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The transit or passage of a planet across the disk 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 1999 November 15, Mercury will transit the Sun for the first time since 1993. This is an especially unique event called a grazing transit. Mercury's path across the Sun follows a short chord near the Sun's northeastern limb. In fact, the planet hovers so close to the solar limb that southern most observers will see only part of Mercury superimposed on the Sun at greatest transit. Further north, the planet's entire disk will be visible but will remain within arc-seconds of the Sun's limb.
The event will be widely observable from the South Pacific as seen in the accompanying map 1999 Transit of Mercury. The partial transit will be visible from most of Australia, New Zealand1, and Antarctica. The total transit will be visible from Papua-New Guinea, northeastern Australia, Hawaii, western South America and most of North America2 where the event occurs shortly before sunset.
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. For southern locations in the zone of partial transit, there are no contacts II or III. Position angles for Mercury at each contact are measured counterclockwise from the north point on the Sun's disk.
Geocentric Phases of the 1999 Transit of Mercury
Event Universal Position
Time Angle
Contact I 21:15:01 32°
Contact II 21:29:44 27°
Greatest 21:40:53 -
Contact III 21:52:03 19°
Contact IV 22:06:47 14°
The above contact times are for a geocentric observer. Due to the nature of the grazing transit, the actual contact times may differ by tens of minutes depending on the observer's actual geographic coordinates. From North America, ingress (contacts I and II) will occur earlier while egress (contacts III and IV) will occur later. In mid Pacific, the contacts approximate the geocentric times listed above. As one approaches the southern limit of the total transit, contacts II and III will converge and vanish while the period between contacts I and IV decreases.
Since Mercury is only 1/194 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 sunspots. However, the most valuable scientific contribution the amateur can make is to time 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, you should make an estimate of the possible error associated with each timing. Your observations and your geographic coordinates (measured from a topographic map) should be sent to: Almanac Office, U. S. Naval Observatory, Washington, D. C. 20390, 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. All these events will be especially prolonged due to the grazing geometry of 1999's transit.
1 North Island sees a total transit while South Island sees a grazing or partial transit.
2 Unfortunately, the transit occurs after sunset from the northeastern third of North America.
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. The following table lists all transits of Mercury from 1970 through 2150.
Transits of Mercury: 1970-2050
Date Universal Separation*
Time
1970 May 09 08:16 114"
1973 Nov 10 10:32 26"
1986 Nov 13 04:07 471"
1993 Nov 06 03:57 927"
1999 Nov 15 21:41 963" (graze)
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"
* distance (arc-seconds) between the centers of the Sun and Mercury
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 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. The following table lists all transits of Venus during the 300 year period from 1901 through 2200.
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 visible from eastern Canada and the USA. Since Venus will subtend 61 arc-seconds, it should be visible to the naked eye using suitable filtration. The first transit of Venus in 121 years is most eagerly anticipated.
The 1999 transit predictions were generated on a DEC VAX 11/785 computer using algorithms developed from the Explanatory Supplement [1974]. Ephemerides for the Sun and Mercury were generated from the JPL DE-200. The next transit of Mercury occurs on 2003 May 07 and is visible from Europe, Africa, Asia and Australia.
The author wishes to thank Goddard's Laboratory for Extraterrestrial Physics for several minutes of computer time. All calculations and diagrams presented in this section are those of the author and he assumes full responsibility for their accuracy.
WebMaster: Fred Espenak e-mail: espenak@gsfc.nasa.gov Planetary Systems Branch - Code 693
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