Talk:Longitude by chronometer

Comments

I have changed "Long by Chrom" to "Long by Chron" because that is what we called it when I was a Midshipman. The Real Walrus 23:14, 19 February 2006 (UTC)

Longitude by chronometer

Longitude by chronometer is a method of calculating a longitude at an assumed latitude though which a position line can be drawn . The observer is somewhere along the position line. To carry out this calculation an accurate time is needed so the sun's (or other bodiies') hour angle can be obtained. The hour angle feeds into the calculation. A latitude and longitude cannot be determined by a single 'sight' with a sextant. Simon JD (talk) 14:10, 30 July 2016 (UTC)

Corrections: Unfortunately, the sun does not make a perfect apparent orbit around the earth. Due to the elliptical nature of the earth’s true orbit around the sun, the speed of the sun’s apparent orbit around the earth varies throughout the year and that causes it to appear to speed up and slow down very slightly. Consequently, noon at the Prime Meridian is rarely if ever exactly at 1200 UTC, but rather it occurs some minutes and seconds before or after that time each day. This slight daily variation has been calculated and is listed for each day of the year in the Nautical Almanac under the interesting title of “Equation of Time”. This variation must to be added or subtracted to the UTC of local apparent noon to improve the accuracy of the calculation. Even with that, other factors, including the difficulty of determining the exact moment of local apparent noon due to the flattening of the sun’s arc across the sky at its highest point, diminish the accuracy of determining longitude by chronometer as a method of celestial navigation. Accuracies of less than 10 nautical miles in position are difficult to achieve using the longitude by chronometer method. --Navigator999 (talk) 04:57, 5 June 2009 (UTC)

<ref> Sailing Alone Around the World by Joshua Slocum

References

A new longitude methods page?

This article needs to be completely re-written. It is confusing and does not clearly explain long by chron. For example the article discusses longitude by equal altitudes which has nothing to do with long by chron. I suggest that there needs to be a Longitude Methods page. Specific methods could still have their own article. For example a time sight worked by two latitudes is a Chord Sumner. This probably does not belong in a Longitude Methods page but rather in a Long by Chron page. On the other hand two sights worked by the intercept method determine longitude so belong on a longitudes methods page.

I am tempted to begin this process by adding the following paragraph at the very start of the long by chron article but will think about it first.

"Lon by Chron (British) aka Time Sight (US) involves measuring the altitude of the sun with a sextant and noting time on a chonometer. From the observed alrtitude, the declinaton of the sun (from an almanac) and the DR latitude, local apparant time can be calculated and hence longitude. The closer the sun is to the prime vertical the less the error caused by an incorrect DR latitude." — Preceding unsigned comment added by Llama234 (talk • contribs) 21:37, 17 August 2020 (UTC)