Sky & Telescope - June 2023 - 50

JUNE 2023 OBSERVING
Celestial Calendar
Action at Jupiter
AS JUNE BEGINS, Jupiter rises in the
predawn less than two hours before
the Sun. By month's end the situation
improves, and the planet is up nearly
3½ hours before sunrise, at which time
it hangs some 38° above the eastsoutheastern
horizon - high enough
for some fine telescopic views. It won't
be until mid-August, however, that
Jupiter climbs to the meridian before
sunrise. Even so, it's likely you'll get
your first good views of Jupiter during
June. How will its various cloud features
have changed since you last saw it? Will
the Great Red Spot still be reasonably
prominent? These are the kinds of questions
that make one's first glimpse of
this dynamic planet so exciting.
Any telescope reveals the four big
Noctilucent Clouds Arrive
with the Solstice
ON JUNE 21ST at 10:58 a.m. EDT,
the Sun will reach the summit of its
yearly hike around the ecliptic, better
known as the summer solstice. Although
in the Northern Hemisphere things
may warm up on the ground, the
upper atmosphere reaches extremely
low temperatures around this time.
In particular, the mesosphere, a scant
layer of air some 80 km (50 mi) high,
drops from its usual -90°C (-130°F)
to around -128°C (-198°F). That
extra chill coaxes what little water
vapor that's present at that altitude to
condense around the dust of vaporized
meteoroids and form noctilucent clouds
(NLCs). Because of their composition
and position near the edge of space, I
like to think of NLCs as having dual
citizenship - part Earth, part celestial.
Most common at latitudes higher
than 50° north and south, the fragile
strands and wavelets of NLCs have been
sighted at lower latitudes in recent years,
perhaps as a consequence of climate
change. NLCs appear low in the north50
JUNE 2023 * SKY & TELESCOPE
pLuminous blue and wispy noctilucent clouds
reflect in Eagle Lake north of Duluth, Minnesota,
late in evening twilight on June 27,
2021. Look for the clouds starting an hour after
sunset low in the northern sky. They're typically
brightest in the direction of sunrise or sunset.
ern sky (between about 5° and 20° altitude)
about 60 to 90 minutes past sunset
or before sunrise - long after more
familiar clouds have gone gray. From
my location in Duluth, Minnesota, I see
NLCs about half a dozen times each year
from late May through July.
The clouds coalesce at an altitude of
around 85 km and shine well into deep
twilight, when the first stars appear.
An unobstructed northern horizon is
essential for success. If you can spot
Capella, located below Polaris this time
of year, you're good to go. The clouds'
bluish tint comes from the ozone layer
located from 15 to 30 km high. Ozone
filters out red light, allowing the cooler
blue to reach our eyes. I'm sure you'll
agree that the fragile forms and color of
NLCs are absolutely entrancing.
Galilean moons, and binoculars usually
show at least two or three. The moons
orbit Jupiter at different rates, changing
positions along an almost straight line
from our point of view on Earth. Use the
diagram on the facing page to identify
them by their relative positions on any
given date and time. All the observable
interactions between Jupiter and its satellites
and their shadows are tabulated
on the facing page. Find events timed
for when Jupiter is at its highest.
Here are the times, in Universal
Time, when the Great Red Spot should
cross Jupiter's central meridian. The
dates, also in UT, are in bold. (Eastern
Daylight Time is UT minus 4 hours.)
May 6: 9:32, 19:28; 7: 5:23, 15:19;
8: 1:15, 11:11, 21:07; 9: 7:03, 16:58; 10:
2:54, 12:50, 22:46; 11: 8:42, 18:37; 12:
4:33, 14:29; 13: 0:25, 10:21, 20:17; 14:
6:12, 16:08; 15: 2:04, 12:00, 21:56; 16:
7:51, 17:47; 17: 3:43, 13:39, 23:35; 18:
9:31, 19:26; 19: 5:22, 15:18; 20: 1:14,
11:10, 21:05; 21: 7:01, 16:57; 22: 2:53,
12:49, 22:44; 23: 8:40, 18:36; 24: 4:32,
14:28; 25: 0:23, 10:19, 20:15; 26: 6:11,
16:07; 27: 2:02, 11:58, 21:54; 28: 7:50,
17:46; 29: 3:41, 13:37, 23:33; 30: 9:29,
19:25; 31: 5:20, 15:16
June 1: 1:15, 11:11, 21:07; 2: 7:03,
16:59; 3: 2:54, 12:50, 22:46; 4: 8:42,
18:37; 5: 4:33, 14:29; 6: 0:25, 10:21,
20:16; 7: 6:12, 16:08; 8: 2:04, 12:00,
BOB KING
Sky & Telescope - June 2023

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