Sky and Telescope - September 2017 - 50

SEPTEMBER 2017 OBSERVING
Celestial Calendar

Jupiter's Moons

Moon Occults Aldebaran

Sept 1

ON TUESDAY MORNING September

a.m. CDT; Toronto, d. 8:49, r. 10:00 a.m.
EDT; Atlanta, d. 8:56, r. 9:52 a.m. EDT;
Pittsburgh, d. 8:51, r. 10:01 a.m. EDT;
Washington, DC, d. 8:58, r. 10:05 a.m.
EDT; New York, d. 8:59, r. 10:07 a.m. EDT;
Boston, d. 9:01, r. 10:08 a.m. EDT.

3

Predictions for hundreds of locations, including the altitudes of the Sun
and Moon, are at lunar-occultations.
com/iota/bstar/bstar.htm. (The page
for each star displays three long tables
with less-than-obvious divides: for the
disappearance, the reappearance, and
the locations of cities.)
In the hours leading up, the Moon
will also mow down several stars along
the southern branch of the Hyades. The
brightest of these, magnitudes 3.4 to 3.8,
are Gamma Tauri and the Theta1 and
Theta2 Tauri pair. Timetables for each are
at the link above, but you'll need a pretty
dark sky with the Sun at least 12° or so
below the horizon ("Sun Alt -12").

7

12th for North America, the Moon once
again occults the brightest star it ever
can. The event happens in daylight for
the eastern and central parts of the continent, and before or during dawn for
the West. Aldebaran will vanish on the
bright limb of the waning gibbous Moon
and will reappear up to an hour or more
later from behind the Moon's dark limb.
For the disappearance you'll need a
telescope no matter where you are, and
you'll also need one for any chance at
seeing the reappearance unless you're
near the West Coast. Hawai'i sees both
events in darkness.
Here are some times:
Los Angeles, disappearance 4:34 a.m.
PDT, reappearance 5:49 a.m. PDT; Seattle,
d. 4:43; a.m., r. 5:54 a.m. PDT; Phoenix,
d. 4:48, r. 6:00 a.m. MST, Denver, d. 6:05
a.m., r. 7:24 a.m. MDT; Chicago, d. 7:37,
r. 8:51 a.m. CDT; Austin, d. 7:39, r. 8:16

2
EAST

WEST

4

Callisto

5
6

8

Europa

9
10
11
12
13
14
15

Ganymede

16
17
18

Io
19
20

Minding Mercury

Minima of Algol

22

MERCURY OFFERS its best morning

apparition of the year for northern
observers this month. This fast-moving
world is only observable low to the
horizon, where the seeing is seldom
good, but morning views are often better than evening ones. The angle of the
ecliptic affects visibility, and this is the
time of year when the ecliptic tilts most
sharply up from the dawn horizon. So
this is when our littlest planet reaches
almost its highest dawn elevation.
Watch for Mercury coming into view
as early as September 5th, when it glimmers at magnitude 1.4. It reaches greatest elongation, 18° west of the Sun, on
September 12th. That morning it's a
resplendent -0.3 magnitude. It continues to brighten for the next two weeks,
reaching -1.3 as it becomes unobservably low around the 26th or 27th.
50

21

Sept.

UT

Oct.

UT

23

3

5:15

1

21:22

6

2:04

4

18:10

8

22:53

7

14:59

11

19:41

10

11:48

26

14

16:30

13

8:36

27

17

13:18

16

5:25

28

20

10:07

19

2:14

29

23

6:56

21

23:03

26

3:44

24

19:52

29

0:33

27

16:40

30

13:29

These geocentric predictions are from the
heliocentric elements Min. = JD 2452500.179 +
2.867335E, where E is any integer. For a nakedeye comparison-star chart and more information
about Algol and its history, see skyandtelescope.
com/algol.

S E P T E M B E R 2 0 1 7 * SK Y & TELESCOPE

24
25

30
Oct 1

The wavy lines represent Jupiter's four big satellites.
The central vertical band is Jupiter itself. Each gray or
black horizontal band is one day, from 0 h (upper edge
of band) to 24h UT (GMT). UT dates are at left. Slide a
paper's edge down to your date and time, and read across
to see the satellites' positions east or west of Jupiter.
http://skyandtelescope.com/algol http://skyandtelescope.com/algol
Sky and Telescope - September 2017

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