Introduction
I just finished a very interesting book about Capt. Cook’s third and final voyage – “The Wide Wide Sea” by Hampton Sides (Ref.1). So I thought it would only be appropriate on Victoria Day to take a lunar measurement as Capt. Cook might have taken. Apparently he also took a K1 chronometer made by Larcum Kendall. This was a less expensive version of the famous Harrison H4. Spoiler Alert: Victoria Day was overcast, so my measurements were taken the day after.
Lunar Distance Method
The principle behind the Lunar Distance method is to find an accurate time when your watch is not calibrated. Early navigators used this method when reliable time pieces were not available. Tables were included in the Nautical Almanac giving GMT time vs. Lunar Distance. I find doing a Lunar brings together all the principles of Celestial Navigation and is an excellent exercise to do on a periodic basis to keep the ideas fresh in your mind (Ref.2/4).
Lunar Measurements
| Time EDT (UTC-4) | Arc Distance to Left Limb of Moon | Apparent Lunar Distance |
| 21:45:00 (01:45:00) | 38deg 32′ | 38deg 49.2′ |
| 21:48:00 (01:48:00) | 38deg 40′ | 38deg 57.2′ |
| 21:46:30 (01:46:30) Average | 38deg 36′ Average | 38deg 53.2′ Average |
| Index Error 2′ off Moon SDapp = 15.2′ Add 17.2′ Arc Distance | Lunar Distance Geo = 38deg 0min 10sec Calculated (NavAlgos) | Lunar Distance App = 38deg 53.2′ Measured Lunar Distance App = 38deg 48.2′ Calculated Error = +5′ |
Figure 1 shows a diagram of the lunar measurement. The angle is measured between the centre of the CB Celestial body (in this case Arcturus) to the closest limb of the Moon. Figure 2 shows the night horizon with Stellarium (Ref.5) and Figure 3 with NavAlgos (Ref.6). Figure 4 is a table of the two arc distances measured with the average. The apparent Moon SD semi-diameter and Index error are added to give the Lunar Distance Apparent as seen from the Observer’s location on the Earth surface. Since it was a near full moon, measurements were made to the left hand limb which is clearly defined. I found the best way to take the measurement was to aim the scope at Arcturus and bring down to a tangent position on the left limb of the Moon. Since the Moon was so bright, I had to use a sun shade.
I followed the same procedure as in Ref.3 since I don’t have a water horizon at my home location (use Hc instead of Ho). My apparent Lunar Distance was off by 5′ (I am getting rusty!). Figures 5 & 6 show the geocentric lunar distance for Arcturus at the exact time and for the whole day (Ref.6). Figure 7 is a spreadsheet with all the CelNav calculations as per Ref.7.
Please send your comments, questions and suggestions to:
contact:
References
#1. – “The Wide Wide Sea”, Hampton Sides
https://www.amazon.ca/dp/0385544766/
#2. – “CelNav Sept Full Moon Lunar Distance”
https://jeremyclark.ca/wp/nav/celnav-sept-full-moon-lunar-distance/
#3. – “Lunar Distance DST to EST”
https://jeremyclark.ca/wp/nav/lunar-distance-dst-to-est/
#4. – “Lunar Distance as Seen in Toronto”,
https://jeremyclark.ca/wp/nav/lunar-distance-as-seen-in-toronto/
#5. – “Stellarium Astronomy Software”
http://stellarium.org/
#6. – “Navigational Algorithms”, Andres Ruiz Gonzalez,
https://sites.google.com/site/navigationalalgorithms/
#7. – “Celestial Navigation Basics & Equipment”
https://www.clarktelecommunications.com/celnav.htm
