Celestial Navigation Session #2
In the first session of Celestial Stories we discussed the coordinate system and units of measure. Now we must discuss what to do with these numbers.
The beauty of celestial navigation is that we can preplan and preposition the telescope or sextant for future viewing, or describe past observances. The astronomical telescope and the sextant are the same in this respect. Once found we can write down our knowledge, document, and publish.
Most telescope mounts when purchased have a set of setting circles attached. These components may look nice, but functionally they are practically useless for our purpose. One remedy is to go to a stationary store like Office Depot, and purchase at least four six-inch 360-degree protractors. Attach one protractor perpendicular onto each axis of the telescope mount, with a suitable pointer to read out the pointing angles. The craftsman must orient these to properly indicate the parameters. We need one protractor for plotting. We could also make an astrolabe using one protractor a soda straw for an alidade and some string with a weight attached.
The best reference for celestial body Geographic Position (GP) is the inexpensive Commercial Edition of the Nautical Almanac. This text is readily available, and revised annually by Her Majesty's Nautical Almanac Office, and the U.S. Naval Observatory.
We also need a good clock. Any digital watch will work just fine. However, the watch must be calibrated and then referenced from our zone time (ZT) to Greenwich Mean Time (GMT). Exact time setting is not necessary. The best way to calibrate the watch is to tune in a shortwave radio to station WWV and set the watch to the WWV tone, then record the watch error.
In session 1 we learned that only three parameters, our latitude, LHA, and declination of the celestial body, are needed to determine the telescope pointing angles. Calculate the value of each of these three parameters using data from the Nautical Almanac. Then calculate the axis pointing angles depending on the type mount, rotate the telescope axis to these coordinates and the object of our desires should at least be in the spotting scope. If calculations and pointing are done carefully the treasure will be centered in the eyepiece! We now have on our mount measurement instruments for the required parameters to be indicated.
On Equatorial mounts, the Right Ascension (RA) axis is aligned to the North Star Polaris. Locate the star Polaris by referring to the end stars of the Big Dipper as shown. Corrections for the position of Polaris are included in the Almanac. With the telescope pointing straight up, mount a protractor to the RA axis and adjust it to read 0 degrees with 270 degrees to the east and 90 degrees west. That protractor will now indicate Local Hour Angle (LHA) when rotating about this polar axis. This protractor can also be labeled to indicate RA, when set to a body of known RA.
On the declination axis, adjust that protractor to read 90 degrees when pointing to Polaris, and 0 degrees with the telescope pointing to the celestial equator. The protractor must now read the value of your latitude when the telescope is pointing to the vertical that is our local meridian. With the calculated values of LHA and declination, we rotate the axis to the direction of LHA and raise the telescope to the declination angle.
On Altitude/Azimuth mounts, the axes are aligned with the vertical and the horizon. With the telescope pointing to Polaris, mount and adjust one protractor on the base vertical axis to read 0 degrees True North, with the protractor scale reading clockwise. Rotating about this axis clockwise will indicate direction Zn from True North. On the horizon axis, mount and adjust the protractor to read 0 degrees in the pointing direction of the telescope when the telescope is level horizontally and 90 degrees when the telescope is pointing to the vertical. This protractor will indicate either observed elevation Ho or the calculated elevation Hc as needed. With the calculated values we then rotate the base to the direction Zn, and raise the telescope the height Hc.
Now let us take a look at the sextant and procedures used for celestial navigation. The sextant has mirrors and a telescope to accurately measure the observed height Ho of a celestial body. With the sextant set to zero, sight on the selected celestial body. Adjust the index arm by pinching the clamp and push outward while keeping the body centered in the telescope to bring it down to the horizon. Rotate and adjust the sextant a bit so as to ensure that the proper Ho is indicated. Read the exact time first and then the sextant's angle of elevation. It is necessary to maintain a log of sight data. Do this several times and record the average readings of Ho and time of sight to achieve the best results.
Since we may not know our actual position, assume our predicted Dead Reckoning (DR) longitude and latitude, calculate LHA from that position, and then determine declination. It is best to fill out a form to guide us in the process to calculate the Hc, and Zn. By comparing the difference between the observed height Ho with the calculated height Hc and Zn we determine our actual latitude and longitude. We then will not be lost!
It must be noted that accuracy of time and of the sight measurement of observed height (Ho} is crucial to determining position. On the surface of the earth one minute of latitude is 1 nautical mile. At the equator the surface of rotating earth moves eastward about 1 nautical mile in 4 seconds. When the sight reduction is completed using the same procedures as for the telescope pointing, our calculated position should be accurate to a few tenths of a nautical mile or minutes of angle. The navigator strives to achieve an error of position of less than 2 nautical miles in order to reach safe harbor. With practice the navigator can easily achieve this.
In the next month, celestial session 3 we will examine the methods and sequence of filling out the forms and completing the data reduction. To directly determine our position and prove our skills as a navigator, we could refer to the handheld GPS hidden in our pocket.
End of Session Two
