Welcome to the NavList Message Boards.


A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Add Images & Files
    Re: Using any star for a lunar
    From: George Huxtable
    Date: 2005 Mar 12, 17:50 +0000

    Jim Thompson asked-
    >I am undoubtedly revealing my failure to spend enough time learning lunars
    >(my limp defense is that it is still bloody cold around here at night), but
    >can any navigational star or planet be used to work a lunar distance, as
    >long as the altitudes of the body and moon are within the window for a given
    >method?  I know the old Almanacs had lunar distance data for only a few
    >selected stars, and I think that Frank's online calculator has solutions for
    >some selected stars, but can I just go outside, shoot a set of sights
    >between the moon and a convenient body, and then come back indoors to reduce
    >the sight?  Or do I have to select from a short list of nagivational bodies?
    >I have a run of sights on a star from last fall, taken with chilled but not
    >frozen fingers, but I recall that when I tried to do a preliminary reduction
    >using Frank's online calculator, that star was not available.
    >What am I missing?
    If it's at a useful separation from the Moon, any identifiable planet can
    be used to work a lunar distance, because all planets are close to the
    plane of the ecliptic, as is the Moon, so that the planet always lies close
    to the path of the Moon. Two effects need to be considered in obtaining the
    apparent position of the planet in the sky, however.
    First, parallax can be appreciable, especially for Venus and Mars when they
    get close to the Earth. The planet correction table in the almanac allows
    for this.
    Second, there's a small "phase correction", because the surface of a planet
    is only partly-lit by the Sun. It appears to the eye to be at its "centre
    of brightness", which is slightly displaced from the geometrical centre.
    The almanac allows for this; a computer prediction of the planet may or may
    not have done so.
    As for stars, any identifiable star at a useful separation that's somewhere
    near the path of the Moon (i.e. near to the ecliptic) will do fine. You can
    find the ecliptic marked on Norton's Star Atlas, for example. Otherwise, if
    it's well out of that plane, then the Moon won't be travelling toward it
    (or away from it) but at an oblique angle, in which case the effective
    speed of the Moon will be significantly reduced.
    Historically, to minimise the labour of computation and the space in the
    almanac, only a minimum number of stars were selected, chosen for their
    brightness and identifiability and for roughly equal spacing around the
    ecliptic. And really, there's no great call for a wider selection of stars
    than those chosen in Maskelyne's day, though it's true that patchy cloud
    may hide some for a time, some nights.
    Nowadays, computation being so easy, any star near the ecliptic can be
    chosen, but make sure that it has been correctly identified and named!
    Having found its coordinates at certain moments near the time of a lunar
    distance observation (and the Moon's) then calculating the predicted lunar
    distances between them is no big deal.
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.

    Email Settings

    Posting Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site