Understanding stellar and solar observation in surveying

The observations made to the sun are called the solar observations whereas those on the star are called stellar observations. 
If very high degree of precision is not required the stellar observations may be made like the solar observation. The stellar observations are made for achieving higher degree of precision. In stellar observations, refined instruments and methods are used and special care is taken to eliminate all systematic errors.

            While making solar observations the sun should never be sighted with naked eye directly through telescope as it may cause serious injury to the eye. A piece of colored or smoked glass should be used between the eye and the eyepiece. Generally the instruments are equipped with a sun glass which may be attached to the eyepiece which is attached to the telescope for making solar observations when the sun’s altitude is high.

            Stellar observations do not have the above problems. On the other hand, since the stellar observations are made during the night, artificial illumination is required to make the cross hair visible. Some instruments are equipped with a reflector sleeve which can be slipped over the objective like a sunshade. A flash light is then held to one side of the reflector to illuminate the cross hairs. Modern optical theodolites are equipped with built in illumination system for illumination of the scales and the cross hairs.

            There are large number of stars in the sky and out of these, a suitable star is selected which is quite favorable for the precise determination of azimuth, latitude or longitude. While selecting a particular star, the following considerations should be made.

For determination of azimuth

1.     By measuring altitude, the star should be far enough above the horizon to reduce the uncertain refraction. The star should be far east or west of the meridian so as to form strong astronomical triangle.

2.     By measuring hour angle, a circumpolar star should be selected. The azimuth of a circumpolar star changes very slowly. Hence an error in hour angle will have less effect on the computed azimuth.

For determination of latitude

          This requires the altitude of the star at culmination. The stars should have a fairly high altitude so that the uncertainty of the refraction correction is small. Moreover, the rate of apparent movement of the star should be small to have a series of observations without any appreciable change in its altitude. The stars which are near the pole satisfy the above conditions.

For determination of longitude or time

          The most suited stars for determination of longitude or time are those which are near the celestial equator as they apparently move rapidly and therefore, more accurate results are obtained.

          Star charts show the various constellation of stars and should be used to identify the stars. It is not necessary to distinguish star from among the neighboring stars. The published directions and altitude of a particular star are set off on the theodolite with sufficient precision and the star is brought into the field of view at a given time.

DETERMINATION OF TIME

The local mean time is required to find the error of the chronometer or watch which is read at the instant of making observations. If the chronometer keeps the sidereal time, it is required to determine the hour angle of the vernal equinox or a star, at the time of observation. Similarly, if the chronometer keeps the solar time, then the hour angle of the sun (its center) at the instant of observations is required to be determined.

          The difference between the chronometer time and the time determined from the observations is known as chronometer error.  The correction for the chronometer error is applied algebraically to the chronometer time to give the true time at the instant of observation. The correction is positive when the chronometer is slow and negative when it is fast.

          The following are the methods usually employed for the determination of time:

1.     Meridian observation

2.     Ex- meridian observation

3.     Equal altitudes.