Introduction to Special Celestial Techniques

Determining Availability of Celestial Bodies

By doing a quick comparison of Greenwich hour angle (GHA) to the observer’s position, it is easy to determine the availability of celestial bodies. For example, the observer anticipates being at 18°N 135° W at 0015Z on 28 September 1995. There are several bodies listed in the Air Almanac, but not all of them are available for observation. To determine availability, take the observer’s longitude and look 80° either side of it. Within this range, compare the GHA of a body. Looking at Figure 12-1, we see that the sun, moon, Venus, and Jupiter are within the 80° range and are therefore usable. Saturn is outside of the 80° range, so it is not usable. The declination (Dec) of a body is not normally a factor; however, at high latitudes a body may not be available when its subpoint is near the pole opposite the observer.

Figure 12-1. A quick check of body availability.

Figure 12-1. A quick check of body availability. [click image to enlarge]

Latitude by Polaris

Polaris is the polestar, or North Star. Because Polaris is approximately 1° from the North Pole, it makes a small diurnal circle and seemingly stays in about the same place all night. This fact makes Polaris very useful in navigation. With certain corrections, it serves as a reference point for direction and for latitude in the Northern Hemisphere. Latitude by Polaris is a quick method of obtaining a latitude line of position (LOP); only the tables given in the Air Almanac are needed.


Obtaining Latitude by Polaris

A latitude by Polaris LOP is obtained by applying the Q correction to the corrected observed altitude. [Figure 12-2] This adjusts the altitude of the pole, which is equal to the navigator’s latitude. The Q correction table is in the back of the Air Almanac. The entering argument for the table is exact local hour angle (LHA) of Aries. The effect of refraction is not included in Q correction, so the observed altitude must be fully corrected. When refraction is used for a latitude by Polaris LOP, it is applied to the observed altitude and the sign of the correction is negative. A Polaris LOP can also be plotted using the intercept method. In this case, the Hc is computed by reversing the sign of the Q correction and applying it to the assumed latitude (rounded off to the nearest degree). Refraction is positive when applied to get an Hc for the intercept method.

Figure 12-2. Polaris Q correction and azimuth tables from the Air Almanac.

Figure 12-2. Polaris Q correction and azimuth tables from the Air Almanac. [click image to enlarge]

Obtaining Azimuth of Polaris

For either method, the azimuth of Polaris is obtained from the Azimuth of Polaris table found in the Air Almanac or in the Pub. No. 249. [Figure 12-2] Whether plotted as an intercept or a latitude, the assumed position should be corrected for Coriolis, or rhumb line, and precession, or nutation. The resulting LOPs should fall in the same place for either method. To plot the LOP using the latitude method, choose the longitude line closest to the DR and plot perpendicular to the longitude line. For the intercept method, use the assumed latitude and plot the intercept normally using the azimuth of Polaris.


Latitude by Polaris Example

On 18 April 1995 for Greenwich mean time (GMT) 1600 at 23° 10′ N 120° W, with an observed altitude 23° –06′ at 31,000′. When doing a latitude by Polaris you must use the exact latitude and longitude. See Figure 12-3 for plotting.

GHA 086° –18′
Longitude (West) –120° –00′
LHA 326° –18′
True Course (TC) 090°
Groundspeed (GS) 400 knots
Coriolis/rhumb line 7R
Corrected Observed Altitude 23° –06′
Q (based on LHA 072-44) –15′
Refraction –01′
Latitude 22° –50′
Azimuth (LHA 326° –18′, Latitude 23° N) 000.8°
Figure 12-3. Plotting the Polaris LOP.

Figure 12-3. Plotting the Polaris LOP.

NOTE: If the Q correction table in Volume 1 is used, precession and nutation (P/N) and Coriolis, or rhumb line, must be used in plotting the LOP. This is because the Pub. No. 249 covers a 5-year period, and the further the years get from the Epoch year, the greater the error is when using the Polaris table. P/N compensates for this error.

Intercept Method Example

Refer to the previous problem and Figure 12-3 for plotting. NOTE: Applying 10A to assumed latitude gives 22° –50′ N, which is the same the answer in the latitude by Polaris example.

Azimuth of Polaris 359.5
Coriolis/rhumb line 7R
Assumed Lat (rounded off) 23° –00′ N
Q (reversed sign) +15′
Refraction +01′
Hc Polaris 23° –16′
Ho Polaris 23° –06′
Intercept 10A

NOTE: In these examples, all information was taken from the Air Almanac. No P/N is required.