Sky & Telescope - August 2021 - 50

AUGUST 2021 OBSERVING
Celestial Calendar
Action at Jupiter
To locate the fi ve brightest Saturnian
moons, use the interactive Saturn's
Moons app, found on the Tools page at
skyandtelescope.org.
Compared to Saturn, Jupiter's disk
overfl ows with detail. The dark North
and South Equatorial Belts (NEB and
SEB) that straddle the bright Equatorial
Zone (EZ) are the easiest features to
pick out. Increasing the magnifi cation
to around 100× will bring out additional
bands. Consult the illustration
on the preceding page to identify these.
In the past few decades, Jupiter's
iconic Great Red Spot (GRS) has been
shrinking and becoming more circular.
The GRS currently bears a lush,
red-orange color, which increases its
visibility. Located just south of the SEB,
the best time to see the GRS is when it
crosses Jupiter's central meridian. Refer
to the listings at right for transit times.
You'll fi nd that a #80A (blue) fi lter will
improve visibility of the GRS as well as
the rust-hued belts.
Jupiter's four bright Galilean satellites
in their myriad confi gurations
around the planet always make for an
enjoyable sight. Each is big enough to
show a tiny disk in a 6-inch scope at
Minima of Algol
Aug.
1
magnifi cations of 200× or greater. From
biggest to smallest, the Galilean moons
are: Ganymede (1.8″), Callisto (1.7″), Io
(1.3″), and Europa (1.1″).
The table on the facing page lists various
satellite events. Of special note are
double-shadow transit events in which
the shadows of two moons cross the
Jovian disk at the same time. Observers
in the western two-thirds of the Americas
have an opportunity to observe a
double-shadow transit on August 1st.
The event begins at 10:08 UT (3:08
PDT) as Europa's shadow lands on the
disk of Jupiter, joining Ganymede's. The
double-shadow transit concludes just 11
minutes later when Ganymede's shadow
departs the planet's disk.
In addition to their interactions
with Jupiter, the moons can also eclipse
and occult one another. Although
the " mutual satellite event " season is
winding down, in August there are two
occurrences visible for observers in the
Americas. Ganymede eclipses Europa
on the 9th (from 3:37 to 4:45 UT), and
once again on the 16th (from 10:03
to 10:35 UT). The combined light of
the two satellites will dim by about 0.5
magnitude during the eclipses.
29
UT
3:35
40:24
621:12
918:01
12
15
18
21
24
26
29
18
30
PERSEUS
38
Algol
14:49
11:38
8:27
5:15
2:04
22:52
19:41
These geocentric predictions are from the
recent heliocentric elements Min. = JD
2445641.554 + 2.867324E, where E is any
integer. For a comparison-star chart and
more info, see skyandtelescope.org/algol.
50 AUGUST 2021 * SKY & TELESCOPE
21
The Jupiter-observing season is in its
prime as the giant planet reaches opposition
on the night of August 19-20. As
a result, you can observe Jupiter from
dusk to dawn throughout the month
and will have plenty of opportunities to
look in on the comings and goings of its
four brightest moons and check out its
famous Great Red Spot. At mid-month
Jupiter gleams at magnitude -2.9 near
the border between Aquarius and Capricornus
and presents a 49″ disk.
Any telescope reveals the four big
Galilean moons, and binoculars usually
show at least two or three. Use the
diagram on the facing page to identify
them by their relative positions on any
given date and time. All the observable
August interactions between Jupiter
and its satellites and their shadows are
tabulated on the facing page.
Features on Jupiter appear closer to
the central meridian than to the limb
for 50 minutes before and after transiting.
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.)
July 1: 0:13, 10:09, 20:05; 2: 6:00,
TRIANGULUM
34
Perseus approaches the zenith during predawn
hours in August. Every 2.87 days, Algol
(Beta Persei) dips from its usual magnitude 2.1
to 3.3 and back. Use this chart to estimate its
brightness in respect to comparison stars of
magnitude 2.1 (Gamma Andromedae) and 3.4
(Alpha Trianguli).
15:56; 3: 1:51, 11:47, 21:43; 4: 7:38,
17:34; 5: 3:29, 13:25, 23:21; 6: 9:16,
19:12; 7: 5:07, 15:03; 8: 0:59, 10:54,
20:50; 9: 6:45, 16:41; 10: 2:37, 12:32,
22:28; 11: 8:24, 18:19; 12: 4:15, 14:10;
13: 0:06, 10:02, 19:57; 14: 5:53, 15:48;
15: 1:44, 11:40, 21:35; 16: 7:31, 17:26;
17: 3:22, 13:18, 23:13; 18: 9:09, 19:04;
19: 4:59, 14:56; 20: 0:51, 10:47, 20:42;
21: 6:38, 16:34; 22: 2:29, 12:25, 22:20;
23: 8:16, 18:12; 24: 4:07, 14:03, 23:58;
25: 9:54, 19:49; 26: 5:45, 15:41; 27:
1:36, 11:32, 21:27; 28: 7:23, 17:19; 29:
3:14, 13:10, 23:05; 30: 9:01, 18:57; 31:
4:52, 14:48
August 1: 0:46, 10:42, 20:38; 2:
6:33, 16:29; 3: 2:24, 12:20, 22:16; 4:
8:11, 18:07; 5: 4:02, 13:58, 23:54; 6:
9:49, 19:45; 7: 5:40, 15:36; 8: 1:31,
11:27, 21:23; 9: 7:18, 17:14; 10: 3:09,
13:05, 23:01; 11: 8:56, 18:52; 12: 4:47,
14:43; 13: 0:38, 10:34, 20:30; 14: 6:25,
16:21; 15: 2:16, 12:12, 22:08; 16: 8:03,
http://www.skyandtelescope.org http://www.skyandtelescope.org/algol
Sky & Telescope - August 2021

Table of Contents for the Digital Edition of Sky & Telescope - August 2021
