INTRODUCTION
This Fall we will experience the last transit of a planet across the Sun until that
of Venus in 2012, and the last transit of Mercury until 2016. Thus we are fortunate
that the 2006 transit is timed so that observers anywhere in the Americas will be
able to see at least part of the event, and those on the Pacific coast will be able
to watch all of it.
TRANSIT PARAMETERS
Below are some descriptive statistics for the transit, where all times are in UT
(Universal Time) and PA stands for position angle, measured in degrees
counterclockwise from celestial north. These are geocentric
(observer-at-the-center-of-the-Earth) data, but, as described below, the values are
not much different wherever you observe from:
Contact 1 (begin ingress) -- Nov 08 19h 12m 01.7s, PA 140.9
Contact 2 (end ingress) ---- Nov 08 19h 13m 54.6s, PA 141.2
Least Distance ------------- Nov 08 21h 41m 01.7s, 422.9 arcseconds
Contact 3 (begin egress) --- Nov 09 00h 08m 13.6s, PA 269.0
Contact 4 (end egress) ----- Nov 09 00h 10m 06.5s, PA 269.3
Thus the event's total duration is slightly under five hours from ingress, the
entrance of Mercury onto the Sun's disk, to egress, its exit from the Sun. Ingress
and egress themselves take only 1m 53s.
In terms of the apparent sizes of the bodies, Mercury's disk will appear only
1/195th as wide as that of the Sun. (Sun's angular diameter = 1937.48 arcseconds
(0.54 degree); Mercury's angular diameter = 9.96 arcseconds.)
Mercury's apparent motion relative to the Sun will be 5.92 arcseconds per minute
(0.099 arcseconds per second).
Our final statistic is Mercury's differential parallax (apparent shift) relative to
the Sun, which is 4.14 arcseconds. Th
is is the maximum shift of the observed path of Mercury from its geocentric path.
Some implications of the above data are:
(1) Wherever you are in the visibility zone, the times of the transit events and the
path of Mercury will be almost the same as for the hypothetical geocentric view.
Anywhere on Earth, contact times will occur within 0.8 minute of the geo-centric
times, the transit duration will be within 1.2 minutes of the geocentric duration,
and the durations of ingress and egress will be within 0.5 seconds of the geocentric
durations. Also, the position angles of contacts will be within 1/4 degree of the
geocentric position angles. Nonetheless your location is still critical, as the
times of local sunrise and sunset will determine whether you can see the transit at
all, and if so which part of the transit you can see; indeed some observers will be
able to watch the entire transit from beginning to end.
(2) Mercury's disk will be very small, both absolutely and in relation to the Sun --
you will need to look carefully simply to identify the planet.
(3) Compared with its tiny disk, the planet will move very rapidly across the Sun's
limb, limiting to a few seconds the length of video sequences that can be stacked
without blurring the planet, the solar limb, or both.
WHERE CAN THE TRANSIT BE SEEN?
As the transit takes place in late northern-hemisphere Fall, the southern hemisphere
is favored for this transit. (The Earth's northern hemisphere had the best views of
the May 2003 transit of Mercury and the June 2004 transit of Venus.) As with any
transit, there are four visibility zones, depending largely on your longitude:
(1) Localities that miss the transit completely because it starts after local sunset
and ends before local sunrise. In 2006 these include central, southern and western
Asia and all of Africa and Europe.
(2) Areas where the Sun rises before Mercury's ingress starts, but where the Sun
sets with the transit still going on. Most of the Americas falls in this category.
(3) A region centered on the Pacific basin, but including New Zealand, easternmost
Australia and the Pacific coast of North America where ingress occurs after sunrise
and egress before sunset; thus the entire transit will be visible.
(4) Finally, eastern Asia and most of Australia will see the Sun rise with the
transit already in progress, but will see the transit end before the Sun sets.
Although observers in western Canada and the United States are fortunate in being
able to watch the whole transit, it will end with the Sun close to their
southwestern horizon.
OBSERVING THE TRANSIT
You can't observe a transit without observing the Sun, so you must take all the
precautions you would when observing the partial or annular phases of a solar
eclipse. There are only three safe ways to observe the Sun:
(1) Use a small aperture with a specially-designed narrow-band solar filter, most
often one that passes light within only a few tenths of an Angstrom unit centered on
the hydrogen-alpha wavelength of 6562.8 Angstrom units.
