Step-by-step for stars and planets
Star sight reduction by HO 249 Vol. 2
1. When correcting Hs to Ho, make sure to use the middle column on the inside cover of the Nautical Almanac marked “Stars and Planets” for the apparent altitude correction. Otherwise proceed just as for a sun sight.
2. Instead of finding GHA in the column marked “Sun” in the Nautical Almanac, it is in the column marked “Aries” at the far left of the left-hand page. There is also a special column in the increments pages in the back of the Nautical Almanac labeled “Aries.”
3. Find the SHA of the star on the daily page and add that to the GHA of Aries. This gives you the GHA of the star that then can be combined with the assumed longitude to yield the LHA of the star. The declination of the star is found next to the SHA, and there is no d correction.
4. Enter HO 249 with the assumed latitude, declination of the star and the LHA of the star.
5. Extract Hc, d and Z. Convert Z to Zn. Use d and the minutes of declination in Table 5 to find a correction for Hc. Determine the difference between Ho and Hc, and label that intercept “toward” if Ho > Hc or “away” if Hc > Ho.
6. Plot using the assumed longitude, assumed latitude, azimuth (Zn) and intercept.
Star sight reduction by HO 249 Vol. 1
Overview: HO 249 Vol. 1 is set up to cover all the latitudes that a boat is likely to sail on. The only entering arguments are assumed latitude and the LHA of Aries.
1. Correct Hs to Ho using the column labeled “Stars and Planets” on the inside front cover of the Nautical Almanac for the apparent altitude correction.
2. Find the GHA of Aries and correct using the Aries column in the increments section of the Nautical Almanac. Combine this with the assumed longitude to produce the LHA Aries.
3. Enter HO 249 Vol. 1 with assumed latitude, LHA Aries and the name of the star.
4. Extract Hc and Zn. No further corrections are needed. Find the difference between Ho and Hc and label that intercept “toward” if Ho > Hc or “away” if Hc > Ho.
5. Plot using the assumed longitude, assumed latitude, azimuth (Zn) and intercept.
Planet sight reduction
Overview: The planets are usually the brightest objects in the sky at dawn and dusk, and they are easy targets for accurate sights. Because their orbits are influenced by each other, there is a special v correction that is applied to GHA the same way that d is applied to declination.
1. When correcting the sight from Hs to Ho, make sure to use the column marked “Stars and Planets” for the apparent altitude correction. Also, Venus and Mars have special corrections that must be applied in addition to the apparent altitude correction.
2. Each of the navigational planets has its own column on each daily page for GHA and declination. At the bottom of the GHA column is the value for v, and at the bottom of the declination column is the value for d.
3. On the increments and corrections pages, find the increment in the column labeled “SUN PLANETS.” You will find v in the same correction column as d. The v correction is subtracted from GHA if v is negative, and added if v is positive. The value for d is negative if the declination decreases from hour to hour, and positive if the declination increases from hour to hour — you have to determine the sign of d for yourself.
4. Find the LHA of the planet by applying the assumed longitude and enter HO 249 with the assumed latitude, declination and LHA. Extract Hc, d and Z. Convert Z to Zn. Correct Hc for minutes of declination (Table 5 at the end of HO 249). Find the intercept (difference between Ho and Hc).
5. Plot using assumed latitude, assumed longitude, azimuth (Zn) and intercept.