Planet Mercury

Observing Mercury

Because Mercury is an inner planet it will never resolve to a whole disk in a telescope. When Mercury is in the position to be "Full" it is located on the far side of the Sun relative to the Earth. When viewing Mercury the planet will appear as a crescent or gibbous shape. It will appear pinkish with gray markings.

Where To Look


If you were to trace the path of the Sun across the sky, the Sun’s path is a line called the Ecliptic.  The Ecliptic rises and falls during the year:  The highest point is the Summer Solstice and at the lowest point, 6 months later, occurs on the Winter Solstice.  Once you get a feeling of where the Ecliptic lies, you might discover that the moon and all the planets, with the exception of the former planet Pluto lies within a few degrees of the Ecliptic.  The Ecliptic represents the edge view of the Solar System.

Scanning the Ecliptic will help you locate the moon and planets. To pinpoint a specific planet at a specific time, you may want to: consult magazines like Sky and Telescope or Astronomy, or use software (see below), or one of the new handheld computerized realtime gadgets (see below), or consult a website like Astro Planet.[ :-) Ed.]

Optimal Times

There are certain times in a planets orbit when a planet is “optimal for viewing.”  For the inner planets: Mercury and Venus the best time to observe is at the Elongations.


Elongations occur when an inner planet's position, in its orbital path, is at tangent to the view from Earth. Because these inner planets are inside the Earth's orbits their positions as viewed from the Earth are never very far from the position of the Sun. When a planet is at Elongation, it is furthest from the Sun as viewed from Earth, so it's view is best at that point. There are two kinds of Elongations: The Eastern Elongation occurs when the planet is in the evening sky and the Western Elongation Occurs when a planet is in the morning sky.



The apparent motion of objects in the sky due to the rotation of the Earth is 15 degrees per hour. Mercury is not visible, due to the brightness of the Sun until 45 minutes after sunset or before sunrise, therefore Mercury must be at least 11 degrees 15 minutes away from the Sun before it is visible from the glare of the Sun. At the Greatest Eastern and Western Elongations, Mercury is between 18 to 28 degrees. At 28 degrees Mercury moves 1 hour 52 minutes behind or in front of the Sun. At the very best position Mercury would only be visible about 1 hour and 7 minutes for a given day.

Equipment Needed


Because planets are bright, though tiny in size, a large telescope isn’t necessarily required for viewing planets like Mercury, Venus, Mars, Saturn and Jupiter.  Large aperture telescopes are very beneficial to make dim things bright, like nebulae, galaxies, star clusters and the far outer planets: Uranus, Neptune and Pluto.

Almost any telescope capable of magnifying 100 to 200 times is great for viewing planets and our moon.


Eyepieces control magnification, field-of-view and eye relief.  You can consider the eyepiece half of your optical system.  Typically you will want high magnification eyepieces (100x-200x) for the moon and planets, while low power, wide field eyepieces are used for deep sky objects.

Calculating eyepieces

Each telescope is designed with a focal length.  Eyepieces also have a focal length. This value is usually printed on the side or top of the eyepiece. If you divide the focal length of the telescope by the focal length of an eyepiece that will give you the power or magnification that eyepiece will give with that telescope.

For example: An 8-inch Schmidt-Cassegrain telescope has the focal length of around 2000mm.  If you use a 10mm eyepiece with that telescope you will have 200 magnification (2000/10). A 30mm eyepiece in the same telescope will produce 67 power (2000/30).

So the lower the focal length of an eyepiece, the higher the power.

Sometimes eyepieces are also specified with Apparent Field-of-View measured in degrees.  If you were to divide the Apparent Field-of-View by the power you will calculate the Actual Field-of-View that that eyepiece would have with the telescope.

To compare various eyepieces click here

Planetary Filters

Planetary Filters Because they reflect light directly from the Sun, most planets are very bright.  If you think of a telescope as a light amplifier, then most telescopes will produce an image of a planet that is too bright to pick out subtle details.  There are two ways to adjust the light: planetary Filters and Off-axis masks.


Wratten System

The Kodak company developed a numbering system to specify color filters for use with black and white film.  This is known as the Wratten System.  Astronomy uses the same numbering system to specify planetary filters.

Because observational astronomy lacks color in the views of astronomical object until one gets into very large aperture telescopes (greater than 10 inches), using a planetary filter is like using a color filter with black and white film.  They will reduce the brightness and enhance various features seen on the planetary disk.

For more information on Planetary Filters, click here.

