The Planet Mercury
In The Sky

from Hermograph Press

Having discussed the physical characteristics of Mercury in our first orbit, we now seek to understand what the casual observer from Earth would see.

Note: This page is under continual construction.


Finding the Planet

Mercury is actually not all that hard to observe. It is brighter than most stars. Unlike the other planets, though, Mercury isn't as obvious and is more often not visible than it is visible. There are usually few nearby sky "landmarks" to help you find it. The reason for these paradoxical statements is that, as the innermost planet, it is never more than about 27o from the sun, less than the width of two outstretched hands side-by-side. Since true evening astronomical twilight ends (or begins, for the dawn sky) when the sun is 18 degrees below the horizon, this doesn't leave much clearance for Mercury to appear above any local horizon obstacles like trees, hills, houses. Also, this 'height' is usually even smaller because Mercury is not usually directly "above" the setting or rising Sun, but is some distance left or right of it as well. Thus it is a non-obvious morning star or evening star on occasion, and only for short timespans. In fact, the ancient Greeks called it "Apollo" during its morning sightings and "Hermes" in the evening, not realizing it was the same object. 

There are four distinct orbital configurations for Mercury, as seen from Earth:

  1. Superior Conjunction -- When Mercury is behind the Sun, as seen from Earth.

    It is quite invisible at that time. If we could see it in a telescope, it would be a "full" Mercury, a round, bright disk. It would also appear at its smallest size and dimmest brightness, because it is at its farthest distance from us. At this time it is appearing to move from West to East, from the right side of the Sun to the left side.

  2. Greatest Eastern Elongation -- The maximum angular distance from the Sun, its largest distance from the Sun's eastern or left side.

    At this time Mercury follows the Sun's daily motion as our star seems to move from East to West because of Earth's rotation. The Sun precedes it down under the horizon, leaving Mercury visible for a time in the evening twilight, an "evening star." Orbit-wise, it appears to us stationary, not shifting among the stars in the background; in fact, it is "turning the corner" and for a short while, coming right at us. It appears in a telescope like a half-moon shape.

  3. Inferior Conjunction -- Invisible, in front of the Sun.

    Following Greatest Eastern Elongation, Mercury reverses itself, and heads back towards the Sun, moving from East to West. When it passes between Earth and Sun, it is at Inferior Conjunction. It may not be exactly between us, perhaps a few degrees above or below the Sun's disk, because of its inclined orbit, but it is still between us and the Sun and not viewable!

  4. Greatest Western Elongation -- Now to the extreme right of the Sun.

    Mercury rapidly moves out of the solar glare and now precedes the Sun across the sky, the Sun follows the planet! At this time, Mercury is a "morning star." It appears as a tiny third-quarter moon shape in a telescope, and orbit-wise, it turns the other corner. In other words, at Greatest Western Elongation, it starts to move directly away from us along our line of sight. Then it dives back towards the solar glare to start the cycle all over again.

The entire cycle of visibility, called the synodic period takes all of 116 days, almost exactly 1/3rd of our year. In most years, there are three (sometimes part of a fourth) evening appearances of the planet, and three (sometimes part of a fourth) morning shows.


Transits of Mercury

Most of the time, Mercury's tilted orbit (it is tipped about 7o compared to Earth's orbital plane) means Mercury doesn't pass exactly behind or in front of the Sun during the above conjunction times. Instead it passes above or below the solar disk as seen from Earth. On rare occasions, it passes, or transits, in front of the sun. Then it is a tiny, sharp, jet-black dot taking up to several hours to move across the Sun's face, like a sunspot with a mind of its own. During Mercury's transits, when it actually reaches or departs a solar edge, there is a "black drop" effect, an illusion that makes it appear that Mercury is attached to the sun's limb, as if it had some sticky gum. This visually neat effect does make it hard to time the exact moments of contact which is helpful for those who keep track of the orbits of planets and how they change over the years by small amounts. The next transit is November 11, 2019. To get some practice observing and measuring Mercury during a transit, there is a lab exercise in Issue 11 of the Classroom Astronomer magazine.

2017 Visibilities

In 2013 the appearances (visible at least 40 minutes before sunrise or after sunset) are:


Morning Dates (Gr. Elong. W.)
Evening Dates (Gr. Elong. E.)
 Dec 2016 - Feb. 10 (Jan 20)
April 3 - June 20 (May 17)

**Aug. 31 - Sept. 29 (Sept. 12)
Dec. 5 - into Jan. '18)
 **Mar. 14 - Apr. 14 (Apr. 1)
June 27 - Aug. 9 (July 30)
Nov. 5 - Dec. 7 (Nov. 24)

** Best chances for Northern Hemisphere. Southern Hemisphere observers usually have opposite experiences; when good for North Americans, it's lousy for Australians. Greatest brilliancy usually occurs 1-7 days earlier or later than greatest elongation. They are nearly the same day for all the above apparitions except for the May and July events  (almost a month apart!).

Not all elongations are created equal. One reason is Mercury has that decidedly elongated orbit. It can be at its greatest elongation point as see from the Earth but if that occurs when Mercury is at perihelion -- closest distance in miles to the Sun -- it will appear much closer (18o)than when Mercury is at aphelion, the farthest point (then it is as much as 27o from the Sun).

Then, there is the fact that, like all planets, Mercury is only found near the Ecliptic, the orbital plane of the Earth in particular, the exact apparent path of the Sun in front of the stars as seen from Earth, and the orbital plane of all planets in general. The Ecliptic is tilted. Sometimes it goes almost straight up from a mid-northern hemisphere horizon. Other times it is close to parallel with the same horizon. When a Mercurial elongation occurs as the ecliptic rises vertically (spring evenings in the northern hemisphere, fall mornings) Mercury is at its best, almost directly above the rising or setting sun. When it is on a horizontal ecliptic, it sets very soon after the sun or rises just minutes before the morning sun, even if it IS far from the solar disk.

Nevertheless it is possible, and a good challenge, to locate Mercury during any of its elongations.

Conjunctions

It is easiest to spot Mercury when it is near a crescent Moon or one of the other bright planets, most notably Venus or Jupiter. These are much easier landmarks to spot and from them you can look a short distance in some direction and find Mercury. Binoculars are always a help to first spot the planet, then try with your eyes. These close meetings are also called conjunctions, though they should not be confused with Inferior or Superior Conjunctions with the Sun. For scale, one full moon size is 0.5o, a closed fist at arm's length is about 10o, a finger width at arm's length is about 2o.


March 18--near
Venus
May 24--1.6o N of Moon
July 25--0.9o S of Moon, occulted from N. Europe and N. Asia
July 26--1.1o S of Regulus
Sept 10--0.6o S of Regulus
Sept 16--0.3o or less from Mars, depending on where you are on Earth.
Sept 18--0.03o N of Moon, occultation in E. Asia and Pacifica
Nov 19-20--7o S of Moon
Nov 28--3o S of Saturn
Dec. 6
-- near Saturn



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