Despite several attempts, I've never
been able to do so, but I've read
numerous reports of crescent sightings online and elsewhere. Some
sound credible, others less so.
I've yet to see anything approaching a deﬁnitive study of the question, so let's see if we can get some
answers. How about a Venus viewing
party during the second half of
May? Invite your astronomy buddies,
neighbors, and family members. To
avoid glare, which tends to expand
the planet into a brilliant disk and
render its shape more difﬁcult to
perceive, observe from early- to midtwilight. Try the method Fred Schaaf
describes on page 46. And be sure to
view with your unaided eyes before
conﬁrming your observation with
binoculars.
You may not be able to perceive
a distinct crescent. Even when
Venus is at its biggest this month,
it remains slightly below the 60″
threshold mentioned earlier. Instead,
look for an elongated shape or an
indentation on the top edge of the
planet. Send your observations to me
at nightsky55@gmail.com and I'll
include them in a future edition of
Celestial Calendar.
s k y a n d t e l e s c o p e . o r g * M AY 2 0 2 0

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Sky & Telescope - May 2020

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