A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2021 Oct 21, 20:21 -0700
The phase of Venus this week, as seen through a backyard telescope is approximately half full. You won't see that through a sextant scope (or in any wide-angle photo) since the angular diameter of Venus is only about 0.4 minutes of arc. Jupiter right now is twice that size, and that's only barely recognizable as a disk in a sextant with a high-power scope. As Venus approaches inferior conjunction, passing within a few degrees of the Sun in the sky, its angular diameter will increase but the crescent will become thinner. This is part of the reason that we reach a point of maximum brightness in early December.
On the last day of December Venus will be a thin crescent just 13° from the Sun. By then Venus will have an angular diameter slightly greater than 1.0 minute of arc. Though on that date, Venus is really too close to the Sun to use for navigation, it would be possible two weeks earlier when the elongation is near 30°. There's a debatable question here for sextant sights. If your magnification is too low to resolve the disk of the planet, then should you use the "center of light" for Venus? This is basically the limb of that thin crescent which is about 0.5 minutes from the center of the disk. But then again, if your magnification is so low, does half a minute of arc make a difference in the first place? And if your scope's magnification is higher, you should be able to make out the little crescent visually, and then you can easily shoot the center of the disk, as is normal.