Hermograph Press presents...

The Planet Mercury


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 more often not visible than visible. There are usually few nearby sky "landmarks" to help you find it.

Because Mercury is the innermost planet it is never more than about 27o from the sun, about 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. Thus it is a morning star or an evening star on occasion, but only for short appearances. In fact, the ancient Greeks called it "Apollo" during its morning sightings and "Hermes" in the evening. (Click here for Mercury as written in other languages.)

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, it would be "full" Mercury, fully illuminated, a round bright disk. It would also appear at its smallest size and dimmest brightness, because it is at its farthest from us. At this time it is in motion 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. It therefore follows the Sun's daily motion as our star seems to move from East (in the morning) to West (in the evening) 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. At this time it appears in a telescope like a half-moon shape.

  3. Inferior Conjuction -- 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, invisible to us.

  4. Greatest Western Elongation -- 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.

A site that will generate astronomical coordinates you can use to chart Mercury yourself is at Harvard

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 like 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 last transit was in 2006. The next transit is N May 9, 2016

For detailed information, go to the US Naval Observatory website.

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.

2011 Visibilities

In 2011 the appearances (visible at least 40 minutes before sunrise or after sunset) are:

Morning Dates (Gr. Elong. W.)
 
Evening Dates (Gr. Elong. E.)
 
**Dec 23, 2010 - Feb 1 (Jan 9) 
  Apr 21 - May 30(May 7)
**Aug 22 - Sept 19 (Sept 3)
**Dec 12 - into 2012!
 
**Mar 7 - Apr 5 (Mar 22)
  Jun 20 - Aug 1 (Jul 19) 
  Oct 29 - Nov 28 (Nov 13)
 
** Best chances for Northern Hemisphere. Southern Hemisphere observers 
 usually have opposite experiences; when good for North Americans, 
 it's great for Australians.

Greatest brilliancy usually occurs 1-7 days earlier than greatest elongation
for morning events and 1-7 days later for evening events. They are nearly the same day
for all the above apparitions except for the June-July and Sept-Oct morning ones.

Not all Elongations are created equal. One reason is Mercury has a decidedly elongated (highly eccentric) orbit. It can be at a greatest elongation point as seen from Earth but if that point occurs when Mercury is at perihelion -- closest distance in miles to the sun -- it will appear much closer (18 o) 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 and all planets in general. The Ecliptic (also the exact apparent path of the Sun in front of the stars) is tilted. Sometimes it appears 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 position. 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 eye. 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.

Jan 2--4o North of Moon (evening)
Mar 16--2o North of Jupiter (evening)
April 19--0.8o North of Mars (morning)
April 9->13, within 3o of Venus and Jupiter, M and V 1.4 degrees apart, Apr 8, 2 deg from Jupiter on the 10th. (morning)
April 19-23--Within 3o of Mars and Venus, 1.4 degrees from Venus on the 18th, 2 degrees South of Mars on the 20th (morning)
July 6--.3o of Beehive cluster (evening)
Aug 1--1.5o N of Moon (evening)
Aug 28--3 degrees N of Moon
(morning)
Sept 9--0.7o N of bright star Regulus (evening)
Oct 27--0.2o N (above) of Moon (evening)
Oct 27->~Nov 15--Within 2.0o of Venus, exactly that much on Nov. 2nd (evening)
Nov 9-10--Between Venus and Antares, exactly 1.9o North of Antares on the 10th, 2 degrees from Venus on the 13th (evening)
Nov 26--1.7o S of Moon (evening

NEXT ORBIT: Mercury in the Telescope
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