Tiptoe Into The Twilight Zone To See Mercury At Its Best

Mercury stands alone low in the sky over grain elevators and freeways in this picture taken last night Jan. 27, 2014 in Duluth, Minn. Credit: Bob King

This week and next Mercury will be brightest and highest in the evening sky. Not until May will skywatchers in mid-northern latitudes have as good an opportunity to spy the planet that spins closest to the sun. That’s what makes it so tricky to see in the first place. Mercury never gets far enough from the sun to appear in a dark sky, forever lurking in the twilight zone.

Mercury will be visible for the next week low in the west-southwest sky at dusk. Start looking about 40-45 minutes after sundown. On Friday, a thin day-old moon will join the scene. Stellarium

Still, I was surprised how easy it was to see last night. Higher up than expected too. I bundled up and went out to look 45 minutes after sunset. Nothing. Where was Mercury? It turned out I was looking too low. Once I raised my gaze a bit, a solitary “star” popped into view about a fist above the southwestern horizon.

While the planet shines tonight at magnitude -0.5 (brighter than Vega and Arcturus) the hazier, thicker air near the horizon robs it of 1.2 magnitudes, dimming Mercury to magnitude +0.7 or about as bright as Altair in the Summer Triangle.  Still plenty easy to see with the naked eye.

I kept the planet in view until around 6:20 p.m. or more than an hour past sunset before subzero temps and 20 mph winds forced a retreat back into the car. If you’re in a mercurial mood, start looking about 45 minutes after sunset to the upper left of the brightest part of the lingering solar glow in the southwest. The planet hovers about 10-12 degrees (a little more than one fist held at arm’s length) high. Since Mercury has no bright company, if you see a single star in that direction, you’ve nailed it.

Only a spruce tree separates Venus from the crescent moon this morning Jan. 28, 2014. A similar but thinner crescent will be near Mercury in the evening sky on Friday Jan. 31. Credit: Bob King

To be on the safe side, you might consider toting along a pair of binoculars. I guarantee that once you find it with optical aid, you’ll quickly see Mercury with the naked eye.

Once you’ve fixed in your mind where Mercury is located along your local horizon, get ready for a really fine event. This Friday the 31st, an incredibly thin one-day-old moon will sidle up some 5 degrees to the lower right of the planet. Are you thinking pictures? So I am.

Place your camera on a tripod – or at least wedge it firmly against something – compose a scene including moon and planet and take a series of photos with your lens set anywhere from f/2.8 to 4.5. ISO 400 speed should be fine with exposures ranging from 2 seconds to 1/15 second. While you’ll get a decent photo with a standard lens, a 100-200mm telephoto lens will make for a tighter, more dramatic image.

As you stand in the cold clicking away or simply admiring Mercury, here are some interesting facts about the planet to warm your brain cells:

Color image of Mercury made by the MESSENGER probe. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

* At 3,032 miles (4,880 km) across, Mercury is smaller than Jupiter’s moon Ganymede and Saturn’s moon Titan.

* Mercury’s orbit is the most eccentric or least circular of all the planets. Its distance from the sun ranges from 29 to 44 million miles (46-70 million km).

* While Venus is slightly hotter, Mercury has one of the most extreme temperature ranges of any body in the solar system. With virtually no atmosphere to capture and distribute the sun’s heat, the sun-facing dayside of the planet tops out around 800 degrees Fahrenheit while the nightside dips to -297 F.

All the dayside heat leaks right back into space during the long night. And it is a L-O-N-G night. The sun’s enormous gravity has locked the little planet in a 3:2 rhythm or “resonance”. For every two orbits around the sun, Mercury rotates three times on its axis.

Since the planet completes an orbit in 88 days (one Mercury year), its day is twice as long as its year or 176 Earth days. Mercury’s sunny side bakes for nearly six months and then chills for another six. No wonder it experiences such extremes of hot and cold.

* Mercury’s slight axial tilt of just 0.03 degree means that craters at its poles are steeped in perpetual shadow and never heated by the sun, making them perfect places to trap volatile materials like ice and keep them frozen for a long, long time. New data from the MESSENGER spacecraft now gives strong evidence for ice holed up in some of these craters.

