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Narrow Window of Opportunity
Let's suppose the orbits of Mercury and our planet Earth occupy the same plane. Then every time Mercury passed between the Earth and Sun at inferior conjunction, Mercury would transit, or cross directly in front of the Sun. As things are, however, Mercury's orbital plane is inclined by 7 degrees to the ecliptic (Earth's orbital plane). Therefore, transits of Mercury are quite rare, happening 13 to 14 times per century.
Though Mercury in its orbit can (and does) pass between the Earth and Sun at any month of the year, transits of Mercury can only occur in November or May. That's because for half its orbit, Mercury swings north of the ecliptic (Earth's orbital plane), and during the other half of its orbit, Mercury sweeps south of the ecliptic. At two points, Mercury's orbital path crosses - or intersects - the ecliptic. When the path swings from south to north, this point of intersection is called Mercury's ascending node; and when this path goes from north to south, it's called Mercury's descending node.
Two things are necessary for Mercury to transit the Sun. First, Mercury has to be at inferior conjunction. Second, Mercury has to be appreciably close to one of its nodes at an inferior conjunction. At inferior conjunction, Mercury is only appreciably close to its ascending node in early to middle November, and its descending node in early to middle May.
If Mercury reaches inferior conjunction after the narrow window of opportunity closes in November but before it opens in May, Mercury resides too far north of the Earth's orbital plane to transit the Sun. If the inferior conjunction happens after the May window shuts, but before the November window opens, then Mercury is too far south for a transit to occur.
November versus May Transits
November transits happen twice as often as May transits. This is because of the pecularities of Mercury's eccentric (oblong) orbit. During November transits, Mercury comes considerably closer to the Sun (and goes farther from Earth) than when Mercury transits the Sun in May. As seen from Earth, Mercury's greater distance from the Sun in May displaces Mercury's path away from the Sun, narrowing the window of opportunity.
November transits occasionally recur in periods of 6 and 7 years, and recur more frequently in 13 (6+7) and 33-year periods; May transits only recur in periods of 13 and 33 years. For both November and May transits, 46-year (13+33) recurrences continue uninterrupted for many hundreds of years.
For instance, the Nov. 8, 2006 transit belongs to a long-lasting 46-year series - sometimes called a "panorama" - that started in 1776 and will end in 2604. This is a period of 828 years. May panoramas last about half that long. As an example, the May panorama to which the previous May 7, 2003 transit belongs, started in 1957 and will conclude in 2371, for a total of 414 years. For more on the subject, read what Fred Espenak has to say in his Catalog of Mercury Transits.
In his More Mathematical Astronomy Morsels (chapter 48), Jean Meeus not only refers to these 46-year series panoramas, but to 217-year series panoramas that endure for many thousands of years!
copyright 2006 by Bruce McClure
|November 8-9 Transit of Mercury|
2006 Transit of Mercury by Fred Espenak
Catalog of Mercury Transits by Fred Espenak
Mathematical Astronomy Morsels by Jean Meeus (pages 223-225)
More Mathematical Astronomy Morsels by Jean Meeus (pages 285-288)
Mathematical Astronomy Morsels III by Jean Meeus (pages 277-283)