NavList:

It's absolutely an interesting comparison, but the accuracy is much reduced compared to lunars. The rate of change of a lunar distance relative to the Sun, the planets, and stars is around 13° per day. Short version: the Moon moves by its own apparent diameter in one hour of time (ignoring parallax issues). Meanwhile the Sun moves relative to the planets and stars at a rate of just about 1° per day. At least a dozen times slower so necessarily a dozen times reduced resolution of time. If a navigator on a good day can get UT/GMT to +/-12 seconds by lunar distances, then solar distances would give time only to +/-2.5 minutes of time. The equivalent longitude error is about three miles (minutes of arc of longitude at the equator, of course) for lunars or about three dozen miles by a solar. And that's under the very best of circumstances.

It's true that those 36 miles are better than the historical (18th century) minimum expectations for a practical method of finding longitude. But no matter how much we improve, it hits a system limit very fast. It can't improve. That's as good as it gets, right? And it's only this good if you have an excellent sextant, and if there's no uncertainity in the other body's position.

Unfortunately, the ephemeris data for the planets improved only slowly. It wasn't quite as difficult a problem in celestial mechanics as the Moon's motion, but it still took decades to develop good, predictive models. Consider that the distances of the planets were not included in the British Nautical Almanac --even for lunars-- until 1834. They might have been included to useful practical advantage at least a decade (and maybe two or three decades) earlier, and it seems that their absense was counted as yet more evidence that the Nautical Almanac was in bad need of a revision. Yet even in other nations' nautical almanacs, there were only experiments in planetary lunar distances through the late 1820s.

Have you looked at pure Venus distances? One could measure the angular distance of Venus from a bright star when Venus is near inferior conjunction. It moves significantly faster than the Sun, and the coordinates of the stars were reliable and accurate from the early 18th century. But then we're limited to perhaps 25% of each synodic period for Venus when we can make such observations. In addition Venus has a semi-diameter that is large enough to confuse and reduce accuracy for measured angular distances unless a navigator is quite careful. But they would work --with limited accuracy. These would be "Venereal Distances".

We need to find some fast asteroids! :) I have observed multiple small nearby asteroids with motion visible in seconds at 60x through a telescope. But for sextant observations, we need something bigger and faster. Yeah, that's it. We need more big, fast asteroids buzzing the Earth. Problem solved.

Frank Reed