A FABULOUS SHOW, PORRIMA! 
 -----------------------
 John Pazmino
 NYSkies Astronomy Inc
 nyskies@nyskies.org 
 www.nyskies.org 
 2011 March 15 initial
 2011 March 23 current
Introduction
 ----------
    Among the most fascinating once-in-TWO-lifetime shows I witnessed 
in the 2-thous was that of gamma Virginis (GA-ma VIRR-jih-niss) or 
Porrima (PO-rih-ma). This star is a binary like no other in the sky 
for its 169-year period in a strongly excentric orbit. 
    For most of its cycle the star is a wide pair for small 
telescopes. As gamma Virginis approached its proximity, when the 
components rounded periastron, the pair grew harder and harder to 
split thru a given telescope and eyepiece. At some date before the 
periastron the stars merged into a single point. It passed beyond the 
resolution of the instrument. 
    This merger happened at different dates depending on the scope, 
observer skill and faculties, and state of the air above him. For me 
the merger came in the 2003 apparition of Porrima. I just could not 
see the star as double any more. 
    I continued to inspect Porrima thru my own and other scopes with 
nothing of a duplicity about it. I also followed reports from other 
astronomers who were able to see the two stars well into 2004. 
    I waited two years to try and capture the pair as it widened. At 
first i failed repeatedly. In the 2008 apparition I just barely, with 
long study and patience discerned that the one dot was probably two 
overlapping ones. By the 2009 apparition the star was definitely, tho 
not easily, seen as two in my small scopes. 
    As background and history of gamma Virginis, please see my prior 
articles 'A star on the move' and 'They're closing in!' in the NYSkies 
website at 'www.nyskies.org/articles/pazmino/gam-vir1.htm' and  
'.../gam-vir2.htm'. This here piece here continues the story of gamma 
Virginis thru 2010-2011. 
Some geometry
 ----------- 
    The two stars of Porrima orbit in Kepler ellipses but seen at a 
148-degree angle against our sightline from Earth. The angular closest 
approach of the stars, peoximity, is really not the linear closest 
approach, periastron, an effect entirely of our perspective. 
    Periastron is the near end of the orbital ellipse, analogous to 
perihelion or perigee in the solar system. Proximity is the angular 
closest separation on the sky as seen from Earth. Think of a planet or 
comet in apparent conjunction with the Sun. 
    By the geometry of the Porrima orbit, periastron and proximity are 
almost coincident, causing many authors to use them as equivalent. 
In the general binary star they are wildly different. 
    Periastron occurred in about 2005.4, meaning 4/10 way thru year 
2005. In double star work we cite dates as a year and decimal, rather 
than calendar dates or Julian day numbers.
    The inclination of 148 degrees is interpreted like a comet orbit's 
inclination. It is retrograde in the sense that the companion star 
rotate thru north, then west, south, and east. The direction, position 
angle, is conventionally dimensioned from north, thru east, south, 
west, so the comes (KO-mess) continuously decreases its position angle. 
Apparition
 --------
    Porrima's apparition follows that of other late spring stars by 
coming into the eastern sky in fall after the Sun moved from Virgo 
into Libra. It is then in the dawn sky before sunrise. All of this 
description applies to New York City, a typical mid-northern latitude. 
    As the Sun marches eastward thru the zodiac, Virgo, with Porrima, 
drifts into night sky in owl-hours and midnight, than into the late 
evening sky. 
    gamma Virginis drifts gradually from east thru south thru west 
during spring and summer. It souths during April after nightfall. By 
late summer, as the Sun approaches Virgo out of Leo, the star sets 
into evening twilight. 
    The blackout, the Sun gap, spans September thru November when 
observations of Porrima are not feasible from the ground. With the 
excitement of periastron in the 2-thous many readers watched gamma 
Virginis thruout its entire apparition,
    In the 2011 apparition Porrima is joined by Saturn as a marker in 
poorer skies. The two are a pair of handy targets for demonstrating 
astronomy at public stargazing sessions. 
