Figure A - This figure shows the path of Mercury across the Sun's disk on 2006 Nov 08. Mercury appears only 1/194 the size of the Sun, so its tiny disk will require a small telescope to see (using a special solar filter to protect the eyes).
Figure B - This map shows the visibility of the 2006 Transit of Mercury from the U.S.. Depending on the time zone, the transit begins at: 2:12 pm EST, 1:12 pm CST, 12:12 pm MST and 11:12 am PST (9:12 am in Hawaii). Observers in the eastern U.S. will see the first 2 to 3 hours of the transit before the Sun sets. From the central U.S., about 4 hours of the transit will be visible before sunset. The transit in its entirety will be seen from the western quarter of the country. The transit ends at 4:10 pm PST (2:10 pm in Hawaii)
On Wednesday, 2006 Nov 08, Mercury will transit the Sun for the first time since 2003. The transit or passage of a planet across the face of the Sun is a relatively rare occurrence. As seen from Earth, only transits of Mercury and Venus are possible. There are approximately 13 transits of Mercury each century. In comparison, transits of Venus occur in pairs with more than a century separating each pair.
The principal events occurring during a transit are conveniently characterized by contacts, analogous to the contacts of an annular solar eclipse. The transit begins with contact I which is the instant when the planet's disk is externally tangent with the Sun. Shortly after contact I, the planet can be seen as a small notch along the solar limb. The entire disk of the planet is first seen at contact II when the planet is internally tangent with the Sun. During the next several hours, the silhouetted planet slowly traverses the brilliant solar disk. At contact III, the planet reaches the opposite limb and once again is internally tangent with the Sun. Finally, the transit ends at contact IV when the planet's limb is externally tangent to the Sun. Contacts I and II define the phase called ingress while contacts III and IV are known as egress. Position angles for Mercury at each contact are measured counterclockwise from the north point on the Sun's disk.
Geocentric Phases of the 2006 Transit of Mercury
|
Event |
Universal Time |
Position Angle |
|
|
|
|
|
Contact I |
19:12:04 |
141° |
|
Contact II |
19:13:57 |
141° |
|
Greatest Transit |
21:41:04 |
205° |
|
Contact III |
00:08:16 |
269° |
|
Contact IV |
00:10:08 |
269° |
Table 1 above gives the times of major events during the 2006 transit. Greatest transit is the instant when Mercury passes closest to the Sun's center (i.e. - minimum separation). At this time, the geocentric angular distance between the center's of Mercury and the Sun will be 423 arc-seconds. The position angle is the direction of Mercury with respect to the center of the Sun's disk as measured counterclockwise from the celestial north point on the Sun. Figure 1 shows the path of Mercury across the Sun's disk and the scale gives the Universal Time of Mercury's position at any instant during the transit. The contact times are listed along with the celestial coordinates of the Sun and Mercury at greatest transit. Since the contact times are geocentric they are only correct for an observer stationed at Earth's center. The contact times for any given location may differ from the geocentric times by up to a minute. This is due to the effect of parallax since Mercury's 10 arc-second diameter disk may be shifted up to nearly 13 arc-seconds from its geocentric coordinates depending on the observer's exact geographic position.
The transit will be widely visible from the Americas, the Pacific Ocean, eastern Asia, and Australia as shown in Figure 2. Observers throughout most of the Americas will witness the beginning of the transit but the Sun will set before the event ends. Similarly, Asia and most of Australia will see the end of the event since the transit will already be in progress as the Sun rises. Regions where the entire transit is visible include western North America, eastern Pacific, New Zealand, southeastern Australia and Antarctica. The transit will not be visible from anywhere within Europe, Africa or western Asia. Table 2 lists predicted contact times and the corresponding altitude of the Sun for over a hundred cities around the world with an emphasis on Canada and the United States.
Since Mercury is only 1/194 of the Sun's apparent diameter, a telescope with a magnification of 50x to 100x is recommended to watch this event. The telescope must be suitably equipped with adequate filtration to ensure safe solar viewing. The visual and photographic requirements for transit are identical to those for observing sunspots and partial solar eclipses. Amateurs can make a useful contribution by timing the four contacts at ingress and egress. Observing techniques and equipment are similar to those used for lunar occultations. Since poor seeing often increases the uncertainty in contact timings, an estimate of the possible error associated with each timing should be included. Transit timings and geographic coordinates of the observing site (measured from a topographic map or GPS) should be sent to Dr. John Westfall (johnwestfall@comcast.net), A.L.P.O. Mercury/Venus Transit Section, P.O. Box 2447, Antioch, CA 94531-2447.
White light observations of contacts I and IV include a small bias since Mercury is only visible after contact I and before contact IV. However, if Hydrogen-alpha filtration is available, the planet may be visible against either prominences or the chromosphere before and after contacts I and IV, respectively. Observations of contacts II and III also require amplification. They're often mistaken for the instant when the planet appears internally tangent to the Sun. However, just before contact II, the so-called black drop effect is seen. At that time, the transiting planet seems to be attached to the Sun's limb by a thin column or thread. When the thread breaks and the planet is completely surrounded by sunlight, this marks the true instant of contact II. Contact III occurs in exactly the reverse order. Atmospheric seeing often makes it difficult to measure contact timings with a precision better than several seconds.
