Key Takeaways:
It doesn’t happen often that Mercury ranks as the planetary highlight of the evening sky, but July is an exception. The solar system’s smallest major planet reaches greatest elongation July 4, when it lies 26° east of the Sun and stands 11° high in the northwest an hour after sunset. Mercury shines at magnitude 0.5, making it an easy target in the gathering twilight.
Take some time each evening to observe the innermost planet through your telescope. The first half of July finds Mercury drawing closer to the Sun, so we see more of its night side and thus a pleasing crescent phase. The inner world also is approaching Earth, so it grows larger in the eyepiece. On July 1, Mercury appears 7.6″ across and 46 percent lit. On the 15th, it spans 9.9″ and the Sun illuminates 22 percent of its Earth-facing hemisphere. The planet disappears in the Sun’s glow in late July.
Unlike Mercury, Mars remains on view all month. The Red Planet drifts slowly eastward through Leo during July, crossing into western Virgo in the month’s final days. Mars glows at magnitude 1.5 and appears more conspicuous for its ruddy color than for its brightness. A telescope reveals a disappointing sight: a featureless disk only 5″ across.
Only an hour or so after Mars sets in the west, Saturn rises in the east. The ringed planet lies against the backdrop of southern Pisces the Fish. Its eastward motion comes to a halt July 14, when it begins to move westward in anticipation of its September opposition.
The best views of Saturn through a telescope occur when it climbs high in the sky during the early morning hours. The planet shows an 18″-diameter disk surrounded by a ring system that spans 41″ and tilts 3.6° to our line of sight. Also keep an eye out for Saturn’s brightest moons: Titan, Tethys, Dione, and Rhea.
You can use Saturn as a guide for finding Neptune in early July. On the 1st, the ice giant planet lies 1.0° north of the ringed world. Neptune glows at magnitude 7.8 so it’s bright enough to see through binoculars, though you’ll need a telescope to capture its blue-gray hue.
Early risers can enjoy wonderful views of Venus in the northeast before daybreak. The planet shines at magnitude –4.1 among the background stars of Taurus, appearing 100 times brighter than the Bull’s luminary, 1st-magnitude Aldebaran. The planet passes 3° north of the star July 14, but a better view comes the morning before when Venus appears nearly on top of Epsilon (ε) Tauri and marks the northern tip of the V-shaped Hyades star cluster. At month’s end, the planet makes a brief and rare excursion into far northern Orion.
As Venus’ orbit carries it away from Earth this month, the inner planet appears smaller and more fully illuminated through a telescope. It begins July sporting a disk 18″ in diameter and 64 percent lit. By month’s end, its disk spans 14″ and appears three-quarters lit.
Jupiter remains lost in the Sun’s glare in early July but climbs into view to the lower right of Venus after midmonth. Shining at magnitude –1.9, it easily pierces the twilight glow. Although a telescope shows the planet’s 33″-diameter disk at month’s end, poor seeing near the horizon will render details hard to see. Better views await in the coming months.
A waxing gibbous Moon occults 1st-magnitude Antares on July 7. Observers in the western parts of Western Australia, the Indian Ocean, and parts of South Africa can see this event. From Perth, Antares disappears at 19h24m UT (on the morning of July 8) and reappears at 20h19m UT.
The Starry Sky
Midwinter is a great time to observe the far southern sky. The spectacular Milky Way arcs high in the south, passing through Crux the Cross with Carina the Keel to its right and the Pointers (Alpha [α] and Beta [β] Centauri) in Centaurus the Centaur to its left.
Alpha Cen makes a fine telescopic object. The third-brightest star in the night sky quite easily resolves into a double star. The primary one is a near twin to the Sun, while the secondary is a bit smaller and cooler. A third member of the system, the 11th-magnitude red dwarf Proxima Centauri, lies some 2° away from the main pair. Proxima’s main claim to fame is as the closest star to the Sun. Father Jean Richaud discovered the duplicity of Alpha Centauri from Pondicherry, India, in 1689. Robert Innes first spotted Proxima in 1915.
Surprisingly, perhaps, Beta Cen — whose proper name is Hadar — also is a triple system. Dutch astronomer Joan Voûte discovered Beta’s main companion in 1935. Beta Centauri B lies 1.3″ from the primary. Glowing at 4th magnitude, it’s extremely hard to spot next to the magnitude 0.6 primary.
In the 1960s, observations of the primary showed variations in its light curve that suggested it was a member of a binary system, making Beta a triple system. To me, R.R. Shobbrook and J.W. Robertson wrote the most significant paper on the star’s duplicity in 1968. They used the famous 74-inch telescope at Australia’s Mount Stromlo Observatory, which was destroyed by devastating fires in 2003. I was reflecting on those observations last year when I attended the observatory’s centenary celebration.
Through those observations and the work of many others, astronomers have established that Beta Centauri A is indeed a close double star. Both suns are blue-white giants that orbit each other every 357 days.
Star Dome
The map below portrays the sky as seen near 30° south latitude. Located inside the border are the cardinal directions and their intermediate points. To find stars, hold the map overhead and orient it so one of the labels matches the direction you’re facing. The stars above the map’s horizon now match what’s in the sky.
The all-sky map shows how the sky looks at:
9 p.m. July 1
8 p.m. July 15
7 p.m. July 31
Planets are shown at midmonth
