Sky and Telescope - July 2017 - 46
JULY 2017 OBSERVING
Sun, Moon & Planets by Fred Schaaf
Four Out of Five
Whether you're a night owl or an early riser, you can observe a bright planet this month.
hough Mars remains too close to
the Sun for observation in July, four
other relatively bright planets make
their appearances this month. Mercury
shows up low in the west during evening twilight. Jupiter, too, comes into
view at dusk, low in the west-southwest,
and doesn't set until around midnight.
Saturn shines at its highest in the south
not long after dusk, setting a few hours
before the Sun comes up. Brilliant Venus
appears about three hours before July's
early sunrises, and it dominates the eastern view during the early stages of dawn.
Mercury fades only slightly this
month, dropping from magnitude -1.0
on July 1st to +0.4 by the 31st. Still,
binoculars will help, especially through
On the evening of July 25th, with
a crescent Moon about 8° to its upper
left, 1st-magnitude Regulus twinkles
just 1° upper right of Mercury. This may
be the last convenient opportunity to
see the star until August 21st, when it
appears about 1° from something else -
the totally eclipsed Sun!
DU S K
Mercury offers a fairly poor apparition
for observers at mid-northern latitudes.
Although it reaches a greatest elongation of 27° east of the Sun on July 30th,
it's highest for observers around latitude
40° north on July 19th. Even then, it's
only about 8° above the west-northwest
horizon just 30 minutes after sunset.
Dusk, July 5-7
1 hour after sunset
Jupiter dominates the low southwest
at dusk. It sets around 1 a.m. (daylightsaving time) as July begins but about 2
hours earlier as the month ends. The
giant planet dims subtly from magnitude
-2.0 to -1.9 during July and shrinks from
37″ to 34″ wide at the equator.
Worse for telescopic observations is
that by late in the month, Jupiter is little
Dawn, July 14
more than 20° high in the west-southwest an hour after sunset. Even so, Jupiter reaches eastern quadrature (90° east
of the Sun) on July 6th, so its shadow
falls far to the east side all month, facilitating views of Jovian satellites emerging
from their eclipses (see p. 51). Naked-eye
observers can enjoy Jupiter slowly closing
the gap between itself and Spica from
11° to 8° through July.
Saturn is at its highest around
midnight on July 1st but before 10 p.m.
by the month's ﬁnal days. Shining in
southeastern Ophiuchus above the
body of Scorpius, Saturn is almost as
far south as it can be in the zodiac, so
even at its highest it's 25° to 30° above
the south horizon for viewers around
latitude 40° north. Still, that's high
enough on summer nights of good seeing to try glimpsing extra detail on the
planet's globe and in its rings - which
are now open by 26.7°, very nearly their
greatest possible tilt.
1 hour before sunrise
TA U R U S
S A G I T TA R I U S
C a t 's
J U L Y 2 0 1 7 * SK Y & TELESCOPE
These scenes are
drawn for near the
middle of North
40° north, longitude
90° west); European
move each Moon
symbol a quarter of
the way toward the
one for the previous date. In the Far
East, move the Moon
symbol halfway. The
blue 10° scale bar
is about the width
of your fist when it's
held at arm's length.
For clarity, the Moon
is shown three times
its apparent size.