Today is going to be the shortest day of the year in terms of hours of daylight in the Northern Hemisphere. With only 10 hours (approximately) I’ve got a lot to do. That means this post needs to be succinct.
So let’s start with a quick question. Do you know where to locate planets in the night sky? I often do not. But tonight, just a bit to the right of the moon when it gets dark, there will be two planets almost lined up together. Jupiter and Saturn, the two largest planets, will be passing each other in orbit for the first time in about 8 decades. They are both bright enough to be seen without the aid of a telescope, although not as bright as Venus which also usually occupies a spot low on the horizon.
When I was navigating at sea I did find occasions when I would use a planet for one of my sextant observations. I would go out on deck with a helper to keep track of the time and record my observations. I’d usually select a set of stars or planets that would be visible at around twilight in advance. That way I knew which direction and elevation I would expect to see the star. Once located, I’d view the reflection of the star in the mirror of the sextant, and move the angle of the mirror until that reflection was just touching the line of the horizon. Then I’d rock the sextant slightly from side to side to insure the measurement was being made perpendicular to the horizon. My helper would record the time and angle measured. Then we’d proceed to find another star or planet. I always tried to use 3 or 4 stars that were seen in different directions. Normally, I’d start looking for one that was away from the direction where the sun had set. The horizon is not as clear in that direction. After making all the observations we’d go inside and compute the direction and distance from where the star would be positioned on the surface of the earth. Each star made a short line on the chart, and ideally, the lines would all intersect at the place from which I had made the observations. My plots rarely made a perfect intersection, but they did make a little triangle that was good enough to use as the position of the ship out in the middle of a vast trackless ocean with no traffic signs.
Wetherhorn 