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!

How to See Planet Mercury at its Best in 2014

Looking west on January 31st 30 minutes after sunset. (Created using Stellarium).

 There’s an often told anecdote that astronomer Nicolaus Copernicus never spied Mercury. And while this tale is almost certainly apocryphal, it does speak to just how elusive the innermost planet of our solar system really is.

Never seen Mercury for yourself? This final week of January offers a good time to try, as Mercury reaches greatest elongation east of the Sun on Friday, January 31st.

This will offer northern hemisphere viewers one on the best chances to spot the fleeting world low to the west immediately after local sunset. And although we get on average six apparitions of Mercury per year – three each in the dawn and dusk – all apparitions aren’t created equal.

The approximate moment of greatest elongation occurs on January 31st at 10:00 UT / 5:00 AM EST, when Mercury is 18.4 degrees east of the Sun. This is only 0.5 degrees shy of the smallest elongation for Mercury that can occur, as the planet reaches perihelion just three days later on February 3rd at 0.3075 Astronomical Units (AUs) from the Sun. The last time this was surpassed was the evening elongation of February 16th, 2013th, and the next time it’ll be topped is October 16th, 2015 at just 18.1 degrees from the Sun.

Path of Mercury from January 27th to February 12th. (Created using Starry Night).
Path of Mercury from January 27th to February 12th as seen from latitude 30 degrees north. (Created using Starry Night Education Software).

And though this elongation is closer than usual, this also works in the Mercury-spotter’s favor. At greatest elongation, Mercury will present a 50% illuminated 7 arc second disk, readily apparent in a small telescope. But a also means that Mercury will appear almost a full magnitude brighter than it does when it reaches greatest elongation near aphelion, as it last did on March 31st of last year, and will do again on March 14th of this year.

Mercury will shine at magnitude -0.4 low towards the west into this coming weekend. We managed to pick up Mercury with binoculars on January 16th and have since managed to start tracking the planet unaided since January 18th.

Mercury also has another factor going for it, in terms of the angle of the evening ecliptic. Following ahead of the Sun, Mercury occupies a space that the Sun will trace up its apparent path along the ecliptic as it begins its long slow crawl northward towards the Vernal Equinox on March 20th. This means that Mercury is almost perpendicular above the western horizon at dusk and is currently getting a maximum boost above the atmospheric murk.

Mercury also gets joined by a razor thin waxing crescent Moon just over 24 hours past New sliding by it on the evening of Friday, January 31st. Look for the Moon five degrees to the right of Mercury on the 31st. The Moon will be a much easier catch on the February 1st when its 10 degrees above Mercury. And can you spy the +1 magnitude star Fomalhaut in the constellation Piscis Austrinus just 20 degrees to the south of Mercury?

The orientation of the Moon and Mercury on the evening of February 1st. Credit: Stellarium.

And speaking of the Moon, this week’s New Moon is the second of the month, a feat that repeats in March 2014 and leaves the month of February “New Moon-less…” such an occurrence in either instance is informally known as a Black Moon.

Orbiting the Sun once every 88 days, Mercury completes about 4.15 circuits of the Sun for every Earth year. From our Earthbound vantage point, however, Mercury seems to only orbit the Sun 3.15 times a year. Thus 6 elongations (3 in the dusk and 3 in the dawn) will occur every year, through 7 can occur, as last happened in 2011 and will occur again next year in 2015.

August 15th, 2012.
Mercury (to the lower left) and the Moon on August 15th, 2012. (Photo by author).

After this weekend, Mercury will resume its plunge towards the horizon through early February. Mercury will begin retrograde (westward) apparent motion against the starry background on February 6th before resuming direct (eastward motion) on February 27th. And although astrologers may  find that “Mercury in retrograde” is a convenient “blame magnet,” they’re also falling prey to a logical fallacy known as retrofitting, as Mercury spends a longer fraction of time than any other planet “in retrograde” at about 20%!

From there, Mercury heads towards inferior conjunction between the Earth and the Sun on Saturday, February 15th, passing just 3.7 degrees north of the solar disk. You can catch Mercury entering into the field of view of the Solar Heliospheric Observatory’s (SOHO) LASCO C3 camera from February 12th to February 18th.

And although Mercury misses this time, we’re not that far away from the next transit of Mercury across the face of the Sun on May 9th, 2016.

Up for more? An even tougher challenge is to attempt to spot Mercury… in the daytime. We’ve noted this possibility before as Mercury reaches maximum elongation from the Sun while still in the negative magnitude range. Of course, you want to physically block the Sun out of view, and don’t even try sweeping the sky near the Sun visually with binoculars or a telescope! You’ll need a clear, blue sky for maximum contrast and a polarizing filter may help in your quest… but this should be possible under exceptional conditions.

Good luck, and be sure to send those Mercury pics in to Universe Today!