Welcome to the NavList Message Boards.

NavList:

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

Compose Your Message

Message:αβγ
Add Images & Files
    Name or NavList Code:
    Email:
       
    Reply
    Re: Mars sight reduction
    From: Frank Reed
    Date: 2014 Nov 19, 09:11 -0800

    The only additional correction for Mars is parallax. Short answer for observations this month: add 0.1' to the observed altitude.

    Long answer: for Mars, the horizon parallax can be estimated with very good accuracy by comparing the planet's brightness to other stars since the brightness of Mars is related to the distance from the Earth (unlike Venus, phase factors for Mars are small). If Mars is about first magnitude or fainter (as it is now), the parallax correction is 0.1 minutes of arc. If Mars is about as bright as Vega or Arcturus (as it was a couple of months ago), then the parallax is 0.2'. If Mars is about as bright as Sirius, then the correction is 0.3'. If it's as bright as Jupiter, then it's 0.4'. And on those rare occasions when Mars rivals Venus in brightness, its parallax correction can be 0.5'. The advantage of these rules based on brightness is that they are permanent, and you can determine the correction by direct observation without reference to any ephemeris data. If you don't like estimating the parallax from the brightness, the values are also tabulated in current editions of the Nautical Almanac. If you don't have a current Nautical Almanac, you can also get the distance to Mars in astronomical units or "AUs" from many different sources and divide 0.15' by that number. For example, current distance to Mars is 1.77 AU so the parallax correction is 0.08' or, nearly enough, a tenth of a minute of arc.

    For current observations of Mars, based on any of the methods above, the horizon parallax of the planet is a mere 0.1'. The actual parallax in altitude is determined by multiplying the horizon value by the cosine of the altitude. It's important to remember that the navigational parallax for any body is opposite the refraction. Refraction lifts the stars and planets and other bodies, so it has to be subtracted from any altitude. Parallax lowers the Moon's observed altitude by as much as a degree, the Sun's altitude by as much 0.15', and the planets Venus and Mars by varying amounts from 0.1' to 0.6', and the correction for it has to be added onto the observed altitudes. For current evening observations, the altitude of Mars is low enough so that the cosine factor is effectively 1.0. The parallax correction is +0.1'.

    -FER

     

       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    NavList is a community devoted to the preservation and practice of celestial navigation and other methods of traditional position-finding. We're a group of navigators, navigation enthusiasts and hobbyists, mathematicians and physicists, and historians interested in all aspects of navigation but primarily those techniques which are non-electronic.

    To post a message, if you are already signed up as a NavList member, start a new discussion or reply to any posted message and use your posting code (this is a simple low-security password assigned when you join). You may also join by posting. Your first on-topic messsage automatically makes you a member, and a posting code will be assigned and emailed to you for future posts.

    Uniquely, the NavList message boards also permit full interaction entirely by email. You can optionally receive individual posts or daily digests by email, and any member can post messages by email (bypassing the web site) by sending to our posting address which is "NavList@NavList.net". This functionality is similar to a traditional Internet mailing list: post by email, read by email, reply by email. Most members will prefer the web interface here for posting and replying to messages.

    NavList is more than an online community... more about that another day.

    © Copyright notice: please note that the rights to all messages and posts in this discussion group are held by their respective authors. No messages or text or images extracted from messages may be reproduced without the explicit consent of the message author. Email me, Frank Reed, if you have any questions.

    Join / Get NavList ID Code

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

    A NavList ID Code guarantees your identity in NavList posts and allows faster posting of messages.

    Retrieve a NavList ID Code

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

    Email Settings

    NavList ID Code:

    Custom Index

    Subject:
    Author:
    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