Catch Mercury Brushing Past Venus in a Spectacular Dusk ‘Quasi-Conjunction’ This Week


Missing Venus? The third brightest natural object in the heavens returns to prime time dusk skies in 2015 after being absent and lingering in the dawn for most of 2014. But there’s another reason to hunt down the Cytherean world this week, as elusive Mercury chases after it low in the dusk. If you’ve never seen Mercury for yourself, now is a great time to try, using brilliant Venus as a guide.

The circumstances surrounding this pairing are intriguing. We have to admit, we missed this close conjunction whilst filtering through research for the Top 101 Astronomical Events for 2015 due to those very same unique attributes until an astute reader of Universe Today pointed it out.

December 31st Bob King
Venus and Mercury setting over the Duluth, Minnesota skyline on December 31st. Credit and copyright: Bob King.

On the evening of January 5th, Venus shines at magnitude -3.3 and sits about 18 degrees east of the Sun in dusk skies.  You’ll have a narrow window of opportunity to nab Venus, as it’ll sit only 10 degrees above the southwestern horizon as seen from latitude 40 degrees north about an hour after sunset. Make sure you have a clear, uncluttered horizon, and start sweeping the field with binoculars about half an hour after sunset.

Do you see a tiny point of light about a degree and a half to Venus’s lower right? That’s Mercury, just beginning its first dusk apparition of seven for 2015, the most possible in a calendar year. Shining at -0.7 magnitude, Mercury is currently about 8 times fainter than Venus, and drops to +1.4 magnitude by late January.

If you watch the pair on successive evenings, you’ll see Mercury — aptly named after the fleet-footed Roman god — racing to rapidly close the gap. Mercury crosses the one degree separation threshold from January 8th through January 12th, and sits just 39’ — slightly larger than the apparent size of the Full Moon — right around 7:00 PM EST/Midnight Universal Time on January 10th, favoring dusk along eastern North America just a few hours prior.

Credit Jose
Venus and Mercury as seen from Venezuela on January 2nd. Credit and copyright: Jose Rozada.

This also means that you’ll be able to squeeze both Mercury and Venus into the same low power telescopic field of view. They’ll both even show the same approximate gibbous phase, with Venus presenting a 10.5” sized 95% illuminated disk, and Mercury subtending 6” in apparent diameter with a 74% illuminated visage. Venus will seem to be doing its very own mocking impersonation of the Earth, appearing to have a single large moon… Neith, the spurious pseudo-moon of Venus lives!

One curious facet of this week’s conjunction is the fact that Venus and Mercury approach, but never quite meet each other in right ascension. We call such a near miss a “quasi-conjunction.” This is the closest pairing of Venus and Mercury since 2012, though you have to go all the way back to 2005 for one that was easily observable, and the last true quasi-conjunction was in October 2001. Miss this week’s event, and you’ll have to wait until May 13th 2016 to catch Mercury — fresh off of transiting the Sun a week earlier — passing just 26’ from Venus only 6.5 degrees west of the Sun. This is unobservable from your backyard, but SOHO’s LASCO C3 camera’s 15 degree wide field of view will have a front row cyber-seat.

Venus 2015
The dusk path of Venus through early 2015. Credit: Starry Night Education software.

In 2015, Venus will become ever more prominent in the dusk sky before reaching greatest elongation 45.4 degrees east of the Sun on June 6th, 2015. The angle of the January ecliptic at dusk is currently shoving Mercury and Venus southward for northern hemisphere observers, though that’ll change dramatically as we head towards the March equinox. Venus reaches solar conjunction sans transit (which last occurred in 2012 and won’t happen again til 2117 A.D.) on August 15th before heading towards its second elongation of 2015 on October 26th in the dawn sky.  And don’t forget, it’s possible to see Venus in the daytime as it approaches greatest elongation. Venus is also occulted by the Moon 4 times in 2015, including a fine daytime occultation on December 7th for North America.

Mercury through January 2015.
The path of Mercury through January 2015. Credit: Starry Night Education software.

This month, Mercury reaches greatest eastern elongation on January 14th at 18.9 degrees east of the Sun.  Mercury begins retrograde movement later this month — one of the prime reasons this week’s conjunction is quasi — before resuming direct (eastward) motion as seen from our terrestrial vantage point. Though it may seem convenient to blame your earthly woes on Mercury in retrograde as astrologers will have you believe, this is just an illusion of planetary orbital motion. And speaking of motion, Mercury transits the Sun next year on May 9th.

John Barantine
Venus and Mercury as seen from Tucson, Arizona on January 3rd. Credit and copyright: John Barentine (@JohnBarentine)

Mercury and Venus factor in to space exploration in 2015 as well. NASA’s MESSENGER spacecraft wraps up its successful mission  in orbit around Mercury in a few months, and the Japanese Space Agency takes another crack at putting its Akatsuki spacecraft in orbit around Venus this coming November.

So don’t fear the bone-chilling January temps (or Mercury in retrograde) but do get out there these coming evenings and check out the fine celestial waltz being performed by the solar system’s two innermost worlds.


Mercury Pierces the Zodiacal Light at Dawn this Weekend

Credit: Stellarium

Psst! Ever spy the planet Mercury? The most bashful of all the naked eye planets makes its best dawn appearance of 2014 this weekend for northern hemisphere observers. And not only will Mercury be worth getting up for, but you’ll also stand a chance at nabbing that most elusive of astronomical phenomena — the zodiacal light — from a good dark sky sight.

