THE USM SOUTHWORTH PLANETARIUM
207-780-4249     www.usm.maine.edu/planet
70 Falmouth Street     Portland, Maine  04103
43.6667° N                   70.2667° W
Altitude:   10 feet below sea level
Founded January 1970
Julian date:  2458646.5
           "I have not failed.  I've just found 10,000 ways that won't
work."
                              -Thomas Edison



THE DAILY ASTRONOMER
Wednesday, June 12, 2019
Explaining a Transit Series

152!
That is the number of days currently separating today and November 11,
2019, the date of the next Transit of Mercury.    Provided the skies are
clear, we here in the eastern US will be able to watch the entire event.
Conveniently, the transit begins in the early morning and ends in the early
afternoon.   We're truly hoping for clement weather as we eastern US
observers won't see another Mercurian transit until May 7, 2049.    For
this reason, we will periodically revisit this event over the next five
months. In so doing, we hope to be able to explain the many facets involved
in such a rare celestial event.

[image: tm2019-Fig01b.gif]


Referring to the graphic above, look at the information listed in the lower
left hand corner:   Transit Series  =  247     Sequence No = 11 of 19

What precisely does that information tell us and why is it helpful to us
when attempting to predict other transits?

A transit of Mercury series refers to a sequence of transits, each of which
is separated by almost exactly 46 years.     After this period of time, a
transit will recur with nearly the same type of path around the same time
of year.        This repetition occurs because 46 revolutions of Earth is
approximately equal to 191 revolutions of Mercury.   So, after 46 years,
Earth and Mercury will almost occupy the same positions relative to each
other.

The November 11, 2019 Transit will be the 11th transit within the Transit
Series 247*

If we look at the dates of the previous transits within the series, we can
see how successive dates are quite close, but deviate after a few
iterations:

[10] November 10, 1973
[Historical note:  NASA launched Skylab 4 less than a week later]

[9] November 10, 1927
[Historical note:  Leon Trotsky was expelled from the Soviet Communist
Party two days later.]

[8] November 6 - 7, 1881
[Historical note: the gunfight in the O.K. Corral in Tombstone, AZ happened
about two weeks earlier.]

[7] November 7, 1835
[Historical note:  Halley's Comet reached perihelion (closest distance to
the Sun) about nine days later.]

[6]  November 5, 1789
[Historical note:  About two weeks later, the State of New Jersey became
the first state to ratify the Bill of Rights]

[5]  November 5, 1743
[Historical note:  French astronomer Joseph-Nicolas Delisle organized
worldwide coordinated observations of this Mercurian transit in an effort
to determine the solar parallax.]

[4]  November 3, 1697
[Historical note:   St. Paul's Cathedral in London opened a month later.]

[3] November 3 - 4, 1651
[Historical note:  the second transit of Mercury to have been observed.
Jeremy Shakerly observed the transit from the Indian city of Surat.  Note
on this note: Pierre Gassendi observed the 1631 transit of Mercury.]

[2]  November 1, 1605
[Historical note:  Guy Fawkes' "Gunpowder Plot" to blow up the English
Houses of Parliament was foiled four days later.]

[1] November 1, 1559
[Historical note:  Elizabeth I was crowned earlier that year.]


Based on this information, can you predict when the following two transits
in this series will occur?

November 11, 2065

November 13, 2111

The final transit in the 247 series will occur on November 20, 2387.
As each transit in a given series occurs, the transit path is slightly
displaced from the other path.   The very first transit of a series will
happen along either the upper or lower limb, depending on the node around
which the transit occurs.**  The 247 Transit Series, as we can see on the
graphic, occurs at the ascending node.  Therefore, the first transit in
Series 247 (in 1559) brought Mercury along the Sun's lower limb. As this
transit is nearly at the midpoint (11 out of 19), the transit path is
almost central.  During the final 247 Transit (in 2387), Mercury will just
skim the Sun's upper limb.     Forty six years later (2433), Mercury will
be too far from the node around November 20th for a transit to occur.  The
transit series will have ended.
Transit series, like Saros series for eclipses, provide a handy way for us
to predict when future celestial events will occur.    They are also a very
disquieting reminder of our mortality, as we can speak with conviction
about events scheduled to transpire centuries in the future.




*Yes, perhaps you are find this number a bit puzzling.     The transit on
November 7, 1631 was the first transit of Mercury to have been observed.
Transit series are listed in a 100,000 year canon of Mercurian transits.
They are numbered according to when they reach their midpoint.

**Just a note about "nodes."   When a planet crosses the "ascending node,"
it is moving north of Earth's orbital plane.  When a planet crosses the
"descending node," it is moving south of Earth's orbital plane.