Gas giants take center stage
Although bright planets continue to avoid the early evening sky, the viewing situation improves dramatically in the ensuing hours. And best of all, the first solar system world to appear happens to be the favorite of many observers.
Saturn rises in the east around 9 p.m. local time in early July and some two hours earlier by month’s end. The ringed planet resides in eastern Capricornus, where it moves slowly westward (retrograde) relative to the Sea Goat’s stars. It passes 1.5° north of magnitude 2.8 Delta ( δ) Capricorni, the constellation’s brightest star, during July’s second week. At magnitude 0.4, Saturn shines nine times brighter than its companion. The planet’s retrograde motion signals that opposition is approaching. It will reach this point of peak visibility in mid-August.
Saturn climbs high enough to deliver stunning telescopic views within two hours after rising. Even small scopes at low magnifications show the ring system nicely. At midmonth, the rings span 42" and tip 13° to our line of sight. Centered within the rings lies Saturn itself — an oval disk measuring 18" across the equator.
An even brighter planet enriches the late evening sky by midmonth. Jupiter rises around 12:15 a.m. local time July 1 and a half-hour earlier each week thereafter. It brightens from magnitude –2.4 to magnitude –2.6 during July, making it the sky’s most dominant point of light until Venus rises before dawn. Jupiter spends the month in a corner of the non-zodiacal constellation Cetus the Whale, beginning its own retrograde loop July 29 in anticipation of a late September opposition.
Jupiter’s disk appears noticeably flattened when viewed through a telescope. It’s equatorial diameter spans 42.7" at midmonth while the polar diameter measures only 40.0". Look for intricate details in the giant planet’s atmosphere during moments of good seeing. Also take some time to admire Jupiter’s four bright Galilean moons: Io, Europa, Ganymede, and Callisto.
While Saturn’s and Jupiter’s visibilities improve markedly during July, Mars operates on a slower schedule. It rises at
1:45 a.m. local time July 1 and only about 25 minutes earlier by month’s end. The Red Planet crosses from Pisces the Fish into Aries the Ram on July 8. Shining at magnitude 0.3 at midmonth, it easily outpaces the stars in these constellations.
Despite its slow progress, Mars continues to pull steadily closer to Earth and thus looks larger through a telescope. Its 8"-diameter disk should show some subtle details through medium-sized scopes during moments of good seeing.
Venus rises a bit more than two hours before the Sun in early July. That gives you plenty of time to enjoy a rare celestial alignment on the 1st, when the brilliant planet becomes the second eye of Taurus the Bull. Blazing at magnitude –3.9, Venus shines nearly 100 times brighter than the Bull’s permanent eye: 1st-magnitude Aldebaran. The planet’s westward motion against the stars carries it through northern Orion the Hunter for 50 hours in mid-July before it moves into Gemini the Twins for the rest of the month. When viewed through a telescope, Venus spans 11" and appears about 90 percent lit.
You might catch a glimpse of Mercury in the northeast shortly before dawn in early July or in the west-northwest at dusk late in the month, but the planet’s low altitude makes observations difficult. You’ll be better off waiting for the inner world’s best evening appearance of the year in August.
The starry sky
As twilight fades to darkness these July evenings, the constellations of the zodiac span from Cancer the Crab in the westnorthwest to Capricornus the Sea Goat in the east-southeast. These star patterns follow the ecliptic plane, the apparent path of the Sun across our sky that coincides with the projection of Earth’s orbit onto the celestial sphere.
When the International Astronomical Union adopted official constellation borders in the 1920s, the length of the ecliptic in each constellation varied. Our current evening sky boasts the constellations having both the longest and shortest ranges of ecliptic longitudes cutting through them: Virgo and Scorpius, respectively. This naturally corresponds to the longest and shortest amounts of time that the Sun spends within their borders.
Astronomers measure longitude along the ecliptic starting from the Sun’s position at the vernal equinox (in Pisces), the point where the ecliptic crosses the celestial equator moving from south to north.
The ecliptic enters Virgo at a longitude near 174° some 3° west of Beta ( β) Virginis. It then passes slightly south of Gamma ( γ) Vir and a bit north of Virgo’s brightest star, 1stmagnitude Spica, before exiting the Maiden 1° southeast of Lambda ( λ) Vir at a longitude of 218°. So, an ecliptic span of 44° lies in Virgo. Virgo wins the prize for the biggest expanse because it is the second-largest constellation and the ecliptic passes almost centrally through its length.
Scorpius tells a much different story. The ecliptic enters the Scorpion 0.6° east-southeast of Lambda Librae and exits it just 7° later some 2° west of Omega ( ω) Ophiuchi. Scorpius wins the booby prize despite ranking a respectable 33rd in size among the 88 constellations. Its downfall arises because, even though it is a sprawling constellation, the ecliptic cuts through its narrow northern section.