July's planetary lineup is changing — and Venus is the last one standing

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The three-planet evening show that graced June's twilight sky has now dwindled to one lone survivor.

Mercury and Jupiter are now swinging behind the sun and are lost in the bright solar glare. Only Venus remains evident after sunset, and even here the dazzling evening star itself is showing subtle signs of slowly dropping down into the sunset fires; it is getting noticeably lower in the western sky with each passing week. It will pass close to the brightest star in Leo the Lion, Regulus, during the second week of July.

Saturn is in a fine position to be viewed just before the break of dawn in the east-southeast. The famous rings are now tilted some 9 degrees toward Earth, once again making this planet a showpiece for telescopic observation.

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• Why does Venus appear lower in the sky each week?
• When will Mercury become visible in the morning sky?
• Why is Uranus difficult to see without a telescope?

Mars rises about 2.5 hours before sunrise but is ever-so-slowly becoming more conspicuous as it brightens while climbing a bit higher in the eastern sky. It will also serve as a benchmark to locate the much fainter planet Uranus early on the morning of July 4; this distant greenish world will then be positioned only about 7 arcminutes (less than one-quarter of the apparent width of the moon) above Mars. Certainly, if you have never seen Uranus before, this will be an excellent opportunity to sight it, either with a small telescope or binoculars, or even possibly with your unaided eyes.

In our schedule, remember that when measuring the angular separation between two celestial objects, your clenched fist held at arm's length measures roughly 10 degrees. Here's where and when to look for each planet during July.

Mercury

Mercury passes inferior conjunction and enters the morning sky on July 12. It's too low and dim to detect until about July 26 or maybe later. By Aug. 2, Mercury will reach a greatest western elongation of 19 degrees from the sun.

Venus

Venus is conspicuous as July's long evening twilights fade. If you live at mid-northern latitudes and look to the west about 45 minutes after sunset, you'll find Venus shining brightly about 15 degrees high this month. This is noticeably lower than it was in June.

The planet starts July by setting more than two hours after the sun and follows the sun down to the horizon somewhat sooner by month's end. Venus is still rather small in a telescope this month, but there's no doubt that it has a gibbous phase. It will be larger in apparent diameter but smaller in phase, down to about half lit, when it reaches greatest elongation from the sun in mid-August.

On July 9, you will notice a much dimmer but still reasonably prominent point of light near Venus: the bluish 1st magnitude star Regulus. Venus burns only 1 degree to the upper right of Regulus, which is 150 times fainter. They're less than 3 degrees apart from July 7 through July 12. On July 17, the 3.5-day-old crescent moon will sit 5.5 degrees to the left of Venus.

Earth

Earth reaches aphelion, its farthest point from the sun in its orbit, on July 6 at 1:30 p.m. EDT (1730 GMT). Our planet is then 94,502,962 miles (152,087,775 km) from the sun (measured center to center), which is 3.28% farther from the sun than we were at perihelion last Jan. 3; a change of only one part in 30.

Mars

Mars rises around 2:50 a.m. local daylight time and is positioned between the Hyades and Pleiades star clusters, low in the east-northeast sky by dawn. At magnitude +1.3, it remains rather inconspicuous, but its luminosity continues to increase as we gain on it in our smaller, faster orbit.

On Independence Day morning (July 4), Mars points the way to Uranus, which is only 1/63 as bright. But with good binoculars or a small telescope, this distant world should appear like a tiny greenish star, hovering just 0.1 degree above Mars. Uranus is 168 light-minutes from Earth and more than 9½ times farther than Mars.

On the morning of July 11, about 90 minutes before sunrise, look low toward the east-northeast horizon to see a waning crescent moon. About 5 degrees below it, you'll find Mars and about the same distance below the Red Planet, you'll find a similarly hued and somewhat brighter star, orange Aldebaran in Taurus the Bull.

Jupiter

Jupiter comes to conjunction with the sun on July 29; it's on the far side of the sun during July and can't be seen.

Saturn

Saturn comes up in the east around 1 a.m. local time at the beginning of July and 11 p.m. at month's end. It's located near the border separating the constellations Pisces and Cetus. By month's end, the rings are tilted 9.1 degrees from horizontal — their greatest tilt of the year. Early on the morning of July 7, you'll find Saturn mimicking a bright star shining with a sedate yellow-white hue, sitting well to the lower left of the moon.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, Sky and Telescope, The Old Farmer's Almanac and other publications.