Facts and figures
 ---------------
    Here are some facts & figures about Porrima. These are updated 
from my prior articles. First up are Porrima's other names: 
    --------------------------
    catalog      | designation
    -------------+------------ 
    3rd Fundamtl | FK3-477 
    Aitken       | ADS1630 
    Bayer        | gamma1 & gamma2 Virginis 
    Bonner Durch | BD+00:2601 
    Boss         | GC17170 
    Flamsteed    | 29 Virginis 
    Henry Draper | HD110379 & HD110380 
    Hipparchos   | HIP61491 
    Hubble       | GSC4949-1113 
    Pos Pro Mot  | PPM400177 
    Proper name  | Porrima, Arich 
    Smithsonian  | SAO138912 & SAO138917 
    Struve       | [Sigma]1670, STF1670 
    Tycho        | TYC4949-1120-2 
    Washington   | WDS12417-0127  
    Yale         | HR4825 & HR4826 
    Zodiacal     | ZC1819 & ZC1821 
    -------------------------
    Because for the most of the 19th and 20th century the stars were 
well separated, they commonly each earned their own designation. It 
can be ambiguous in catalogs which component is which. 
    The celestial coordinates are, for epoch 2000: 
    ------------------
    Right ascension | 12h 41.6m 
    Declination     | -01d 26.9m 
    Ecliptic long   | 190.13 
    Ecliptic lat    | +02.79 
    Galactic long   | 297.77 
    Galactic lat    | +61.33 
    ------------------------
    The two stars are nearly equal in size, brilliance, diameter, and 
other properties: 
    --------------------------------------------
    Property  | A & B |   A   |   B   | comments
    ------------+-----+-------+-------+---------
    Diameter  |       | 1.3   | 1.3   | Suns 
    Mass      |       | 1.3   | 1.3   | Suns
    Surf temp |       | 7,100 | 7,100 | Kelvin 
    Spectrum  |       | F0-V  | F0-V  | yellow-white
    App magn  | +2.7  | +3.4  | +3.5  |
    Abs magn  | +2.4  | +3.1  | +3.2  |
    Luminos'y |       | 4.3   | 4.2   | Suns
    Distance  | 38.6  |               | ly, 11.8pc 
    Sun magn  | +5.2  |               | seen from star 
    Rad Vel   | -20   |               | km/s  
    ----------------------------------------------
    Slight variations in these values occur in the litterature. 
What happened? 
 ------------
    Porrima is a binary whose orbit hangs the pair for decades on end 
near apoastron but whips it around near periastron. The previous 
instance, the first witnessed, was in 1836. The instant one, only the 
second to be observed, was in 2005. The two stars collapsed into a 
single dot thru most home telescopes by 2003. 
    Periastron occurred in May-June of 2005 with a separation of about 
0.35 arcseconds. There is a minor discrepancy among observers in the 
exact date, with all within about 40 days. 
    Only really large telescopes under truly stable and still air 
could resolve Porrima into two stars. The star was broken apart by 
various interferometric techniques at campus observatories. 
    By 2008 gamma was pulling apart. First the larger scopes and then 
gradually the smaller ones were able to show it as a pair, not just a 
congealed image. In 2009 pretty much any good quality home telescope 
revealed the star as double.
    For the next 160 or so years, gamma Virginis will be a wide double 
star, a pleasant sight in any scope. The farthest separation, at 
apoastron, is about 6 arcseconds in around year 2090. If you want to 
watch the next periastron, hang around until 2174.. 
Stars in motion!
 --------------
    Of all the stars in the heavens, only Porrima allowed us to see 
within months or years real stars in real gravitational motion. There 
are a few other stars that show large proper motion, like Barnard's 
star, but in gamma we saw the influence of one star's gravity upon an 
other in a vivid manner.
    We will not ever see this again in our lifetime and, with a 
plausible odd, nor will our grandchildren. Of course, the decades to 
come may bring new devices to the home astronomer that can reveal 
gravitational stellar motion elsewhere in the sky, but this is 
speculation to the max. 
    The situation of gamma is quite different from other binaries . 
Even short period binaries require decades to discern the orbital 
movement. It's the cometary nature of the orbit in Porrima that lets 
the stars whirl so rapidly near pariastron within a couple years. For 
the rest of the orbital period the star changes lazily thru the 
decades. 