During the present era, transits of Mercury fall within several days of May 8 and November 10. Since Mercury's orbit is inclined seven degrees to Earth's, it intersects the ecliptic at two points or nodes which cross the Sun each year on those dates. If Mercury passes through inferior conjunction at that time, a transit will occur. During November transits, Mercury is near perihelion and exhibits a disk only 10 arc-seconds in diameter. By comparison, the planet is near aphelion during May transits and appears 12 arc-seconds across. However, the probability of a May transit is smaller by a factor of almost two. Mercury's slower orbital motion at aphelion makes it less likely to cross the node during the critical period. November transits recur at intervals of 7, 13, or 33 years while May transits recur only over the latter two intervals. Table 3 lists all transits of Mercury from 2001 through 2100 (Meeus, 1989).
Transits of Mercury: 2001-2100
|
Date |
Universal Time |
Separation |
|
|
|
|
|
Contact I |
19:12:04 |
141° |
|
2003 May 07 |
07:52 |
708" |
|
2006 Nov 08 |
21:41 |
423" |
|
2016 May 09 |
14:57 |
319" |
|
2019 Nov 11 |
15:20 |
76" |
|
2032 Nov 13 |
08:54 |
572" |
|
2039 Nov 07 |
08:46 |
822" |
|
2049 May 07 |
14:24 |
512" |
|
2052 Nov 09 |
02:30 |
319" |
|
2062 May 10 |
21:37 |
521" |
|
2065 Nov 11 |
20:07 |
181" |
|
2078 Nov 14 |
13:42 |
674" |
|
2085 Nov 07 |
13:36 |
718" |
|
2095 May 08 |
21:08 |
310" |
|
2098 Nov 10 |
07:18 |
215" |
Edmund Halley first realized that transits could be used to measure the Sun's distance, thereby establishing the absolute scale of the solar system from Kepler's third law. Unfortunately, his method is somewhat impractical since contact timings of the required accuracy are difficult to make. Nevertheless, the 1761 and 1769 expeditions to observe the transits of Venus gave astronomers their first good value for the Sun's distance.
Because Venus's orbit is considerably larger than Mercury's orbit, its period is also longer making transits of Venus are much rarer. Indeed, only seven such events have occurred since the invention of the telescope (1631, 1639, 1761, 1769, 1874, 1882 and 2004). During the current era, transits of Venus are only possible in early December and early June when Venus's orbital nodes pass across the Sun. Venus transits show a clear pattern of recurrence at intervals of 8, 121.5, 8 and 105.5 years. Table 4 lists all transits of Venus during the 300 year period from 1901 through 2200.
Transits of Venus: 1901-2200
|
Date |
Universal Time |
Separation |
|
|
|
|
|
2004 Jun 08 |
08:19 |
627" |
|
2012 Jun 06 |
01:28 |
553" |
|
2117 Dec 11 |
02:48 |
724" |
|
2125 Dec 08 |
16:01 |
733" |
The 2004 transit of Venus was the first one since 1882. The upcoming 2012 transit will be visible from all of North America and Asia as well as parts of Europe and Africa (Espenak 2002). Since Venus will subtend 58 arc-seconds, it can be seen with the naked eye (using suitable filtration) as a tiny black disk against the Sun. This final transit of Venus during the 21st century is most eagerly anticipated. For more details, see:
http://eclipse.gsfc.nasa.gov/transit/venus0412.html
The next transit of Mercury occurs on 2016 May 09 and is visible from the Americas, Europe, Africa and central Asia.
NASA's Eclipse Home Page features two catalogs listing dates and details for all transits of Mercury (AD 1600 to AD 2300) and Venus (2000 BC to AD 4000). The transit catalogs were generated using elements published by Meeus (1989). Both catalogs can be accessed from the top transit page at:
http://eclipse.gsfc.nasa.gov/transit/transit.html
To determine whether a transit is visible from a specific geographic location, it is simply a matter of calculating the Sun's altitude and azimuth during each phase of the transit. As an aid to historical research, the transit catalogs have also been ported into several Microsoft Excel 97 spreadsheet files which perform the these calculations automatically. Upon entering the desired latitude and longitude, each Excel file calculates the altitude of the Sun at that location for every transit in the file and at each of the four contacts . To simplify the spreadsheet calculations the Sun's geocentric coordinates at greatest transit are used. This results in an accuracy in the Sun's altitude to within one degree. The Excel files can be accessed at:
http://eclipse.gsfc.nasa.gov/transit/catalog/Visible.html
The 2006 transit predictions were generated on an Apple G4 iMac computer using algorithms developed from Meeus [1989] and the Explanatory Supplement [1974]. Ephemerides for the Sun and Mercury were generated from VSOP87. The author wishes to thank Goddard's Living with a Star program for support of this work. All calculations, diagrams, tables and opinions presented in this paper are those of the author and he assumes full responsibility for their accuracy.
Special thanks to National Space Club summer intern Sumit Dutta for his valuable assistance in preparing the web page (July 2005).
Espenak, F., 2002, "2004 and 2012 Transits of Venus", Proceedings for Scientific Frontiers in Research on Extrasolar Planets, PASP.
Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac, 1974, Her Majesty's Nautical Almanac Office, London.
Meeus, J., 1989, Transits, Willmann-Bell, Inc., Richmond.
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