Planetpalooza: All Bright Planets Visible in the July Dusk Sky

Moon and Venus
Moon and Venus
Venus and the waxing crescent Moon above the Grand Palais in Paris, France from May 17th. Image credit: Gwenael Blanck

Missed the planets in the dusk sky in early 2018? This summer’s astronomical blockbuster sees the return of all the classical naked eye planets in the dusk sky, in a big way.

The Sky Scene in July

This coming July 2018 features a rare look at the solar system in profile: you can see Mercury and Venus low in the dusk looking westward immediately after sunset, with Jupiter high to the south, Saturn rising in the east, and Mars rising just behind. This isn’t a true grouping or grand conjunction, as the planets span a 170 degree swath of the ecliptic from Mercury to Mars (too bad they’re not in orbital order!) but a product of our Earthly vantage point looking out over the swath of inner solar system in the evening sky.

Can you manage a “planetary marathon” and collect all five this coming Fourth of July weekend? Here’s a quick rundown of all the planetary action from west to east:

Mercury
An amazing view – Mercury through the telescope from May 5th. Image credit and copyright: Roger Hutchinson.

Mercury’s July apparition – fleeting Mercury is always the toughest of the planets to catch, low to the west. -0.3 magnitude Mercury actually forms a straight line with the bright +1st/2nd magnitude stars Castor and Pollux in Gemini the Twins later this week on the evening of June 27th. Mercury reaches greatest elongation 26 degrees east of the Sun on July 12th, presenting a half illuminated, 8” disk. The angle of the evening ecliptic is canted southward in July, meaning that the position of the planets in the evening sky also favors southern viewers. July also presents another interesting mercurial challenge, as Mercury passes in front of the Beehive Open cluster (Messier 44) in the heart of the constellation Cancer on the night of July 3rd/4th.

planets
The span of the planets through late-July at dusk. Credit: Stellarium.

Venus this summer – higher up at dusk, brilliant Venus rules the evening sky, shining at magnitude -4. Venus is so bright that you can easily pick it up this month before sunset… if you know exactly where to look for it. Venus reaches greatest elongation 46 degrees east of the Sun on August 17th, presenting a featureless half-illuminated disk 25” in diameter near a point known as dichotomy. Venus also flirts with the bright star Regulus (Alpha Leonis) in July, passing a degrees from the star on July 10th. Fun fact: Venus can actually occult (pass in front of) Regulus and last did so on July 7th, 1959 and will do so next on October 1st, 2044.

Jupiter
Jupiter, with the shadow of Europa in transit from June 6th. Image credit and copyright: Ralph Smyth.

Jupiter Rules – The King of the Planets, Jupiter rules the sky after darkness falls, crossing the astronomical constellation Libra the Scales. Fresh off of its May 9th opposition, Jupiter still shines at a respectable magnitude -2 in July, with a disk 36” across. Jupiter heads towards quadrature 90 degrees east of the Sun on August 6th, meaning the planet and its retinue of four Galilean moons cast their respective shadows off to one side. In fact, we also see a series of fine double shadow transits across the Jovian cloud tops involving Io and Europa starting on July 29th.

saturn
The glorious planet Saturn. Image credit and copyright: Paul Stewart

…and Saturn makes five: Stately Saturn never fails to impress. Also just past its June 27th opposition, the rings are still tipped open narrowing down only slightly from last year’s widest angle of 27 degrees, assuring an amazing view. Shining at magnitude 0 and subtending 42” (including rings) in July, Saturn traverses the star-rich fields of the astronomical constellation Sagittarius the Archer this summer. Look at Saturn, and you’re glimpsing the edge of the known solar system right up until William Herschel discovered Uranus on the night of March 13th, 1781.

The origins of a dust storm: Mars from late May. Image credit and copyright: Efrain Morales.

Enter Mars: We saved the best for last. The Red Planet races towards a fine opposition on July 27th. This is the best approach of Mars since the historic 2003 opposition, and very nearly as favorable: Mars shines at magnitude -2.8 at the end of July, and presents a 24.3” disk. More to come as Mars approaches!

And as with many an opposition, dust storm season has engulfed Mars. Be vigilant, as the ‘Red’ Planet often takes on a sickly yellowish tint during a large dust storm, and this cast will often be apparent even to the naked eye. NASA’s aging Opportunity rover has fallen silent due to the lack of sunlight and solar power, and it’s to be seen if the rover can ride out the storm.

The path of the Moon – The Moon makes a good guidepost as it visits the planets in July. The first eclipse season of 2018 also begins in July, with a partial solar eclipse for Tasmania, SE Australia and the extreme southernmost tip of New Zealand on July 13th and wrapping up with a fine total lunar eclipse favoring Africa, Europe, Asia and Australia on July 27th. Note that this eclipse is only 14 hours after Mars passes opposition… we expect to see plenty of pictures of a ruddy Mars near a Blood Moon eclipse.

