NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: [Fwd: lunars hard to shoot?]
From: Michael Wescott
Date: 2000 Sep 11, 9:24 AM
From: Michael Wescott
Date: 2000 Sep 11, 9:24 AM
"R. Winchurch" wrote: > Excuse my ignorance but would someone give me a primer on lunars. > I understand that prior to the effective use of chronometers lunars were the > only method for determining latitude. As I understand it one measures the > angle between the moon and selected planets (Jupiter and ?) and certain > stars. How does this translate into latitude? You did mean longitude there, didn't you? Longitude and standard time are complementary aspects of the same problem. If you can determine one you can get the other. One way to do this is with a clock. Until the invention of the chronometer, this was not practical for ship-board use. Another option is to use astronomical events that can be predicted with some degree of accuracy. Use the event to set a clock, and use the clock to take some sights. The astronomical events used were the eclipses of the moons of Jupiter. These could be predicted with considerable accuracy. Unfortunately, this requires a high power telescope and therefore a stable platform to observe the eclipses. Not practical for shipboard use, but it was used on land. Since the moon moves across the background of the stars at about 13 degrees a day or 30' per hour, determining its position can yield a fairly accurate estimation of the current time. It is possible to determine the Moon's position by measuring the angle between it and other bright stars or planets. From this you can get time and therefore longitude. That's the theory in a nutshell, anyway. In practice, this is practical on ships and even small boats. Joshua Slocum (the first around alone sailor) seems to have preferred Lunars to keeping his Chronometer on time. The drawbacks are in the difficulties involved with taking three "simultaneous" sights (two altitudes and the angle between them) and in the calculations involved to reduce the sights. Had the Lunar method been perfected earlier, it may well have delayed invention of the Chronometer for some time. As it was, understanding of the motion of the Moon sufficient for accurate tables was achieved at roughly the same time as the invention of the Chronometer so the two methods were competitors for the famous prize. -- -Mike Wescott wescott@conterra.com