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
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:
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.
Morning Dates (Gr. Elong. W.)
Evening Dates (Gr. Elong. E.)
**Dec 23, 2010 - Feb 1 (Jan 9)
**Mar 7 - Apr 5 (Mar 22)
** 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.
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
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
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