Lunar Occultation
ON THE MORNING OF September 22nd,
the last-quarter Moon temporarily
hides Eta (η) Geminorum for observers
in the American Southwest and Central
America. Eta disappears behind the
bright limb of the Moon and reappears
at the dark limb as many as 70 min-
utes later, depending on location. For
most, the reappearance will happen in
a brightening sky, just at the edge of
astronomical twilight.
More familiarly known as Propus,
Eta is an M-type variable, moving from
magnitude 3.2 to 3.9 and back over a
period of 233 days. Occultations by the
Moon can reveal the double nature of
stars — the disappearance of a star is a
more gradual, stepped event when a star
is double — but Eta is already a known
triple-star system. A magnitude-6.2 sec-
ondary, Eta Gem B, can be split with a
6-inch scope at moderate powers under
transparent skies. The tertiary member,
Eta Gem C, was detected only from the
primary’s radial velocity variations.
A grazing occultation is visible
within 2 to 3 km of the predicted50 SEPTEMBER 2019 • SKY & TELESCOPE
northern limit of the path of visibility.
Observers near Austin and San Antonio,
Texas, may be able to see Eta “blink” as
the rugged northern edge of the Moon
glides in front of it.
Although astronomers know Eta is
a multiple-star system, observations of
the event remain highly desired by the
International Occultation Timing Asso-
ciation (IOTA). Timetables are available
from the IOTA website (https://is.gd/
IOTApredict). You can also calculate
predictions and generate maps that
show the precise disappearance-reap-
pearance locations on the lunar limb,
using IOTA’s free software program,
Occult (https://is.gd/IOTAoccult).
Some timings: La Paz, Mexico,
disappearance 4:09 a.m., graze 4:31 a.m.,
reappearance 4:48 a.m. MDT; Guada-
lajara,d. 5:00 a.m., r. 6:11 a.m. CDT;
Mexico City,d. 5:01 a.m., r. 6:23 a.m.
CDT; San Antonio, Texas,d. 5:33 a.m.,
gr. 5:54 a.m., r. 6:12 a.m. CDT; Austin,
d. 5:38 a.m., gr. 5:56 a.m., r. 6:11 a.m.
CDT; Brownsville,d. 5:19 a.m., r. 6:23
a.m. CDT.Action at Jupiter
JUPITER APPEARS out of evening twi-
light on the 1st, already at its highest in
the south. By the 30th, it sets around 10
p.m. daylight-saving time.
Any telescope shows the four big
Galilean moons, and binoculars usu-
ally show at least two or three. Use the
diagram on the facing page to identify
them by their relative positions on any
given time and date.
All of the September 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 in the evening hours.
On nights of steady seeing, look
for the Great Red Spot (GRS). It began
acting up in May, so yo u may be able to
detect changes. Here are the times, in
Universal Time, when the GRS should
cross Jupiter’s central meridian. The
dates, also in UT, are in bold. (Eastern
Daylight Time is UT minus 4 hours.)
August 1: 8:41, 18:36; 2: 4:32, 14:28;
3: 0:23, 10:19, 20:15; 4: 6:10, 16:06; 5:
2:02, 11:58, 21:53; 6: 7:49, 17:45; 7: 3:40,
13:36, 23:32; 8: 9:28, 19:23; 9: 5:19,
15:15; 10: 1:10, 11:06, 21:02; 11: 6:58,
16:53; 12: 2:49, 12:45, 22:40; 13: 8:36,
18:32; 14: 4:28, 14:23; 15: 0:19, 10:15,
20:11; 16: 6:06, 16:02; 17: 1:58, 11:54,
21:49; 18: 7:45, 17:41; 19: 3:36, 13:32,
23:28; 20: 9:24, 19:19; 21: 5:15, 15:11;
22: 1:07, 11:02, 20:58; 23: 6:54, 16:50;
24: 2:46, 12:41, 22:37; 25: 8:33, 18:29;
26: 4:24, 14:20; 27: 0:16, 10:12, 20:07;
28: 6:03, 15:59; 29: 1:55, 11:50, 21:46;
30: 7:42, 17:38; 31: 3:33, 13:29, 23:25.
September 1: 9:21, 19:17; 2: 5:12,
15:08; 3: 1:04, 11:00, 20:55; 4: 6:51,
16:47; 5: 2:43, 12:39, 22:34; 6: 8:30,
18:26; 7: 4:22, 14:17; 8: 0:13, 10:09,
20:05; 9: 6:01, 15:56; 10: 1:52, 11:48,
21:44; 11: 7:40, 17:35; 12: 3:31, 13:27,
23:23; 13: 9:19, 19:14; 14: 5:10, 15:06;
15: 1:02, 10:57, 20:53; 16: 6:49, 16:45;
17: 2:41, 12:36, 22:32; 18: 8:28, 18:24;
19: 4:20, 14:16; 20: 0:11, 10:07, 20:03;
21: 5:59, 15:55; 22: 1:50, 11:46, 21:42;
23: 7:38, 17:34; 24: 3:29, 13:25, 23:21;
25: 9:17, 19:13; 26: 5:08, 15:04; 27:
1:00, 10:56, 20:52; 28: 6:48, 16:43; 29:
2:39, 12:35, 22:31; 30: 8:27, 18:22.SEPTEMBER 2019 OBSERVING
Celestial Calendar
NASA/^ JPL- CA
LTECH/^ SWRI^ /^MSSS/^ GERAL
D^ EICHST
ÄDT^ /
SEÁNDORANNASA’s Juno spacecraft cap-
tured this image of Jupiter’s
Great Red Spot on December
21, 2018. Since then, the giant
storm has begun to unfurl, with
clouds peeling off its fl anks.