Celestial navigation series, part 10
Editor’s note: We’re revisiting this series on navigating by the sun, moon, planets and stars in the age of GPS because celestial nav is not only a viable backup to satellite navigation, but it is also a skill that ocean voyagers should have in their toolkit. In this series, we’ll cover all the basic knowledge you’ll require to get up to speed on this elegant and rewarding technique for finding your way at sea. Click to read Part 1, Part 2, Part 3, Part 4, Part 5, Part 6, Part 7, Part 8 and Part 9.
In this installment, we’ll cover how to reduce a planet sight; we’ll include a step-by-step breakdown of how to reduce star and planet sights, and we will also include a star sight problem to solve as a way to review what you learned in the last installment.
After getting to know the stars in our last installment, we move on to the four navigational planets: Venus, Mars, Jupiter and Saturn. Planets have a big advantage over stars because they are so bright, and unlike stars they don’t twinkle (the technical term is scintillate) but cast a steady light. Because the light from these bodies is relatively bright and steady, it is often possible to shoot planets right after sunset in the evening and almost until sunrise in the morning. These sights are taken with a clear, well-lit horizon and may be of excellent quality. Because there are only four navigational planets, it is possible that there will not be one available for a particular twilight. Or, there may be only one. For this reason, it is common to shoot planets in conjunction with stars and/or the moon.
Small differences from a sun sight
There are only a few differences between a sight reduction for a planet and the sun sight reduction. The sight correction is similar to a star sight correction. The index error and dip corrections are applied in a standard fashion, and the apparent altitude correction is taken from the middle column on the inside front cover of the Nautical Almanac. There are, however, additional corrections for Venus and Mars. These are in the same box.
This diagram (not to scale) shows that since Venus is closer to the sun than the Earth, it is only seen just after sunset or just before sunrise. Mars, Jupiter and Saturn, however, can be seen for long periods of the night. This helps identify them. For example, a very bright planet in the sky at midnight cannot be Venus, and so is likely to be Jupiter.
The columns for the GHA and declination of the four planets are on the left-hand page of the Nautical Almanac’s daily pages. These are used just the same way that the GHA and declination columns are used for the sun. Just as with the sun, there is a d correction at the bottom of the declination column that must be used in the increments and corrections pages to determine the correction to declination. As with the sun, the sign for d must be determined based on whether declination gets larger or smaller during the course of the day. If declination is increasing, the sign for d is positive; if declination decreases, the sign for d is negative.
For planet sights, however, we have a new correction that we haven’t used before in sun sights or star sights. This new correction is the “v factor.” The v factor is found at the bottom of the GHA column. If it has no sign, then it is always positive; it’s only negative if it has a minus sign next to it. The v factor is used in the same manner as the d factor. Enter the increments and corrections pages (the colored pages at the back of the almanac) at the top of the column labeled “v or d” and go down that column to the value of the v factor. The corresponding correction is right next to it. The v correction is applied to GHA. This means that planets have two corrections to GHA: both the increment (from the “SUN PLANETS” column) and the v correction. The v correction is added if v is positive and subtracted if v is negative.
Other than that, planets are just like the sun.
On May 5, you take a sight of Mars at 06:26:10 GMT and get an Hs of 34° 12.4’. There is no index error and a height of eye of 10 feet. Find Ho and GHA and declination for Mars.
Starting with the Hs, the dip table reveals the dip correction is –3.1’.
Ha apparent altitude
Take this Ha to the apparent altitude correction and find –1.4’. Mars also has an additional correction that comes from the same box. In this case, we are between March 8 and Dec. 31 so the correction is +0.1’ (not very significant).
app. altitude correction
Now go to the daily page to find the GHA and declination. There are four pieces of information needed from the daily page: GHA, v, declination and d. GHA is 313° 17.3’; v is +1.2; declination is 1° 25.7’ S; d is –0.3. Now back to the increments and corrections page for 26 minutes. The GHA increment is 6° 32.5’. The v correction is –0.5 and the d correction is –0.1’.
+ 6° 32.5’
N 21° 11.6’
From here on, the planet sight reduction is just like a sun sight reduction. We use our DR longitude to determine the assumed longitude and apply that to the GHA of the planet to get LHA. We also use the DR latitude to determine the assumed latitude. After that we gather the three entering arguments for HO 249: assumed latitude, declination and LHA. With these, we can go into HO 249 and reduce a planet sight just like a sun sight or a star sight. In the next installment we’ll discuss the noon sight.