Sky & Telescope - April 2023 - 51

VENUS UV IMAGES: NIALL MCNEILL; JUPITER: SEAN WALKER / S&T
Seeing
Venusian
Clouds
CHANCES ARE YOU'VE tracked Venus's
changing phases many times in a telescope
or with binoculars. Your scope
may have also shown subtle brightness
differences across the planet's disk from
limb to terminator. But have you ever
seen Venus's sulfuric-acid clouds? Few
amateurs have. To discern these notoriously
low-contrast features, you need
good seeing, filters, and patience.
For imaging, an ultraviolet (UV) filter
such as a Baader U-Venus-Filter will
help. Visually, you don't need a monster
telescope. An 8-inch, good seeing conditions,
and a color filter or two might
be all that's required. Some observers
have had success with instruments
with apertures as small as 4 inches. A
Wratten #47 deep-blue filter is often a
first choice, but some also report success
with #21 orange, #23A red, and the
Baader Neodymium filter, which is said
to reduce glare and improve the contrast
of planetary details.
Plan your session when Venus's
phase is between gibbous and a thick
crescent, and the planet is at a healthy
elongation from the Sun. The middle
of this month is a good time to start.
On the 15th, Venus is 72% illuminated,
15.3″ across, and 40° away
from the Sun. By month's end its
phase wanes to 67%, and its diameter
increases modestly to 17″. Circumstances
remain ideal through midJune.
That should give you plenty of
opportunities to see cloud textures on
this oft-neglected planet.
Try to observe during the daytime
when Venus climbs highest in the sky.
Daylight nixes the planet's overpowering
glare that so often accompanies twilight
viewing, while its greater altitude
means steadier seeing conditions. In
mid-April Venus transits the meridian
around 3:30 p.m. local daylight time at
a satisfying 73° altitude, as seen from
Violet
UV
Blue
Composite
pThese photos taken through violet, UV, and blue filters reveal remarkable detail in Venus's
clouds on September 22, 2021, when the planet was 17.6″ across and 65% illuminated. The three
frames were composited and assigned colors to create the composite color image. Although
Venus takes 243 days to rotate, its dense atmosphere and clouds circle the planet every four days
- a phenomenon known as super-rotation.
a latitude of 40° north. A month later,
it's even higher (76°) during its transit
at around 4:00 p.m.
There are several ways to find the
planet in daylight. Obviously, a properly
aligned Go To scope makes short work
of locating it, but a regular equatorial
mount is also a good choice. Use the
table on page 44 or software to get the
right ascension (R.A.) and declination
(Dec.) of both the Sun and Venus. Next,
approximately polar-align your scope
using a compass, then pop on a solar
filter. After centering and focusing the
Sun, offset in R.A. and Dec. using your
mount's setting circles to locate Venus
in a low-power eyepiece. Remove the
solar filter and, if I've been diligent, the
planet will appear in sharp focus in the
field of view. If it's not, have a look for it
in your finderscope, or sweep around a
bit - you'll get there eventually.
Jupiter is in conjunction with the Sun on April 11th and out of view all month. As
a result, our regular tables showing the transit times of the Great Red Spot and
Jovian satellite events aren't presented in this issue. They will return (along with
Jupiter) next month.
sk yand tele scope .o r g * APRIL 2023 51

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Sky & Telescope - April 2023

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