Sky and Telescope - August 2015 - 50

OBSERVING

Celestial Calendar

Jupiter's Moons
2
3

EAST

WEST

4
5
6

100
80
60
40
20
0

Aug 11

12h

18h

Aug 12

12h

18h

Aug 13

12h

18h

Aug 14

Time (UT)

7
8

Zenithal hourly rate (ZHR)

Aug 1

120

In 2013, a total of 303 meteor counters used standardized International Meteor Organization methods to log 34,805 Perseids across a span of 20 days. Observers contributed from
all around the world, enabling the IMO to create this nearly unbroken activity curve covering
the shower's richest 72 hours. The more counters, the smaller the statistical error bars. The
shower's zenithal hourly rate (ZHR) stayed around 100 or more for 12 hours.

Callisto

9
10
11

Europa

12
13
14
15

16
17
18
19

a measure of the shower itself, rather
than your light pollution or eyesight.
The ZHR is how many meteors a single
observer with a wide-open view would
count if the radiant were at the zenith
and magnitude-6.5 stars were visible.
Part of this process is determining
the faintest naked-eye stars you can see
in the area you're watching. Your rough
latitude and longitude, the time every
half hour or so, any minor obstructions

by clouds or trees, and time spent looking away also need to be recorded.
This way hundreds of observers'
counts from around the world will be
intercompared to track what the shower
is doing, hopefully for many days running as night circles and recircles the
globe. That's how the IMO compiled
the activity curve for the 2013 Perseids
above. Find the instructions and how to
report at imo.net/visual/major.

Saturn at Quadrature

Lunar Occultation

By August Saturn is way past its June 3rd
opposition, but so far is Saturn from Earth's
orbit around the Sun that this hardly matters for good observing. By August 21st,
when Saturn has moved the 90°° from opposition to quadrature (90° from the Sun), it
has shrunk by only 9% from its opposition
diameter. That matters less than a change in
the atmospheric seeing.
Quadrature is very nearly when sunlight
strikes an outer planet from the greatest
angle to Earth's line of sight. So this is when
we see Mars most gibbous, Jupiter's eastern
and western limbs most unequally lit, and
Saturn casting the broadest visible shadow
onto the rings behind it.
That, and the slight unequalness of the
lighting on Saturn's own east and west
sides, make the planet look a little more 3D
than usual around these times.

On the night of July 29-30, telescope
users across most of North America
except the northwest can watch the
almost-full Moon occult Rho1 Sagittarii, magnitude 3.9, the handle end
of the Sagittarius Teaspoon. The star
will snap out of view behind the very
thin dark limb of the 98%-illuminated
Moon: barely oﬀ the edge where a little
bit of shadowing appears on lunar
features.
Some times of the star's disappearance: Toronto, 12:27 a.m. EDT; Montreal, 12:36 a.m. EDT; Washington DC,
12:25 a.m. EDT; Atlanta, 12:05 a.m.
EDT; Miami, 12:08 a.m. EDT; Chicago,
11:11 p.m. CDT; Austin, 10:31 p.m.
CDT; Kansas City, 10:53 p.m. CDT;
Denver, 9:34 p.m. MDT; Los Angeles,
8:03 p.m. PDT.

20
21
22
23
24

Ganymede

25
26
27
28
29
30
31

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 0h (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.

August 2015 sky & telescope

http://www.imo.net/visual/major

Sky and Telescope - August 2015

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