Rich N wrote:
>Late, after midnight I tried splitting Porrima. It was easy to see
>that it was a
>double star but no black line between the Airy discs. It was just
>at the point
>of starting to show a figure eight. I was using well over 400x. One of the
>observers though maybe it would have shown the split if the seeing had been
>dead steady. I was happy the way it looked given it is very close
>for my 6.1"
>scope. Two years ago it was easy to see a black line between the two stars.
I checked the Porrima orbit today and found that the A and B
components are currently only separated by 0.51 arcseconds. That is
well under the Dawes limit for a 6.1 inch scope. Getting Porrima to
a figure 8 at this separation already sounds like you had a very
good night.
Rich also wrote:
>Two years ago a couple of us made two or three trips to the site a
>little below
>Montebello
>to test a new telescope, using Porrima as a target. Porrima was a
>little more open
>giving me a nice clean split in my 155mm.
For others who have looked at this star before and may be interested
in how it is changing, here is the Binary Star Orbit Ephemeris for
the A-B components of gamma Virgo (calculated from SkyTools binary
star orbit tool)
Year/Month Angle Separation distance
1998 Apr 25 267.0 1.73
1999 Apr 25 262.8 1.57
2000 Apr 24 257.6 1.39
2001 Apr 25 250.7 1.20
2002 Apr 25 241.1 1.00
2003 Apr 25 226.1 0.77
2004 Apr 24 196.8 0.51
2005 Apr 25 126.7 0.37
2006 Apr 25 67.3 0.58
2007 Apr 25 44.3 0.86
2008 Apr 24 32.4 1.12
2009 Apr 25 24.8 1.35
Note that Porrima was easily splittable through 2002, but has been
getting tougher in 2003 and 2004. Next year is the year of closest
approach, and will be particularly difficult. As the components
reach their minimum distance, the position angle of the system will
also be changing orientation at over 5 degrees per month. (Systems
in elliptical orbits sweep out an equal area of the orbit each year.
When the components are at minimum separation, they charge past each
very quickly, making up in orbital path length what they lack in
physical separation).
At Porrima's current rate of orbital change, it should be possible
to see a change in position angle of the AB components within the
2004 observing season. I estimated a position angle of 195 degrees
last night, and was happy to see how close that was to the actual
orbit when I checked today in SkyTools. (Position Angle Estimators
Unite! Jamie Dillon recently mentioned on TAC the small observing
rush that comes from making and galaxy position angle estimates in
the eyepiece that can later be confirmed in catalogs or images of the
field. It also increases my confidence that I didn't just
hallucinate the Porrima observation last night with high
magnification in the 7 inch scope). By this summer, the two
boulders of the gamma Virgo snowman may be standing nearly straight
up (position angle 180 degrees) as Porrima drifts west through the
eyepiece.
David Kingsley
Received on Mon Apr 26 18:03:29 2004