Future behavior 
 -------------
    As weird as it sounds, we had no definitive orbit for gamma 
Virginis almost right up thru periastron! In spite of the century and 
half of detailed study since the 1836 round, there were several orbits 
in circulation. A couple, recited in the last years before the 2005 
periastron claimed the closest approach comes by 2008!. 
    Any one adhaering to such wrong orbits -- issued by hefty 
authorities -- may have lost their once and forever chance to observe 
gamma during its fast-forward phase. They sleeped thru 2005 intending 
to go and look in 2007 or so. Perhaps it is my good fortune not to 
personally know of an astronomer fooled by a false prediction. I hear 
anecdotes of such astronomers, but not any I know of for myself. 
    Now with two positively observed periastra (yes, the word got a 
proper plural), a good orbit is in hand. Not a one single solution but 
several that don't differ all that much. In any case, now there's no 
critical phase coming for a LOOoong while off. 
    The orbits here are from Scardia in 2007-2008 and Mason &a in 
2006. The latter is part of an ongoing speckle interferometry project 
on binaries at USNO. The figures are within the range of other data in 
the litterature. 
        --------------------------------------------------
        parameter         | Scardia  | Mason &a | comments 
        ------------------+----------+----------+--------- 
        ascending node    |  35.36   | 213.79   | degrees 
        arg of periastron | 255.03   |  73.78   | degrees 
        lon of periastron | 290.39   | 287.57   | = node + arg 
        inclination       | 140.41   | 148.82   | degrees 
        excentricity      | 0.8815   | 0.8825   | 
        periastron time   | 2005.509 | 2005.438 | 2005 Jun 9-Jul 4 
        semimajor axis    | 3.641    | 3.662    | arcsec 
        orbital period    | 169.101  | 169.93   | years 
        -----------------------------------------------
    Notice how much the longitude of ascending node and argument of 
periastron diverge! Yet their sum, longitude of periastron, is quite 
the same. This case illustrates how tough it is to suss out the 3D 
orientation of a binary's orbit from the one remote viewpoint as a 
plane shape on the celestial sphere. 
    Scardia and Mason also worked out the future aspect of gamma: 
        -------------------------------
               | Scardia      || Mason &a 
               +--------------++--------- 
        year   | sep   | pos  || sep   | pos 
        -------+-------+------++-------+-----
        2010.0 | 1.390 | 23.7 || 1.417 | 22.5  
        2011.0 | 1.592 | 18.5 ||  ---  |  ---
        2012.0 | 1.779 | 14.4 || 1.807 | 13.6 
        2013.0 | 1.952 | 11.2 ||  ---  |  ---
        2014.0 | 2.116 |  8.4 || 2.145 |  7.7
        2015.0 | 2.270 |  6.0 ||  ---  |  ---
        --------------------------------------
    The position angle is the direction from the primary toward the 
secondary. With the two stars almost equally bright, it can be hard to 
tell which is actually the primary. The position angle may be given as 
an strike, like '11.2-191.2' for 2013. This is merely the directions, 
180 degrees apart, as seen from each star in turn. Note the retrograde 
sense, toward lower angles. 
Wild speculations 
 ---------------
    As word of the impending closure of Porrima flashed thruout the 
astronomy world in the 1830s, a panic set in. What will happen to the 
stars?! Mind well that we didn't understand what stars were and 
certainly had no  concept of a collision between two of them. 
    Er figured the stars could be like the Sun, only viewed from a 
humongous remoteness. Big help that was being that we didn't 
understand what the Sun was. A common notion was that the Sun is s 
solid globe like Earth with a luminous cloud deck. Sunspots are holes 
in the clouds to allow peeks of the dark land under them. So, what 
happens if two Suns collided? 
    Will the stars fuse together in one and never again be seen as a 
double star? Will they collide and explode like bombs before our eyes? 
Will they interfuse and fluff out into a nebula? Will they snuff out 
like capped candles, never to shine again? Will they trigger a 
calamity on Earth to end humankind's existence? Will they bounce apart 
like billiard balls and distort the Virgo constellation catchfigure? 
