*THE SOUTHWORTH PLANETARIUM 207-780-4249 <207-780-4249>
 www.usm.maine.edu/planet <http://www.usm.maine.edu/planet> 70 Falmouth
Street  Portland, Maine 04103 43.6667° N,                    70.2667° W
Founded January 1970*
*                 "We are all in the gutter, but some of us are looking at
the stars."*
                             -Oscar Wilde



*THE DAILY ASTRONOMER*

*Wednesday, March 30, 2016*


*A Mercurian Transit Primer*


*About six weeks from now, the planet Mercury will appear to move across
the Sun.  Such direct "passages" of planets against their parent stars are
known as "transits."   Transits are noteworthy evens as they are certainly
not common.     We last observed a transit of Venus in 2012.  The last
transit of Mercury happened in 2006.   Today's DA offers a brief "Transit
of Mercury"  primer that will hopefully prepare us to watch this upcoming
spectacle! *

*We're using the conventional Q/A format for this primer.  If you have a
question we didn't include, please send it to me at
[log in to unmask] <[log in to unmask]>.  *


*When will the next transit of Mercury occur?*

*May 9, 2016*


*How long will it last?*

*Seven and a half hours.  It begins at 7:12 a.m. and ends at 2:42 p.m. *


*Will we eastern seaboard residents see all of this transit?*

*Yes.  The transit starts after sunrise and ends well before sunset.*


*Will the transit be visible to the unaided eye?*


*No.   Mercury's apparent diameter is 1/158 that of the Sun.  We recommend
that one use a telescope with at least a 50 x magnification in order to
observe this transit.   NOTE:  NEVER look at the Sun through a telescope.
Either use a filter or, preferably, project an image of the Sun onto a wall
and observe the transit in this image.*

*What will we actually see?*

*You will see a small black dot moving across the Sun.   During this
transit, Mercury moves "below" the Sun's center.  *


*If I miss this transit, when will the next one occur?*

*November 11, 2019.     Then, unfortunately, the following transit won't
occur until November 13, 2032!*



*Why don't transits of Mercury happen more often?*

*Transits of Mercury can only happen when Mercury is at inferior
conjunction at the same time it is at a node.  A planet is in inferior
conjunction when it is between Earth and the Sun.  (Only Mercury and Venus,
the two inferior planets, can ever be in inferior conjunction)   A node is
the intersection point between two orbits.    Mercury's orbit is inclined
by about seven degrees relative to Earth's orbit, generally called the
"ecliptic."   Consequently, most of the time Mercury will either be "north"
or "south" of the Sun during inferior conjunction.  Only when Mercury just
happens to be at the intersection point during inferior conjunction will we
see a transit.   If Mercury's orbit and Earth's orbit were precisely
aligned, a transit would occur during each inferior conjunction.  *


*Can Mercury and Venus ever transit the Sun simultaneously? *


*They can't at present because transits of Mercury occur only in either May
and November.    Transits of Venus happen only in either June or December.
However, nodes shift around orbits so that these dates will change.
Eventually, Mercury and Venus will transit Sun simultaneously.  Or, more
correctly, observers will be able to see both planets against the Sun.
This will happen in AD 69,163.  A bit of a wait.   *

*Will the Sun's brightness diminish during the transit?*


*Theoretically, yes, it will.  However, the brightness reduction will be
less than one percent and won't be noticeable at all.    Notably,
astronomers can find planets around other stars by noticing how their
brightness diminish when the planets move in front of them.   This planet
detection technique is known as  the "transit method."*

*Is it true that Captain Cook observed both a transit of Venus and a
transit of Mercury?*

*Yes.  He observed a transit of Venus from Tahiti, in a location now known
as "Point Venus."  He also observed a transit of Mercury while in New
Zealand from  what is now known as "Mercury Bay."*


*What is the relation between a transit of Mercury and a former King of
England?*
*A transit of Mercury occurred on May 3, 1661, the same date as Charles
II's coronation. Charles II did not observe this transit, but famed
astronomer   Christiaan Huygens did see it. *