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    Re: venus
    From: Frank Reed CT
    Date: 2004 Oct 9, 17:56 EDT
    Michael D wrote:
    "As I was sitting in the hot tub this AM watching the spectacular heavens with the moon and venus about 15 degrees apart, it occurred to me to wonder about lunar distance measurements using venus."

    A pretty sight, wasn't it? In historical practice, short-distance lunars like this were rarely used. You have to deal with the nuisance of "second-difference" interpolation when the distances get short, and also (when observed and not calculated) the altitudes have to be much more accurate than is commonly the case with lunars. In the almanacs, predicted lunar distances were rarely given when the distance was shorter than 15 to 20 degrees, and when they were it had more to do with consistent page formatting than navigational reality.

    Ignoring the short distance, Venus should be avoided for lunar distance observations because it has a significant phase. Right now, Venus has a phase of about 50%, appearing like a tiny half-moon in a telescope. Through a sextant telescope, this is not quite visible and it throws off the apparent location of the planet's center (that said, Venus is far enough away right now that this isn't a huge problem). With planets, it is customary to place the center of the planet's disk right on the limb of the Moon. For Mars, Jupiter, and Saturn, that's not difficult. For Venus, it's tricky.

    And:
    "I've never tried to measure any lunar distances but I went ahead anyway."

    Maybe try a Sun-Moon lunar next. Assuming the weather holds, there are a few more days before the Moon gets too close to the Sun. If you visit HistoricalAtlas.com/lunars and click on "Predicted Lunars" you can get detailed almanac predictions specifically for your area of the world.

    And:
    "My results were consistently about 5 to 7 minutes lower than predicted by a program I wrote that uses the Mosier JPL AA routines.  I wonder if this is some fault in my measurements, calculations, or the AA routines.
    I'd really appreciate it if someone could independently work out the moon
    venus distance for
    89.50 west 42.85 north
    altitude 290 meters
    temperature 5 degrees C
    tick.usno.navy.mil time 6-13-06 CDT 11-13-06 Greenwich"

    You can do this on my web site HistoricalAtlas.com/lunars by clicking on "Clear a Lunar Online". Even if you haven't measured a lunar, you can get the correct observed distance by trial and error. The correct geocentric distance for that GMT (that your almanac routines should have produced) is 17d 00.7'. The apparent distance for a near limb lunar at that time is 16d 05.5'.

    How do these numbers compare with yours?

    Frank R
    [ ] Mystic, Connecticut
    [X] Chicago, Illinois
       
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