    No one knew. With the small weak scopes of the 1830s the stars did 
blend into one dot for a couple years. What's going on with them? A 
united relief erupted when by 1838 the two stars emerged from 
conjunction! I don't think there was actually a wild cheering or giddy 
celebration, but surely we were glad that nothing disastrous occurred. 
    The blended pair for the year around 1836 did block positive 
assessments of the orbit. That's why the time of the 1836 periastron 
is still a bit uncertain, altho all estimates fall within the merger 
period. This imprecise timing caused the parade of various orbit 
models for the next 160 and more years, until the periastron of 2005. 
Occultations 
 ---------- 
    Porrima is a favorite star for occultations by the Moon. It sits 2 
degrees north of the ecliptic about 60% from Denebola to Spica. Each 
component suffers its own occultation, allowing for tight bounds on 
their orientation and separation. 
    We in 2011 are, as luck falls, between seasons ed Porrima 
occultations. Most of the events of the previous and next season miss 
New York City. 
    We missed an occultation on 2004 April 4 when the Moon slided next 
to the star along her northern limb. A little south of New York there 
was a graze occultation. The two components each skimmed thru valleys 
and mountains on the north edge of th Moon. 
    -------------------
    UT date     | event 
    ------------+------
    1997 Aug  8 | first occultation of old season   
    1998 Jul  1 | first occultation over NYC 
    2003 Aug  3 | previous occultation over NYC 
    2004 Apr  4 | close miss in NYC, graze south of NYC 
    2005 Jan  3 | last occultation of old season 
       ---      | 11 years between seasons 
    2016 Feb 25 | first occultation of new season 
    2017 Jan 18 | next occultation over NYC 
    2022 Mar 19 | last occultation over NYC 
    2023 Jun 26 | last occultation of new season 
    --------------------------------------
    In 2005 the Moon's descending node migrated too far form Porrima 
and the Moon began to miss the star. The ascending node creeps toward 
Porrima by 2016 to start the next season of occultations. There were 
no occultation right around the periastron to help fill in data at 
that critical point. 
    The circumstances of the 2017 occultation for the City are: 
 ----------------------------------------------------------------
 Date        EST      P %Ph Eln Alt Az Cusp Pos Ver LibL LibB 
 ----------- -------- - --- --- ------ ---- --- --- ---- ----
 2017 Jan 18 00:11:04 D 66- 108 15 105 -74N  97 144 +6.1 -3.6 
 2017 Jan 18 01:18:02 R 65- 108 27 118 +72N 310 352 +5.9 -3.6 
 -------------------------------------------------------------
    P is the phaenoenon with D for disappearance, immersion, ingress 
and R for reappearance, emersion, egress. The calculation is for ONE 
component with supplemental information about its duplicity. 
    %Ph is the lighted percent of the lunar disc. Eln is the downrange 
distance of the Moon from the Sun along the ecliptic. 
    Alt & Az are the altitude and azimuth of the Moon at each event. 
    Cusp is the angle around the lunar limb from the north or south 
horn to the star on the dark side (-) or light side (+). 
    Pos is the position angle CCW around the limb to the star from 
celestial north. 
    Ver is the vertex angle CCW around the limb to the star from the 
top of the Moon. 
    LibL & LibB are the lunar libration in longitude and latitude or 
the selenographic lon-lat of the center of the lunar disc. 
Other components 
 --------------
    Besides the two bright components, A and B, Porrima has at least 
four other possible members. They were measured only sporadicly, 
leaving their attachment to the main pair still in doubt. The values 
here are form different years but the scanty reports seem to indicate 
they didn't change much thruout the 20th century. 
    --------------------
    # | magn | sep | pos 
    --+------+-----+----
    C | 15.1 |  53 | 159 
    D | 12.2 | 124 |  88 
    E | 13?  | 252 | 180 
    F | 13?  | 482 | 269 
    --------------------
    I could find no stated brightness for the E and F star. 
Photographs of the field show they are in the 13th magnitude range.  
Porrima chasing? 
 --------------
    The periastron of Porrima is an event litterally as rare as a 
transit of Venus. You can easily live your life between events without 
personally witnessing one. There were in 2004 many trips and special 
viewings to watch the Venus transit. Were there any to chase after 
gamma Vitginis? 
