GREATEST ILLUMINATED AREA OF VENUS
---------------------------------
John Pazmino
NYSkies Astronomy Inc
www.nyskies.org
nyskies@nyskies.org
2012 January 23
Introduction
----------
A few of you folk at the Seminar on 20 January 2012 didn't quite
catch the feature of 'greatest illuminated area' for Venus. It's
simple when the lighted part of Venus's disc has the maximum angular
area and is, more or less, reflecting the most sunlight toward us.
This is not strictly true because the Venus atmosphere does funny
things to reflected light at shallow incidence angles. The compromise
is the use of just the geometric factor. At least this is directly
observable, as by measuring off of pictures taken of the planet.
Simulating Venus
--------------
I did a run with a planet ephemeris generator for five days before
thru five days after the nominal date of greatest illuminated area for
the evening half of the 2012 apparition. The column '%ill' is the
fraction of the disc that is lighted and 'diam' is the angular
diameter of the entire disc in arcsecons.
The last column I added for the illuminated area, the area of the
of the disc that is lighted. The figure in this column is not the
actual area in square arcseconds because I skipped the factors of pi
in the area formula. This column is (diam)^2 * (%ill), with the
percent applied as a decimal. It peaks on April 30, the generally
cited date for greatest illuminated area.
----------------------------------
GREATEST ILLUMINATED AREA OF VENUS
----------------------------------
date, 0h UT %Ill diam IllArea
----------- ---- ----- ------
2012 Apr 25 31.8 34.22 372.38
26 31.0 34.75 374.34
27 30.2 35.28 375.89
28 29.4 35.83 377.43
29 28.5 36.40 377.61
30 27.7 36.97 378.60 <-- maximum illuminated area
2012 May 1 26.8 37.55 377.88
2 25.9 38.15 376.95
3 25.0 38.77 375.78
4 24.1 39.39 373.93
5 23.2 40.03 371.76
-------------------------------
Brilliancy versus area
--------------------
This parameter was promulgated in 2005 after some discussion about
the historical figure 'greatest brilliancy'. This began in about 1920
using the reflective behavior of the Venus atmosphere as ell as the
lighted area. This was done by Russell from an erroneous model for the
atmosphere, which was not yet explored with any substantial detail.
It also was not an observable quantity, which is what a physical
ephemeris should calculate. By dropping the factors from atmospheric
behavior a directly and easily observable parameter was left,
'greatest illuminated area'.
I do recall going thru the computation using the method in the
Explanatory SUpplement in the late 20th century and asking, 'so
what?'. I even did a max-min calculation from the equation of
brilliancy to see how it comes to a peak on a certain date.
No, I did not invent 'greatest illuminated area'. It was reported
in Sky & Telescope in about 2003. I, with most other senior observers,
recognized it as a far better and easier parameter to work with.
In the case of Venus the date of greatest illuminated area is
always within two days of the old greatest brilliancy. There is no
need to alter your observing regimen for the new parameter.
conclusion
--------
S&T asked me, and other senior home astronomers, to get the word
out to drop the brilliancy figure and begin using the area one. This
was supposed to start with the next apparition of Venus in 2005
because almanacs for 2003 and 2004 were already in circulation.
Altho brilliancy was formally dropped in 2005 some authors
continued to cite it, but with the date for the area! That is, since
no one computed brilliancy anymore and the area date is close to the
brilliancy date, some astronomers just stated the area date under the
brilliancy name. Even the RASC Observer's Handbook did this for a
couple more years after 2005.
Without being boastful, on several occasions I had to rattle some
cages each year when ever I found a reference to 'brilliancy' in the
stead of 'area'