DST note: This post was written whilst we we’re visiting Arizona, a land that, we’re happy to report, does not for the most part observe the archaic practice of Daylight Saving Time. Life goes on, zombies do not arise, and trains still run on time. In the surrounding world of North America, however, don’t forget to “fall back” one hour on Sunday morning, November 2nd. I know, I know. Trust me, we didn’t design the wacky system we’re stuck with today. All times noted below post-shift reflect this change, but it also means that you’ll have to awaken an hour earlier Sunday November 2nd onwards to begin your astronomical vigil for Mercury!

Oct21 to Nov14. Created using Starry Night Education Software.
The apparent daily path of Mercury as seen from 30 degrees north from October 21st to November 14th. Created using Starry Night Education Software.

Mercury starts the month of November reaching greatest elongation on Saturday, November 1st at 18.7 degrees west of the Sun at 13:00 Universal Time UT/09:00 EDT. Look for Mercury about 10 degrees above the eastern horizon 40 minutes before sunrise. The planet Jupiter and the stars Denebola and Regulus make good morning guideposts to trace the line of the ecliptic down to the horizon to find -0.3 magnitude Mercury.

Mars, Mercury and the International Space Station.
Mars, Mercury and the International Space Station caught during an evening apparition in 2013. (Photo by author)

Sweeping along the horizon with binoculars, you may just be able to spy +0.2 magnitude Arcturus at a similar elevation to the northwest. The +1st magnitude star Spica also sits to Mercury’s lower right. Mercury passes 4.2 degrees north of Spica on November 4th while both are still about 18 degrees from the Sun, making for a good study in contrast.

Later in the month, the old waning crescent Moon will present a challenge as it passes 2.1 degrees north of Mercury on November 21st, though both will only be 9 degrees from the Sun on this date.

Mercury also passes 1.6 degrees south of Saturn November 26th, but both are only 7 degrees from the Sun and unobservable at this point. But don’t despair, as you can always watch all of the planetary conjunction action via SOHO’s sunward staring LASCO C3 camera, which has a generous 15 degree field of view.

Mercury (the bright ‘star’ with spikes) transits SOHO’s LASCO C3 camera. Credit: NASA/ESA/SOHO.

At the eyepiece, Mercury starts off the month of November as a 57% illuminated gibbous disk about 7” in diameter. This will change to a 92% illuminated disk 5″ across on November 15th, as the planet races towards superior conjunction on the far side of the Sun on December 8th. As with Venus, Mercury always emerges in the dawn sky as a crescent headed towards full phase, and the cycle reverses for both planets when they emerge in the dusk sky.

Why aren’t all appearances of Mercury the same? Mercury orbits the Sun once every 88 days, making greatest elongations of Mercury far from uncommon: on average, we get three dawn and three dusk apparitions of the innermost world per year, with a maximum of seven total possible. Two main factors come into play to assure that not all appearances of Mercury are created equal.

Credit: NASA
A depiction of the evening motion of Mercury and Venus as seen from Earth. Credit: NASA.

One is the angle of the ecliptic, which is the imaginary plane of our solar system that planets roughly follow traced out by the Earth’s orbit. In northern hemisphere Fall, this angle is at its closest to perpendicular at dawn, and the dusk angle is most favorable in the Spring. In the southern hemisphere, the situation is reversed. This serves to place Mercury as high as possible out of the atmospheric murk during favorable times, and shove it down into near invisibility during others.

The second factor is Mercury’s orbit. Mercury has the most elliptical orbit of any planet in our solar system at a value of 20.5% (0.205), with an aphelion of 69.8 million kilometres and perihelion 46 million kilometres from the Sun. This plays a more complicated role, as an elongation near perihelion only sees the planet venture 18.0 degrees from the Sun, while aphelion can see the planet range up to 27.8 degrees away. However, this distance variation also leads to noticeable changes in brightness that works to the advantage of Mercury spotters in the opposite direction. Mercury shines as bright as magnitude -0.3 at closer apparitions, to a full magnitude fainter at more distant ones at +0.7.

In the case of this weekend, greatest elongation for Mercury occurs just a week after perihelion, which transpired on October 25th.

earlier 2014 Curiousity
Mercury transits the Sun earlier this year as captured by the Curiosity rover on Mars. Credit: NASA/JPL.

Mercury is also worth keeping an eye on in coming years, as it will also transit the Sun for the first time since 2006 on May 9th, 2016. This will be visible for Europe and North America. We always thought it a bit strange that while rarer transits of Venus have yet to make their sci-fi theatrical debut, a transit of Mercury does crop up in the film Sunshine.

The first week of November is also a fine time to try and spy the zodiacal light. This is a cone-shaped glow following the plane of the ecliptic, resulting from sunlight backscattered across a dispersed layer of interplanetary dust. The zodiacal light was a common sight for us from the dark skies of Arizona, often rivaling the distant glow of Tucson over the mountains. The zodiacal light vanished from our view after moving to the humid and often light polluted U.S. East Coast, though we’re happy to report that we can once again spy it as we continue to traverse the U.S. southwest this Fall.

The zodiacal light captured by Cory Schmitz over the Drakensberg Mountains in South Africa.
The zodiacal light captured by Cory Schmitz over the Drakensberg Mountains in South Africa. (Used with permission). 

None other than rock legend Brian May of Queen fame wrote his PhD dissertation on the zodiacal light and the distribution and relative velocity of dust particles along the plane of the solar system. Having a dark site and a clear flat horizon is key to nabbing this bonus to your quest to cross Mercury off your life list this weekend!