Suggested Filters for Mercury

Wratten NumberColorFeature
15Yellowfor low atmospheric contrast
21Orangefor low atmospheric contrast
25Redfor daytime observation (to cancel the blue sky)
29Redfor daytime observation (to cancel the blue sky)
57Greenfor low atmospheric contrast
23AOrangefor low atmospheric contrast

Off-axis Masks

Using an off-axis mask on the front of a telescope is another way to reduce the light gathered by a telescope for observing planets.  An off-axis mask is a plate that fits in the front opening of the telescope with a smaller hole located between the center and the edge of the opening (off-axis).  Frequently off-axis masks are built into the dust cover of some Newtonian reflector telescopes.  The hole is placed off-axis to avoid being blocked by the secondary mirror, usually located in the center of the aperture.

Using and off-axis mask has two advantages of filters.  They do not introduce false color and by reducing the usable aperture makes the telescope less sensitive to poor seeing conditions caused by turbulent atmosphere.

Kendrick Kwik-Focus


One way to get an off axis mask is to purchase a Kendrick Kwik-Focus.  The Kwik-Focus is an off-axis mask with three equally separated holes.  It is a tool used by astrophotographers to assist in focusing.  Basically,  if your telescope is out-of-focus the mask will produce three separate images.  The astronomer simply focuses the telescope until all the images are merged together and the telescope is focused, the astronomer removes the plate and starts photographing.

To use the Kwik-focus to observe a planet, simply plug two of the holes with the conveniently supplied plugs supplied with the mask and return the plate to the front of the telescope.

For more information on the Kendrick KwikFocus go here.

Computerized Sky Guides

In the last couple of years a new class of astronomy gadgets have appeared.  These handheld devices integrate GPS, Electronic compasses and motion sensors to create and integrate system that allow you to locate and identify visible objects in the sky without a telescope or other celestial aid.

These devices have three basic functions:

  1. Locate an object from the device’s database. Select the object from the database and follow the arrows to aim the device at the object in the sky.
  2. Identify an object in the sky.  Aim the device at an object in the sky and press the “Identify” button to get a list from brightest to dimmest of candidate objects.
  3. Give visual and audio information about a select object in the Device.

These devices first must sync to the GPS satellites, so they work best when there is a relative clear view of the sky.  Also, these devices are sensitive to electric and magnetic fields, so their battery compartments are shielded or separated from the rest of the mechanism and they work best when you step away from large metal objects like cars and electrical fields like high power lines.

Celestron SkyScout Personal Planetarium

Celestron SkyScout

Announce in 2006 at the Consumer Electronics Show (CES), this is the first-of-its-breed device.  The idea was so novel that it won the “CES 2006 – Best of Innovation Award”, the “Readers Digest – Best 2006 Award”, the “PC Magazine – Last Gadget Standing Award” and “Popular Mechanics- Editor’s Choice Award (CES 2006)”.

This device consists of :

  1. A black & white, backlit, liquid crystal display
  2. Audio port with “ear-buds”.
  3. Memory Card Port
  4. USB Port – for updating firmware.

This device runs on two AA batteries that are place in metal shielded tubes before you install into the device to reduce electrical interference.

To aim the SkyScout, you look through the device that has two rings on either end of the chamber.  The far ring has a ring of LED arrows to help you point your way

For more information on the Celestron SkyScout go here.

Meade mySky - Your Personal Guide for Sky Exploration

Announced in 2007, Meade came out with their mySky device. The mySky is a light weight,  gun-shaped device which sport LED at the top side of the device for aiming and a 2 inch color video screen for visual output.

When you first turn this device on you get an option to watch an instruction video on how to use the device or simply start using the device.

The mySky consist of:

  1. A color video screen
  2. Audio port with “ear-buds”.
  3. Memory Card Port
  4. USB Port – for updating firmware.
  5. A cable port in interface the mySky with a Meade computerized telescope system.  You can tell your telescope to move to a selected object from the mySky.
Meade mySky

This device runs on 4 AA batteries that are located at the bottom of the handle so they do not need to be shielded.

To aim the mySky simply look down the LED gunsite of the device and follow the “real time” star map projected in the video screen.

For more information on the Meade mySky go here.

Computer Software

One of the easiest ways to pinpoint the location of a planet or any celestial object for any given night is to use computer software to simulate the sky. Here are a few examples:

Starry Night

Available for Windows and Macintosh, this software is some of the most popular sky simulators.  There is also a “PRO” version which allows you to control a computerized “GOTO” telescope.

More information for Starry Night can be found here.

Starry Night

The Sky

A beautiful program available for Windows is another popular choice for simulating the sky.

More information for TheSky can be found here.


For those who are into open source software. Stellarium is available from  Stellarium is available for Windows, Macintosh, Unix and Linux.


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