The Moon also makes a handy guide to catch each of the planets in the daytime sky… though you’ll need binoculars or a telescope to nab Mercury or Saturn (also, be sure the Sun is physically blocked out of view while hunting for Mercury in the daytime sky!) Here are the respective passes of the Moon near each planet in July:

Planet Date Time Moon Phase/illumination Distance
Mercury July 14th 23UT/7PM EDT Waxing crescent/5% 2.1 degrees
Venus July 16th 4UT/00AM EDT Waxing crescent/14% 1.5 degrees
Jupiter July 21st 2UT/10PM EDT Waxing gibbous/63% 4.2 degrees
Saturn July 25th 5UT/1AM EDT Waxing gibbous/94% 2 degrees
Mars July 27th 16UT/12 EDT Full Moon/100% 8 degrees

Unfortunately, the telescopic planets Uranus and Neptune are left out of the July evening view; Uranus is currently crossing the constellation Aries and Neptune resides in Aquarius, respectively. Pluto is, however, currently in the direction of Sagittarius, and you can also wave to NASA’s New Horizons spacecraft en route to its New Year’s Day 2019 KBO destination Ultima Thule (nee 2014 MU69) near the waxing gibbous Moon on the night of July 26th.

The Moon, Pluto and New Horizons on the evening of July 26th. Credit: Starry Night

And finally, another solar system destination in Ophiuchus the Serpent Bearer beckons telescope owners in July: asteroid 4 Vesta.

All of this is more than enough planetary action to keep planetary observers and imagers up late on forthcoming July evenings.

Venus Returns to the Dusk Sky

Venus returns  – The changing phases of Venus, from the 2013 apparition. Credit and copyright: Shahrin Ahmad (@Shahgazer)

Where have all the planets gone? The end of February 2018 sees the three naked eye outer planets – Mars, Jupiter and Saturn — hiding in the dawn. It takes an extra effort to brave the chill of a February morning, for sure. The good news is, the two inner planets – Mercury and Venus – begin favorable dusk apparitions this week, putting on a fine sunset showing in March.

Venus in 2018: Venus begins the month of March as a -3.9 magnitude, 10” disk emerging from behind the Sun. Venus is already over 12 degrees east of the Sun this week, as it begins its long chase to catch up to the Earth. Venus always emerges from behind the Sun in the dusk, lapping the Earth about eight months later as it passes through inferior conjunction between the Sun and the Earth as it ventures into the dusk sky.

Follow that planet, as Venus reaches greatest elongation at 45.9 degrees east of the Sun on August 17th. Venus occupies the apex of a right triangle on this date, with the Earth at the end of one vertice, and the Sun at the end of the other.

Not all elongations of Mercury or Venus are equal, but depend on the seasonal angle of the ecliptic, and whether they occur near aphelion or perihelion. Credit: Dave Dickinson

Mercury joins the fray in early March, as the fleeting innermost world races up to meet Venus in the dusk. March 4th is a great date to check Mercury off of your life list, as the -1.2 magnitude planet passes just 66′ – just over a degree, or twice the span of a Full Moon – from Venus. Mercury reaches greatest elongation 18.4 degrees east of the Sun on March 15th.

And the Moon makes three on the evening of March 18th, as Mercury, Venus and the slim waxing crescent Moon form a line nine degrees long.

The Moon, Venus and Mercury – looking west at dusk on March 18th. Credit: Stellarium

It’s a bit of a cosmic irony: Venus, the closest planet to the Earth, is also eternally shrouded in clouds and appears featureless at the eyepiece. The most notable feature Venus exhibits are its phases, similar to the Moon’s. Things get interesting as Venus reaches half phase near greatest elongation. After that, the disk of Venus swells in size but thins down to a slender crescent. Venus’s orbit is tilted 3.4 degrees relative to the ecliptic, and on some years, you can follow it right through inferior conjunction from the dusk to the dawn sky. Unfortunately, this also means that Venus usually misses transiting the disk of the Sun, as it last did on June 5-6th, 2012, and won’t do again until just under a century from now on December 10-11th, 2117.

Small consolation prize: Mercury, a much more frequent solar transiter, will do so again next year on November 11-12th, 2019.

Amateur astronomers have, however, managed to tease out detail from the Venusian cloudtops using ultraviolet filters. And check out this amazing recent image of Venus courtesy of the Japanese Space Agency’s Akatsuki spacecraft:

Venus in the ultraviolet courtesy of JAXA’s Akatsuki spacecraft. Credit: JAXA/Akatsuki/ISAS/DARTS/Damia Bouic

It’s one of our favorite astro-challenges. Can you see Venus in the daytime? Once you’ve seen it, it’s surprisingly easily to spy… the main difficulty is to get your eyes to focus in on it without any other references against a blank sky. The crescent Moon makes a great visual aid in this quest; although the Moon’s reflectivity or albedo is actually much lower than Venus’s, it’s larger apparent size in the sky makes it stand out. Key upcoming dates to see Venus near the Moon around greatest elongation are April 17th, May 17th, June 16th, July 15th, and Aug 14th.

Apparitions of Venus also follow a predictable eight year cycle. This occurs because 13 orbits of Venus very nearly equals eight orbits of the Earth. For example, Venus will resume visiting the Pleiades star cluster during the dusk 2020 apparition, just like it did back before 2012.

Phenomena of Venus

When does Venus appear half illuminated to you? This stage is known as dichotomy, and its actual observed point can often be several days off from its theoretical arrival. Also keep an eye out for the Ashen Light of Venus, a faint illumination of the planet’s night side during crescent phase, similar to the familiar sight seen on the crescent Moon. Unlike the Moon, however, Venus has no nearby body to illuminate its nighttime side… What’s going on here? Is this just the psychological effect of the brain filling in what the eye sees when it looks at the dazzling curve of the crescent Venus, or is it something real? Long reported by observers, a 2014 study suggests that a nascent air-glow or aurora may persist on the broiling night side of Venus.

All thoughts to ponder, as you follow Venus emerging into the dusk sky this March.