    Unlike the transit, or a solar eclipse, there was no need to be at 
a specific place at a specific time to see the periastron. It was 
visible thruout the world and at any time within its apparition. 
    On the other hand, Porrima could have been an extra treat during a 
solar eclipse trip because such trips routinely offer stargazing. As 
far as I found in ads in the 2000-2005 timeframe, there was no mention 
of gamma Virginis as an accessory feature of eclipse trips. 
    It was also possible to organize a viewing of Porrima from a site 
blessed with large telescopes and still quiet air. There were trips 
for photography and imaging, without a particular astronomy event, in 
such locations. The regularly scheduled starparties during this period 
could have highlighted the chance to inspect Porrima. Again, I found 
no word about gamma Virginis to attract astronomers to these trips. 
    A wisely traveler on an astronomy trip in the mid 2-thous could 
have alerted others with him to the spectacle of Porrima. It would be 
a surprise feature of the trip. While I suppose there were such good 
folk who called attention to gamma, I have no personal accounts of 
any. And I heard really few anecdotes of any.
    An example of a surprise treat was a partial solar eclipse during 
a halley's comet trip I was with in Australia. In preparing for the 
trip I had a timetable for this eclipse to watch at one of the camp 
grounds along the itinerary. 
    On the afternoon of the eclipse I set up my scope with solar 
filter and let the other trippers view the Sun. Apart form not knowing 
about the eclipse as part of the comet chase, many travelers remarked 
that this was the very first solar eclipse they ever personally saw. 
    All in all, those following Porrima did so from what ever location 
and with whatever equipment they had to hand, with possibly only 
weak effort to move to a better site or gear. 
Why Porrima?
 ----------
    There are many short-period binaries whose motions can be 
appreciated within an astronomer's career. What makes Porrima so 
special? The two reasons are the strongly excentric orbit and the 
closeness of the star to us. 
    Porrima is only 39 lightyears away. Its stars are angularly set 
apart to see. At a separation of 5 arcseconds, suppose Porrima is 
removed to ten times its distance, to 390 lightyears. The star would 
have separation of only 1/2 arcsecond, out of reach of all but the 
strongest home telescopes. Since this is near the widest stance of the 
members, Porrima would then drop from a target list of double stars 
for small scopes. 
    An other scenario is that Porrima is 390 lightyears away and still 
stands 5 arcseconds apart. The linear distance is then ten times 
greater and the orbital period is about 32 times longer. Porrima would 
then take some 5,300 years to complete one round. It resembles most 
other binaries with little detectable motion in several lifespans. 
    The elongated orbit, the other reason, makes the companion behave 
like a comet, lingering near apoastron for many decades and whipping 
around periastron within one decade. 
    A more circular orbit would still make Porrima a fascinating 
target with a gentle circulation noticeable within a lifespan. It's 
the intense activity near periastron, like that of a comet near the 
Sun, that sets gamma Viriginis apart from other binaries. 
Other rapid binaries
 ------------------
    During and specially since the 2005 periastron readers asked if 
there are other rapidly changing binaries. There are none so amazing 
as gamma Virginis but there are several normal binaries to excite you. 
The gotcha is that you better be in astronomy for the duration, like a 
couple decades. 
    Dr Martin Gaskell, University of Nebraska, in 1995 answered this 
question with some stars to follow. 
    I include those of his that are also in my own list of targets for 
New York City. We differ in selection because of the changing aspect 
of the stars. 'year' is the epoch of the separation and position. 
    All stars are nearby, within 100 lightyears, and have orbits less 
excentric than Porrima's. The orbit of mu Cygni is almost edgeon to 
our line-of-sight. It has two proximities per lap, when the companion 
is at inferior and then superior conjunction as seen from Earth. 
    -------------------------------------------------------------
    Cns Star RtAsc    Decl   Mag Mag colors   sep  pos year  per 
             hr mn    deg mn  A   B           sec  deg       years 
    --- ---- -------  ------ --- ---- ------- ----  --- ---- ------
    CMa alp  06 45.1  -16 43 1.4- 8.5  whi whi   4.6 150 2000   50.1 
    Gem alp  07 34.6  +31 53 1.9  3.0  whi whi   4.6  57 2009  420. 