(2) Use a full-aperture solar filter on the front end of your telescope. The filter
should pass no more than 1/100,000th of the light in the visible, ultraviolet and
infrared bands and should not have scratches or pinholes. Remember to similarly
filter your finder, or cover its lens entirely. If using binoculars, safely filter
both lenses.
(3) Project the Sun's image through the eyepiece on a white screen. This allows
several people to watch simultaneously. There are several disadvantages, however.
First, contrast is low unless the screen is thoroughly shaded. Second, someone may
foolishly try to look directly through the eyepiece. Third, the unfiltered sunlight
inside your telescope will heat up its optics. This may actually shatter the
eyepiece, or the secondary or diagonal mirror. The same may happen with any filter
placed behind the eyepiece; these so-called solar filters should be avoided
entirely.
Mercury can be spotted on the Sun with low-power safely filtered binoculars, but you
will probably need a telescope magnifying at least 50 times, and 60 mm in aperture or
larger, to see the details of ingress and egress and time the four transit contacts.
Contact 1 is the most difficult to time because the planets starts to "notch" the
Sun's disk without any warning. You will need to orient the telescopic field prior
to this event in order to catch it. With an undriven telescope, the Sun's motion
defines celestial west. With an equatorial mounting, you can move the telescope to
determine celestial directions; the position angle of first contact on the solar
limb will be 4/7 of the way from east to south. Also, viewing with a narrow-band
hydrogen alpha filter enables you to see Mercury silhouetted on the Sun's
chromosphere just before it first touches the photosphere (which defines First
Contact).
The remaining contacts would seem much easier to time accurately. However, the
telescopic image is always somewhat blurred by atmospheric seeing and the finite
resolution of one's optical system. This creates the famous "black drop" effect - a
temporary fuzzy appendage connecting the limbs of Mercury and the Sun that makes
exact contact timing impossible.
The ALPO Mercury/Venus Transit Section (P.O. Box 2447, Antioch, CA 94531-2447 or
johnwestfall at comcast.net) would like to receive your contact times in order to
study the effects of the atmosphere and telescopic characteristics on timing
accuracy. Please report times to 1-second UT precision, and include your observing
site's position (to 0.01 degree or 1 arc-minute latitude and longitude); your
telescope's optical type, aperture, and focal length; any stops or filters used; and
atmospheric seeing and transparency, preferably reported on the standard ALPO
scales. Please also give your address -- postal, email, or both -- in case any
questions arise later.
Drawings, or film or electronic images, either still or video, of the transit are a
useful permanent record of the rare event, particularly if the following guidelines
are followed:
-- Submit your drawings, photographs or images to the ALPO Mercury/Venus Transit Section.
-- Image frames showing the entire disk of the Sun will be at too small a scale to
show useful detail for Mercury itself. Use sufficient magnification/EFL to show only
the planet and the area of the Sun immediately surrounding it.
-- Do not employ any digital sharpening algorithm, which can create artifacts such
as a bright halo around the planet or a light patch within its disk.
-- Record the time for each image to 1 second UT. Include the same information as
asked for with contact timings.
-- We need to know the orientation of images or drawings; the direction of celestial
north and east and whether the image is reversed). Also record the mode of imaging
(afocal, prime focus, eyepiece projection, etc.) with the effective focal length or
magnification, along with the image-recording method (camcorder, digital still
camera, etc.).
-- When stacking video frames taken during ingress or egress, avoid compositing so
many frames that the planet moves significantly during the interval. A reasonable
limit might be 5 seconds (50 frames at 10 frames per second), during which the
relative motion will be a half arc-second.
-- To make sure that none of the tonal range is lost, no portions of the image
should be completely black (0% brightness) or completely white (saturated; 100%
brightness).
Transits of Mercury average about thirteen per century and are thus even more rare
than solar eclipses. As with solar eclipses, the most interesting phases of transits
-- ingress and egress -- are compressed into a few minutes and present a challenge
for timing contacts, making drawings, or recording images.
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You can find more information about this transit, including a diagram of Mercury's
passage across the face of the Sun, a map showing where on Earth the event may be
seen, and a table of local circumstances for selected cities, on Fred Espenak's
website: http:/sunearth.gsfc.nasa.gov/eclipse/OH/transit06.html