Topic: Three-body celestial fix in broad daylight
posted September 06, 2004 02:08 AM
As David discusses in his book, it is, rarely, possible to use Venus for a line of position during the daytime. For a couple days this past spring, the afternoon sky afforded three celestial bodies visible in the daytime to the naked eye.
We were sailing from Roche Harbor to Port Townsend aboard Skye Reach, a 35' Fountaine
Pajot catamaran, on the afternoon of April 29th. We were just south of San Juan Island on
a course of 134°T at a speed of 5.1 kts. Visibility was exceptional for the Strait of
Juan de Fuca. We had just finished supper and were getting set up for a round of evening
twilight star sights. We noticed in the Nautical Almanac (and had observed the previous
evening) that Venus was very bright, magnitude -4.5, so as we were setting up we were scanning the sky to try to pick out Venus before the sun set. This proved to be no problem at all. The moon was high in the sky, so we simply followed an arc backwards from the setting sun toward the moon until we saw Venus, a tiny but clear dot against the late afternoon sky.
At this point the sun was very close to setting, so I took a very quick round of sights on all three bodies. The sun was much lower in the sky than one would ever want to use for "real" navigation, and the geometry between the Sun and Venus was marginal at best.
The star sights just a short while afterwards were significantly better, the three-body daylight fix in this particular case was more for novelty than utility. All sights were taken with an Astra IIIb sextant and reduced with Starpilot.
Here's the data--
29 April 2004, all times UT +7 hours (Pacific Daylight Time)
Height of Eye 10', Instrument Correction 0.0'
GPS Position 48°25.3'N 123°03.5'W
Course 134°T Speed 5.1 kts
The computed celestial fix was 48°22.3'N 123°00.9'W, 3.5 nm off from the GPS.
Windworks Sailing Center, Seattle
(and former Starpath instructor)