 # NavList:

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

Message:αβγ
Message:abc
 Add Images & Files Posting Code: Name: Email:
Re: formula for refraction
From: Frank Reed CT
Date: 2007 Mar 21, 02:20 EDT
"Meeus's formula is certainly approximate (and he says so)
but it does not match the almanac table with the precision
Meeus claims.
Chauvenet has complete theory but no useful simple formula.
My question is practical: I want to incorporate automatic refraction
computation in my spreadsheet for star distances."

The refraction tables in the almanacs are derived from numerical integrations which are confirmed by detailed observations at observatories. The resulting table can be matched to numerical series of various flavors. Depending on the spreadsheet you're using, you could just input the whole table giving the refraction for each (exact) degree of altitude, and interpolate (use a "VLOOKUP" function --see detailed formula in PS below).

The formulas given in various sources on the pattern of
Ro = 0d.0167/tan(H + 7.32/(H + 4.32))
are simply numerical "fittings". They're not "derived" from underlying theory. In my code, I use two different equations, one for altitudes above 15 degrees, one for lower altitudes. At higher altitudes, R = 57"/tan(H) is excellent. If you adjust the parameters in the other formula (above), you can get better results at lower altitudes. From the perspective of accuracy and technical quality, this is NOT a better solution than inputting the whole table, but it certainly involves less typing. :-)

-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars

PS: Some spreadsheet details... In cells G10 through G100, fill in integral degrees of altitude 0, 1, 2, ...90. Then in H10 through H100, enter the corresponding refraction in minutes of arc. For altitudes above 15 degrees, you could simply enter the standard formula, rather than typing them in. For lower altitudes, take the values from the almanac, and enter them manually. Note that this block of values can be placed "way out in right field" on your spreadsheet so that it doesn't clutter up your calculating area. Next suppose I have an altitude in degrees in cell B2 for which I want an exact value of the refraction. This would typically be a non-integral altitude, so we need a formula to grab bracketing values from the refraction table (in G10:H100) and interpolate between them. And here's the formula:

=VLOOKUP(INT(B2),G10:H100,1)*(1-(B2-INT(B2)))+VLOOKUP(INT(B2)+1,G10:H100,1)*(B2-INT(B2))

Naturally, it's a linear interpolation, which is more than adequate when the altitude difference is 1 degree. Note that this formula would have to be modified if you decide to use a separation different from 1 degree or a variable separation, as in the Nautical Almanac. Enjoy!

AOL now offers free email to everyone. Find out more about what's free from AOL at AOL.com.

--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To unsubscribe, send email to NavList-unsubscribe@fer3.com
-~----------~----~----~----~------~----~------~--~--- Browse Files

Drop Files ### NavList ### What is NavList? ### Join NavList

 Name: (please, no nicknames or handles) Email:
 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: ### Email Settings

 Posting Code: ### Custom Index

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