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: Calculating Horizontal Parallax
    From: Frank Reed
    Date: 2014 Apr 2, 16:14 -0700


    You can also get HP's for planets by simple inverse proportion in distance, knowing that the Sun's HP is 9 seconds of arc. For example, we see Jupiter and Mars high in the night sky tonight. Jupiter is about 90° away from the Sun and crosses the meridian close to sunset. If you draw a little picture, and you know Jupiter's mean distance from the Sun is about 5.2 AUs (5.2 x the Earth's mean distance from the Sun), then you can see that the HP of Jupiter would be less than 2" of arc (from 9/5 --one significant figure is good enough here). This small angle is irrelevant for celestial navigation. Meanwhile Mars transits right around midnight so it's about 180° from the Sun. Given that the mean distance of Mars is about 1.5 AUs, it must be roughly 0.5 AU from the Earth right now yielding a parallaz of 18" or 0.3' of arc (from 9"/0.5) which is minor for navigation by common celestial altitudes but would be significant for something like lunar distances. So if you know the basic sequence of the planets' distances from the Sun in AUs --0.4, 0.7, 1.0, 1.5, 5.2, 9.5 from Mercury through Saturn-- and if you can picture their positions on an "orrery" or a flat "overhead" view of the Solar System, then you can usually estimate the parallaxes of the planets with a very short calculation: HP=9"/d. And of course, once you have that, then the parallax in the altitude is HP*cos(h) as usual.

    Here's a fun puzzle: you look through a small telescope, and you see that the phase of Venus is very nearly half full. Knowing only that the Sun's HP is 9" and the mean distance of Venus from the Sun is 0.7 AU, what is the HP of Venus on that date? :)


    NavList message boards and member settings: www.fer3.com/NavList
    Members may optionally receive posts by email.
    To cancel email delivery, send a message to NoMail[at]fer3.com

    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