Solar Observations

This feature calculates true north and/or grid north bearings by solar observation. It uses the Local Hour Angle (LHA) method. The routine calculates Ephemeris data, thus alleviating the necessity of obtaining a Solar Ephemeris. The True North option calculates the true north bearing to mark. This option requires no zone/ellipsoid information. The True North & Grid North option calculates both true north and grid north bearings to north. The convergence angle is also shown.

Note: There is a description of solar observation field procedures at the end of this section.

True North Prompts

Calculate true north, or true north and grid bearing (<True north>/Grid Bearing: type T, press Enter
Choose field method (Leading edge/Trailing edge/<Center>): choose method, press Enter
If a Roelofs prism is being used, the Center Method should be selected. If not, select one of the other options. The Trailing Edge Method is the more popular of the two remaining methods.
Date of observation as MM/DD/YY or MM-DD-YYYY: For example 04/08/03.
Enter latitude of instrument point as DD.MMSS: For example 36.0545
Enter longitude of instrument point as DD.MMSS:

The following input loop will begin:

Obs. #1 - Time of observation as HH.MMSS: For example 15.3030
Enter angle to mark as DD.MMSS: Angle in the instrument when backsighting the mark.
Enter angle to sun as DD.MMSS: Clockwise angle from mark to sun.
The angle to the mark always has a default value of the last entered Angle to Mark. Each observation is numbered and the true bearing to the mark will be calculated. There is not limit as to the number of observations that can be made from a setup. After data entry is complete, press Enter.

The following options appear:

[Edit/Ok/Quit] <O>:

If you choose Edit, you will have the following options:

ADD/Change/Delete/eXit:

Add: Allows for addition observation data entry.
Change:

Allows editing of existing data. When selected a prompt for Enter observation to change will be displayed. Choose which observation number to edit. You will then be prompted with the initial input prompts for the observation again. The original input values will be the default values for each prompt. To change the value, simply enter new data.

Once Delete: This will delete the specified observation data. Choose the observation number to delete.
eXit: This exits the change routine.

If you type 0 and Enter or just enter for OK, the bearings from all the observations will be averaged and shown as well as the True Bearing. For example:

No. Time Angle-@-Mark Angle-to-Sun True-Brg-to-Mark

1 12.3030 0°00'00" 20°00'00" N 73°05'43"E
2 12.4456 0°00'00" 21°00'00" N 74°17'15"E
Average True Bearing: N 73°41'29"E

True North & Grid Bearing Prompts

Type of calculation [True-north/true-north-and-Grid-bearing] <T>: G
The following dialog will be displayed.

Select the state in which the observations were made. All fifty states are available, as well as PR for Puerto Rico and UTM for Universal Transverse Mercator.
If the state is divided into zones, you will be prompted for the zone you are working in.

Enter zone (N,S): Enter the zone.

If you are using a UTM , you will see the following prompt:

Enter ellipsoid to use [GRS-1980/Other] <G>:

Type "R" and Enter or just Enter for Reciprocal flattening, "S" and Enter for Semi-minor axis, or "E" and Enter for ellipsoid ECC squared.

if you typed O and Enter for Other, you will see the following prompt:

Ellipsoid constant [Reciprocal flattening/Semi-minor axis/ellipsoid ECC squared] <R>:
Depending on what was entered at the last prompt you will see one of the following prompts:

Enter reciprocal flattening constant: Type the constant.
Enter semi-minor axis: Enter the axis.
Enter ellipsoid ECC squared constant: Enter the constant.

After entering the zone and ellipsoid information (if applicable) the date, latitude, longitude and time input loop will begin (as described above for the True North calculation).

After data entry is completed the [Edit/Ok/Quit] <O>: prompt will be displayed (see the True North section for more details on this prompt).
If you type 0 and Enter of just Enter for Ok, the information for all the observations is displayed along with the Average True Bearing, Average Grid Bearing and the Convergence Angle as follows:

No.       Time       Angle-@-Mark    Angle-to-Sun    True-Brg-to-Mark
1           12.2222       0°00'00"              20°00'00" N        72°57'31"E
2           12.4444        0°00'00"             22°00'00" N         74°20'51"E
Average True Bearing: N 73°39'11"E
Average Grid Bearing: N 72°15'12"E
Convergency Angle: 1°23'59"

Field Procedures for the Local Hour Angle (LHA) method

This section explains Universal Time and then explains two ways of pointing, Trailing Edge Tangency and Roelofs Prism.

Universal Time
The Universal Time can be obtained on certain radio bands. On the radio channel there will be a signal beep every minute. Set a watch to the Universal Time or, when in the field, start a stopwatch at the beep (for a known Universal Time).
In order for the solar observation method to produce accurate results, it is essential that you record the precise Universal Time for an observation. Thus, when making the field observations, record the stopwatch elapsed time in order to calculate the Universal Time or, if you set your watch to Universal Time, record the time directly.

Trailing Edge Tangency
While pointing at the ground mark, set the horizontal circle to read about 00-00-30, perfect pointing. With the scope direct, record the horizontal circle reading to the mark.
Attach the eyepiece filter and sight the sun. After locating the sun, do the following:
Set the horizontal reticle line near the center of the sun’s image with the vertical reticle line leading the trailing edge of the sun (slightly right for a direct image).
Clamp the horizontal motion and watch the image of the sun as the trailing edge approaches tangency with the vertical reticle line.
Stop the timer at the time of tangency.
Record the time and the horizontal circle reading.
Repeat the pointing for a total of four pointings in the direct position.
Unclamp the horizontal motion, rotate the instrument 180 degrees, plunge the scope, and then obtain data for four reverse readings.
Unclamp the horizontal motion, point at the ground mark with the instrument reverse and record the horizontal circle.
The timer must be checked-in on a radio signal. Some quartz regulated electronic watches are accurate for extended periods of time, allowing several hours of check-in to check-out on the radio. Otherwise, most timers should be started and stopped on a radio signal at the beginning and ending of the observation set.

Roelofs Prism
Attach Roelofs prism and sight the sun (you can center the shadow of the telescope between the standards as an aid in locating the sun). Be sure that the hinged tube is closed when pointing at the sun. After locating the sun through the scope, do the following:
   Rotate the prism until the four overlapping images of the sun are symmetrical with the instrument’s reticle lines.
   Point on the ground mark with the instrument direct and the Roelofs prism tube swung open, perfect pointing on the ground mark. Record the horizontal circle reading to the mark.
   Point at the sun with the prism closed. After locating the sun, do the following:
Set the horizontal reticle line near the center of the sun’s pattern with the vertical reticle line leading the center of the moving pattern (slightly to right of the sun for direct optics).

Clamp the horizontal motion and watch the pattern move to the point of coincidence. This is the intersection of the vertical reticle line with the apex of the small dark square formed in the center of the pattern by the overlapping parts of the four images formed of the sun.
Stop the timer at the moment of coincidence.
Record the time and the horizontal circle reading.
Repeat the pointing for a total of four readings in the direct position.
Unclamp the horizontal motion, rotate the instrument 180 degrees, plunge the scope, and then obtain data for four reverse readings.
Unclamp the horizontal motion, point on the ground mark with the instrument reversed and record the angle on the horizontal circle.
The timer must be checked-in on a radio signal. Some quartz regulated electronic watches are accurate for extended periods of time, allowing several hours of check-in to check-out on the radio. Otherwise, most timers should be started and stopped on a radio signal at the beginning and ending of the observation set.

Pulldown Menu Location: COGO
Keyboard Command: cg_solar_obs
Prerequisite: None