A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2015 Aug 29, 12:46 -0700
Mark Coady, you wrote:
"Due to limited availability of sun/moon, a lunar navigator must have had other choice targets."
In theory, yes. In practice, sun-moon lunars were much preferred in the period when lunars were used historically.
"I was eyeballing jupiter last night up there in scorpio and wondering how that might be I have read that the most critical thing in moon and stars/planets is that they track closely to the ecliptic for reasons of accuracy due to divergent angles reducing accuracy."
Jupiter is gone for now, behind the Sun. Venus and Mars are returning to the morning sky, though maybe Mars doesn't count since yet Venus is six magnitudes --over 200 times-- brighter. The only bright planet we can observe right now is Saturn, which is what you're seeing in Scorpius (a little spelling trivia: Scorpio is the zodiac sign, Scorpius is the constellation). You could try a Saturn-Moon lunar tonight. The distance is around 101° so that's a bit tricky, and the Moon will be low. You could also try an Antares-Moon lunar or maybe an Altair-Moon lunar. Antares was a traditional star for lunars, and perhaps surprisingly, so was Altair. Even though it's well off the ecliptic, the angle between the ecliptic and the lunar arc from the Moon to Altair is around 40° tonight which is low enough for reasonable determination of GMT. Or you could wait until the wee hours, and use Hamal or, better yet, Aldebaran which were also both traditional lunars stars. The location of the Moon tonight is a dead zone for bright ecliptic stars. It was an interesting puzzle in the early days of lunars when astronomers thought lunars might be used nearly every day. In practice it mattered very little if navigators had to wait a few days to grab a lunar, and that's important to remember today if you're trying to emulate historical practice: patience counts with lunars.
When observing lunars with the planets, you'll discover that the planets, when they are bright enough to use, have small discernible disks when seen through a 7x or better scope. They are up to a minute of arc across. Instead of worrying about some small, variable semi-diameter value and trying to bring the limb of that tiny disk into contact with the Moon's limb, standard practice with planet lunars is to "split" the disk. Try to place the planet's center right on the limb of the Moon.
There's no particular difficulty clearing planet and star lunars. My online tools, which you have already used, include options for the bright planets and the nine standard lunars stars.
PS: I am running two workshops in October that you might enjoy. CELESTIAL NAVIGATION: 19TH-CENTURY METHODS, October 3-4, 10am-4pm both days, and LUNARS: FINDING LONGITUDE BY OBSERVING THE MOON, October 17-18, also 10am-4pm both days. These classes are a partnership between ReedNavigation (me) and the Treworgy (truer-gee) Planetarium at Mystic Seaport. These are two out of six workshops I'm running there in the next three months. They're the ones most relevant to fans of lunars!