* Mercury has a large (by volume) partially molten iron core and a planet-wide magnetic field, a feature lacking on Venus and Mars.

Sesame Street’s Cookie Monster and a crater on Mercury appear to be related. Credit: NASA

* Mercury is the most cratered planet in the solar system. Unlike Earth, Mars and Venus, which have been extensively resurfaced through volcanic and tectonic processes, Mercury’s retains much of its ancient battered surface.

12 Responses

  1. caralex

    Saw it this evening, Bob! I’m on a Caribbean cruise at the moment, so from San Juan, Puerto Rico, is was an easy sight at dusk, visible until almost 7.30pm. It was really bright too!

    Shifting my gaze around to the south, I also saw Canopus and Achernar, high above the southern horizon! Oh the joys of a) a break from the frigid weather up north, and b) a chance to see southern stars and bright planets!

  2. Sean

    I hope to sight Mercury again in the next few days – it would be a REAL treat to see it beside the moon Friday evening – we shall see! Usually i use the moon to find planets in the daytime, but in this case it might be the opposite! Since i already know pretty much where Mercury will be after my sighting last week. Nice pic of Venus and the moon this AM Bob. I took advantage of their “nearby” locations today and the fact that i didn’t have to work (jury duty) to do some daytime Venus observation. First spotted them when it was pretty dark (around 6:15A) and noted their positions relative to one another for later reference. Then periodically went back and identified them. I ran out to grab some food prior to court, so observed them again while there just past sunrise, then again dropping stuff back off prior to court, then again twice on the way to court. My final observation of both was during AM break (still waiting to be impaneled, which i later was, for the 1st time in my life,) at about 10:07AM. So within 2 hours of solar noon! That is the closest to solar noon i have seen Venus in either this young apparition or in the last! Psyched! By the time we had lunch break after 1PM i couldn’t find either, which is not surprising considering they were much lower by that time and we had some high cloudiness (patches of which had existed all day, actually,) which later overspread the sky. All spottings were naked-eye, and i made no bino observations, since it might have been awkward to explain to the court security people why i was bringing binoculars into the court building (I don’t have a car currently and rely on public transit). Maybe i should have just had the guts to do it.

    1. astrobob

      That’s a satisfying observation, isn’t it? I remember the first time I found Venus in the daylight afternoon sky. Where had it been hiding all my life? You really appreciate both how cloudy the planet is and how much closer to the sun, too.

      1. Sean

        Yeah, thank God for the brightness which results from those clouds. Turns out this was also my record for a midday-ish small crescent observation. i have seen them smaller but around dawn. As far as worlds WITHOUT clouds, Mercury is really getting high like u mentioned in a recent entry. Spotted it today around 5:21 or so, after having looked at 5:15 and failing to find it (no binos until naked eye spotting). When it actually gets this high up in the sky at a time suitable for observation, and u indeed observe it, especially with magnification, it’s kinda surprising to see how almost WHITE it appears, since so often it’s seen at a lower altitude and gets more of that pinkish tinge.

  3. Hi Bob- I’m in the Philadelphia area & I’ve looked eagerly for Mercury the past few nights around 6PM. I’ve found it easily with the naked eye. It seems surprisingly bright & high, to the left of the sunset. It seems a bit reddish to me. I wonder if the maxim that planets don’t twinkle still holds true when it’s Mercury low on the western horizon? Or am I seeing something else? It is the only thing visible in this corner of the sky. Jupiter & Gemini are already visible high overhead & behind me. Thanks! Jason

    1. astrobob

      Hi Jason,
      That’s Mercury for sure. Sometimes planets will twinkle if low in the sky in turbulent air. I’ve seen it happen to Venus a number of times. The reddish color you see it tinting from earthly twilight. The planet appears white in a telescope.

  4. Thanks, Bob! I’m one up on Copernicus! Or was it Galileo? We have had some absolutely amazing sunsets here the past two days, orange sherbet fading to rosy pink then deep purple. I’m glad that Mercury picked up some of the local color. Jason

  5. I saw it on Feb, 1st. I had to go up to the buiding top. After some minutes, it became visible below and left of the moon, between two mountain peaks here in Rio de Janeiro. It was my first sight of Mercure, thanks for the tips.

Comments are closed.