During the early part of August the planets Saturn, Venus and Mars will be visible in the western quadrant of the sky in the late afternoon and evening. For celestial navigators, these planets provide a perfect opportunity to brush up on the observing and the reduction of sights.
There are four visible planets that the navigator can utilize: Jupiter, Venus, Mars, and Saturn. In the early part of August the latter three will be visible near each other in the western sky in the early evening.
Planet sights are not difficult to take; the sight reduction has some little quirks that are easily mastered and compared to a moon observation, they are a piece of cake. Of course, the first thing the navigator must do is identify the planet. This can be accomplished in two ways using the Nautical Almanac.
On page nine of the Almanac is a planet diagram that depicts the path of the planets in relation to the sun, and the local times that the planets achieve meridian passage. The directions for the diagram are on page eight of the Almanac. Another way to see which planets will be visible is to go to the daily planet pages of the Almanac and in the lower right hand corner both the SHA and the meridian passage time of the planets will be given. Either method will enable the navigator to identify the bright object in the sky that is being observed.
A planet, unlike a star, will not twinkle and unlike the sun or moon has no upper or lower limbs, so the planet is brought to the horizon as is a star; that is a point of light on the horizon. As with shooting a star, it is easier to set the sextant to 0° and turn it upside-down, aim it like a gun at the planet, and bring the horizon up to the planet. Then turn the sextant over and fine-tune the reading.
Let’s do an example of the planet Venus. It is August 6, 2010. The time is 0 hrs 36 min 15 sec. The DR is 40° 25’ N by 69° 40’ W. The Hs is 15° 30.9’ and the height of eye is 10 feet. We want to find the Ho.
Go to daily pages and find GHA Venus at 0 hours.
GHA = 135° 38.7’
Increment and correction, + 9° 3.8’
New GHA = 144° 42.5’
The declination at the time is N 0° 12.7’.
At the bottom of the page is the correction (d) for the declination and (v) for the GHA. The d correction is either added or subtracted depending on whether the declination is increasing or decreasing. This can be found by inspection. The v correction is always added, though sometimes for Venus it is subtracted. This will be clearly indicated.
At the bottom of the page the v correction is 0.2’ and the d correction is 1.2’.
On the 36 minute increment and correction page we find that the v correction is 0.1’ and the d correction is 0.7’ thus:
GHA = 144° 42.5’
+v of 0.1’
GHA = 144° 42.6’
The d correction is subtracted from the declination because we can see by inspection that the declination is decreasing.
Declination = N 0° 12.7’
-d of 0.7’
New declination = N 0° 12.0’
Now we can reduce the Hs to Ho.
Hs = 15° 30.9’
-dip of 3.1’
App = 15° 27.8’
3rd – 3.6’
Hs = 15° 24.2’
Now ordinarily this would be the Hs, but Venus has another trick up its sleeve and there is an additional correction that can be found on the same page as the altitude and corrections table at the front of the Nautical Almanac. For the time of year and the heght of the sight the additional correction is + 0.2’ — not critical, but we might as well do it all, so we add that to the Hs and get the final number.
15° 24.2’
+ 0.2’ (additional correction)
Ho = 15° 24.4’
When you have done this you can enter sight reduction tables and calculate the intercept in the usual manner. Have fun!
