THE SOUTHWORTH PLANETARIUM



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       "All those who believe in psychokinesis raise my right hand."
                -British comedian Ronnie Corbett, who died yesterday at the
age of 85


THE DAILY ASTRONOMER
Thursday, March 31, 2016
More on Mercurian Transits!


Answers generate more questions!
Yesterday's article about the upcoming transit of Mercury has prompted more
questions about the transit of Mercury.
We happily address them today and didn't even bother to store them in
Pandora's vessel.

We have fewer questions, but, believe me when I say that these were enough!

However, please keep those cards and letters coming.





*When did people first see a transit of Mercury?*
*Johannes Kepler (1571-1630) was the first scientist to correctly predict
upcoming Mercurian transits, but he never observed them himself.  He
actually predicted that a transit of Mercury would occur on November 7,
1631. The only person to observe this transit -and therefore the first
person to have ever seen a Mercurian transit- was *French
philosopher/mathematician/astronomer  Pierre Gassendi (1592-1655). *Kepler
died the previous year. *

*Why are transits of Mercury more common than transits of Venus?*
Primarily because of the orbital paths.    Mercury is closer to the Sun and
therefore its orbit is much smaller than Venus.   Consequently, Mercury
moves faster than Venus and completes an orbit in much less time.  Mercury
needs about 88 days to revolve around the Sun.  Venus requires 225
days.     These times relate directly to the planets' "synodic periods."  A
synodic period is the time a planet requires to return to the same position
relative to Earth.  Mercury's synodic period is 116 days, which is the time
that separates successive inferior conjunctions.  Venus' synodic period is
584 days.    Mercury will pass through inferior conjunction about three
times a year, but more than a year and a half separates each inferior
conjunction of Venus.    Remember that a transit only occurs when the
planet is at a node the same time it moves through inferior conjunction.
Mercury is in inferior conjunction more often than Venus and its transits
will be more frequent.

*Transits of Mercury can only occur in either May or November, but I heard
that November transits are more common.  Is this true?  If so, why?*
We'll start by listing all 21st century transits of Mercury.

  2003 May 07
  2006 Nov 08
  2016 May 09
  2019 Nov 11
  2032 Nov 13
  2039 Nov 07
  2049 May 07
  2052 Nov 09
  2062 May 10
  2065 Nov 11
  2078 Nov 14
  2085 Nov 07
  2095 May 08
  2098 Nov 10

Five of the 21st century transits of Mercury happen in May.  The other nine
occur in November.    So, yes, November transits are about twice as
numerous as May transits. To explain this difference, we need to discuss
the concept of "Transit series."

We recall that transits only occur when an inferior planet is at a node
during inferior conjunction. A node is an intersection point between two
orbits.  Two planetary orbits will actually have two nodes, ascending and
descending.   When Mercury crosses the "ascending node," it is moving
"north" of Earth's orbit.   When Mercury crosses the descending nodes," it
is moving "south" of Earth's orbit.     Transits can and will occur at both
nodes.     May transits occur when Mercury is at the descending node.
November transits happen at the ascending node.   A transit series consists
of a sequence of transits happening at either node.

For example, Mercury transit series 8 consists of a sequence of November
transits.  The first occurred on November 2, 1776.    That transit just
"grazed" the southern tip of the Sun.    The next transit in that series
happened on November 5, 1822.   During that transit, Mercury moved along a
"higher" chord, though it was still "low."  The following Series 8 transit
was on November 6, 1868.  Mercury's transit path on that date was even
higher than the November 5, 1822 path.     Since November transits occur
when Mercury is at ascending node, each successive transit within a given
series will migrate "northward" until the highest chord is reached, at
which point the series ends.    Mercury transit series 8 concludes on
November 23, 2604.
[Perhaps a couple graphics will help you.   See them on the Daily
Astronomer web-page: www,usm.maine.edu/planet/da-7-december-2015 ]

Transit series 9 consists of May transits.  The first such transit occurred
on May 6, 1957.   This transit path sliced across the northern tip of the
Sun.    The next transit in series 9 happened on May 7, 2003.    Since May
transits occur at the descending node, this transit path was "south" of the
May 6, 1957 chord.  The next series 9 transit -May 7, 2049- will be south
of the May 7, 2003 transit chord.  (Note:  The transit on May 9, 2016 is
part of series 7.)     The Transit series 9 ends on May 16, 2371. This
transit path will cut across a very small chord along the Sun's southern
region.

Now, what does all this have to do with the question as to why November
transits are more frequent than May transits?
Well, first, if you do look at the graphics [www,
usm.maine.edu/planet/da-7-december-2015]  you will remark that the November
transit series has more members than the May transit series.   In fact,
there are about twice as many.      We have to introduce one more moving
part to understand this lack of parity.     A May transit occurs about a
month after Mercury reaches aphelion, the point of greatest distance from
the Sun.  When any planet is at its aphelion, it is moving most slowly in
its orbit.     Conversely, when a planet is at its perhelion, its orbital
velocity is at a maximum.   November transits occur a few days before
Mercury reaches perihelion and therefore it is moving most quickly.

We see that each transit in a series shifts either "northward" (November)
or "southward" (May), because the transits happen at the ascending and
descending nodes, respectively.   The shift of Mercury's position relative
to the Sun will be maximum (about 200 arc-seconds) when Mercury is moving
most slowly, or in May.   This shift will only be about 100 arc-seconds
when Mercury is moving most rapidly, or in November.

So, if you miss this next May transit, you'll have to wait until 2049 to
see another transit in the month of May.



*Could  someone could watch Earth transit the Sun from outer planets?*
Yes, Earth transits are sometimes visible from other planets.    A while
ago, we withdrew this same question from Pandora and will be lazy sods and
repeat the answer:
The last transit of Earth from Mars occurred on May 11, 1984.  The next
transit of Earth from Mars won't happen until November 10, 2084.     Also,
and quite excitingly, the Moon will also appear to transit the Sun, as
well. It will either follow or precede Earth across the Sun.        If you
miss the 2084 Earth transit event, you'll have to wait until November 15,
2163.     Transits of Earth from Mars are presently only possible in May
and November.

The following two transits of Earth from Mars occur on May 10, 2189 and
then again on May 13, 2268.

Transits of Earth from Jupiter are more common.  The last occurred on
January 5, 2014.  The next won't happen until January 20, 2026.   Then,
another occurs on June 24, 2055, followed by another on June 29,
2067.

The next transit of Earth from Saturn occurs on July 20, 2020. The next
transit of Earth from Saturn after the 2020 event occurs on July 16, 2049.