    UMa xi   11 18.2  +33 06 4.3  4.8  yel yel   1.8 273 2000   59.8 
    Sco xi   16 04.4  -11 22 4.9  7.3  yel       7.5  46 2005   45.7 
    Oph  70  18 05.5  +02 30 4.1  6.0  ora whi   5.3 136 2006   88.1 
    Cyg mu   21 44.1  +28 45 4.8  6.9  y-w yel   1.8 311 2005  507.5 
    Aqr zet  22 28.8  -00 01 4.3  4.5  y-w whi   2.2 174 2009  856. 
    ----------------------------------------------------------------
    All of these stars demand good clean collimated optics, good acute 
clear eyesight, good steady quiet air. Alpha Canis Majoris, Sirius, is 
a challenge star because the comes is usually veiled by the glare of 
the primary. 
    Please understand that a short period implies a smaller linear 
distance between the components. Even at their closeness to Earth the 
stars can be angular tight in a small scope. Thee stars do need a 
larger scope, generally 200mm aperture or more, to see a significant 
part of their orbital motion. 
    I remind that binaries are evolving targets. Due to their short 
periods, the aspect of these stars changes quickly over the years. You 
better seek current news about the stars before observing them. In 
some cases, like for Porrima, the components may close in beyond 
resolution of your equipment. 
Cardboard micrometer
 ------------------
    Dr Gaskell promotes a micrometer for double star work made from a 
disc of cardboard. I did say 'cardboard'. He reminded me about it when 
I assembled material for my first Porrima article 'A star on the 
move'. I made a unit during the rampup of excitement for Porrima in 
2000. I explain it in 'The cardboard diffraction micrometer' in the 
NYSkies website at 'www.nyskies.org/articles/pazmino/diffmic.htm'. 
    I tried it on gamma Virginis in 2001 and 2002. In the latter year 
it was tough to visually split the star. Believe it or not, this 
effing thing really worked! It does need the highest power the scope, 
eye, air can stand and the double must actually be resolved opticly. 
But with this cardboard contraption competent and valid measurements 
of separation and position angle are feasible. 
    I didn't try it since then, thru 2010-2011 when the stars are 
getting well separated. In fact, I misplaced the gadget and have to 
hunt for it. Or just make a new one. 
Observing tips 
 ------------
    Naively the stars at periastron should be resolvable in a scope of 
480mm aperture, based on (resolution in arcsec) = (120 arcsex.mm) / 
(aperture in mm) rule. This is an ideal value with the stars as 
perfect optical images. The atmosphere never allows such perfect 
images. The stars may blur to a half to a full arcsecond diameter, 
thoroly smearing them together. 
    The practical limit on resolution is generally taken as one 
arcsecond in normal air. In addition, this limit requires good quality 
optics, well maintained and collimated. It helps to have acute and 
clear vision. 
    You must have a quiet and stable air above you. Viewing gamma 
under a turbulent air will likely end in failure while the stars are 
still close together. Beyond 2012 Porrima becomes an easy binary star 
in the typical small scope under typical air stability. 
    To better appreciate the motion of the stars, keep to a particular 
set of scope and eyepiece,.else the angular extent in a this and that 
instrument can not be correlated properly. 
    The angular rotation is harder to appreciate because of field and 
optical rotation. Unless you fix the field orientation, you won't know 
which way the stars are turning. This situation will for sure prevail 
when inspecting Porrima in telescopes other than your own. 
 
Conclusion 
 --------
    If you are new to astronomy, and missed the Porrima show, you 
still have time to witness the star in action. It is a slower one, 
yes, but still appreciable thru small scopes. Year by year the pair 
separates gently farther apart. It becomes easier to make out the two 
individual stars, using the same telescope and eyepiece. 
    Perhaps with your rig the star still looks single in 2011. 
Eventually with that gear you'll be surprised to look and see two 
stars where in the previous apparition you could make out only one. 
    For those who watched Porrima close in, coalesce, than break 
apart, you are a very privileged person. You're one of the fellowship 
of astronomers who can recount the tales of stars on the move. You are 
one with the original witnesses who walked the planet when the United 
States had only half of its states,