AUGUST 2019 OBSERVING
Celestial Calendar
Moon Blocks Star
THE WANING MOON HIDES a 3rd-magnitude star in Taurus on the morning of
August 25th. Zeta (ζ) Tauri, a spectroscopic binary system that marks the tip of the
Bull’s southern horn, will disappear behind the bright, leading edge of the lunar
crescent and reappear at the dark limb as much as an hour later. The predicted
path of visibility covers a large swath of North and Central America, but for most
observers in those locations, this will be a daylight event.
Observers in the Far West are better situated — the occultation occurs in the
dark, early morning hours with the Moon climbing higher as the event progresses.
San Diego, Los Angeles, San Francisco, and Salt Lake City see the entire occulta-
tion in darkness. For Denver, Phoenix, Las Vegas, and Mexico City, the reappear-
ance of Zeta occurs in a twilight sky. Observers in northern Canada and Alaska
will miss this one entirely.
San Francisco, Sacramento, and other places along the northern limit of the
event path should watch for a grazing event, when Zeta seems to skim by the
Moon’s northern limb. The star will blink off and on, as if controlled by a light
switch, as it’s alternately covered and revealed by lunar mountains and valleys.
Complete occultation timetables for hundreds of cities and towns in the pre-
dicted path of visibility are available from the International Occultation Timing
Association (IOTA). Visit https://is.gd/IOTApredict and click on the link for the
appropriate star and date. The event page includes three tables: for the star’s disap-
pearance, its reappearance, and the locations of the cities and towns in the event
path. The Moon Alt column provides the altitude of the Moon in degrees above the
horizon. The CA° column shows the cusp angle where the star disappears and reap-
pears. Use it to determine how many degrees from the Moon’s northern or south-
ern cusp you’ll need to look to see the event.
Here are a few timings: San Francisco, disappearance 3:59 a.m., graze 4:20 a.m.,
reappearance 4:31 a.m. PDT; Sacramento, d. 4:02 a.m., gr. 4:21 a.m., r. 4:32 a.m.
PDT; Los Angeles, d. 3:46 a.m., r. 4:38 a.m. PDT; San Diego, d. 3:43 a.m., r. 4:39
a.m. PDT; Las Vegas, d. 3:52 a.m., r. 4:44 a.m. PDT; Phoenix, d. 3:45 a.m., r. 4:47
a.m. MST; Salt Lake City, d. 5:05 a.m., r. 5:49 a.m. MDT; Denver, d. 5:04 a.m., r.
6:03 a.m. MDT; Guadalajara, Mexico, d. 5:34, r. 6:42 a.m. CDT; Mexico City, d.
5:41 a.m., r. 6:43 a.m. CDT.50 AUGUST 2019 • SKY & TELESCOPE
Action at Jupiter
JUPITER, ALREADY AT ITS HIGHEST
by the time true darkness falls, bright-
ens in Ophiuchus. The gas giant shines
at magnitude –2.4 at the opening of
August and improves to a luminous
–2.2 by the end of the month. At the
same time, the planet slims at the
equator, contracting from 43′′ across on
August 1st to 39′′ by August 31st. Jupi-
ter’s at the end of its retrograde journey,
reaching its western stationary point on
the 11th. After this apparent pause, it
resumes direct (eastward) motion, edg-
ing ever closer to Sagittarius.
Any telescope shows the four big
Galilean moons, and binoculars usually
show at least two or three. In binocu-
lars, the moons are all but indistin-
guishable from one another. Use the
diagram on the facing page to identify
them by their relative positions on any
given time and date.
All of the August interactions
between Jupiter and its satellites and
their shadows are tabulated on the fac-
ing page. Find events timed for when
Jupiter is at its highest in a dark sky,
just after nightfall.
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: 7:58, 17:53; 2: 3:49, 13:45,
23:40; 3: 9:36, 19:32; 4: 5:27, 15:23; 5:
1:18, 11:14, 21:10; 6: 7:05, 17:01; 7: 2:57,
12:52, 22:48; 8: 8:44, 18:39; 9: 4:35,
14:30; 10: 0:26, 10:22, 20:17; 11: 6:13,
16:09; 12: 2:04, 12:00, 21:56; 13: 7:51,
17:47; 14: 3:43, 13:38, 23:34; 15: 9:30,
19:25; 16: 5:21, 15:16; 17: 1:12, 11:08,
21:03; 18: 6:59, 16:55; 19: 2:50, 12:46,
22:42; 20: 8:37, 18:33; 21: 4:29, 14:24;
22: 0:20, 10:16, 20:11; 23: 6:07, 16:03;
24: 1:58, 11:54, 21:50; 25: 7:46, 17:41;
26: 3:37, 13:33, 23:28; 27: 9:24, 19:20;
28: 5:15, 15:11; 29: 1:07, 11:02, 20:58;
30: 6:54, 16:49; 31: 2:45, 12:41, 22:37.
August 1: 8:32, 18:28; 2: 4:24, 14:19;
3: 0:15, 10:11, 20:06; 4: 6:02, 15:58; 5:
1:54, 11:49, 21:45; 6: 7:41, 17:36; 7: 3:32,
13:28, 23:24; 8: 9:19, 19:15; 9: 5:11,
15:06; 10: 1:02, 10:58, 20:54; 11: 6:49,A So-So Shower
Every August, we’re treated to the meteor shower par excellence — provided
conditions are favorable, of course. Offspring of Comet 109P/Swift-Tuttle,
the Perseids put on a reliably good show every year. However, the shower
is expected to peak on the evening of August 12th this year, just in time to
share the night sky with a 95%-lit waxing gibbous Moon. So it’s unlikely any
of us will see the predicted 100+ meteors an hour near peak in 2019. Thank-
fully, this prolifi c shower is also long-lived, so you’ll certainly see shower
meteors if you’re out under dark skies earlier in the month. In fact, you can
start looking in late July, as soon as the full Moon gets out of the way. Watch
the shower rise toward maximum after the fi rst-quarter Moon on August 7th.