Published in Observer's Handbook 2003, Royal Astronomical Society of Canada
--- Live Webcast of Transit! ---
2003 May 07: Transit of Mercury
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. On the average, there are 13 transits of Mercury each century. In comparison, transits of Venus occur in pairs with more than a century separating each pair.
On Wednesday, 2003 May 07, Mercury will transit the Sun for the first time since 1999. The entire event will be widely visible from the Europe, Africa and Asia as shown in the map in figure 1. Japan Australia, and New Zealand will witness the beginning of the transit but the Sun will set before the event ends. Similarly, observers in western Africa, eastern North America and eastern South America will see the end of the event since the transit will already be in progress at sunrise from those regions.
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.
Table 1 Geocentric Phases of the 2003 Transit of Mercury Event Universal Position Time Angle Contact I 05:12:56 15° Contact II 05:17:24 14° Greatest 07:52:23 333° Contact III 10:27:19 292° Contact IV 10:31:46 291°
Table 1 above gives the times of major events during the transit. Greatest transit is the instant when Mercury passes closest to the Sun's center (i.e. - minimum separation). During the 2003 transit, Mercury's minimum separation from the Sun is 708 arc-seconds. The position angle is defined as 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 celestial coordinates of the Sun and Mercury are provided at greatest transit as well as the times of the major contacts. The world map at the bottom of figure 1 shows regions of visibility of the event.
Note that these times are for an observer at Earth's center. The actual contact times for any given observer may differ by up to several minutes. This is due to effects of parallax since Mercury's 12 arc-second diameter disk may be shifted up to nearly 16 arc-seconds from its geocentric coordinates depending on the observer's exact position on Earth. Table 2 lists predicted contact times and corresponding altitudes of the Sun for major cities around the world with an emphasis on Canada and the United States.
Since Mercury is only 1/158 of the Sun's apparent diameter, a telescope with a magnification of 50x to 100x is recommended to watch this event. Naturally, the telescope must be suitably equipped with adequate filtration to ensure safe solar viewing. The visual and photographic requirements for observing a transit are identical to those for solar viewing. Amateurs can make a scientific 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 receiver) should be sent to: A. L. P. O. Mercury/Venus Transit Section, P.O. Box 16131, San Francisco, CA 94116, USA.
Actually, white light observations of contacts I and IV are not technically possible 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.
All transits of Mercury fall within several days of 8 May and 10 November. 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.
Table 3 Transits of Mercury: 2001-2100 Date Universal Separation Time 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, transits of Venus are much rarer. Indeed, only six such events have occurred since the invention of the telescope (1631,1639, 1761,1769, 1874 and 1882). Transits of Venus are only possible during early December and early June when Venus's orbital nodes pass across the Sun. Transits of Venus 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.
Table 4 Transits of Venus: 1901-2200 Date Universal Separation Time 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 will be the first one since 1882. The final stages of the transit will be visible from eastern Canada and the USA. Since Venus will subtend 58 arc-seconds, it should be visible to the naked eye using suitable filtration. The first transit of Venus in nearly 122 years is most eagerly anticipated. Full details will be published next year in the Observer's Handbook 2004. In addition, the paper 2004 and 2012 Transits of Venus is now online. Finally, special web site for transits can be found at:
The 2003 transit predictions were generated on an Apple iMac computer using algorithms developed from Meeus  and the Explanatory Supplement . Ephemerides for the Sun and Mercury were generated from VSOP87. The next transit of Mercury occurs on 2006 November 8-9 and is visible from the Americas, eastern Asia, Australia and New Zealand.
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.
A special thanks goes to summer intern Holly Schurter (National Space Club) for transferring this document to the web.
We will list links for live web coverage of the transit as they become available.
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Last revised: 2004 Sep 20 - F. Espenak