239 observed minima up to 1959 are collected and discussed. It is found that before 1925 the period may have varied periodically, while after 1925 it has remained practically constant. The appearance of the curve of the residuals as well as rough calculations make it improbable that the period variations could be explained in terms of apsidal or nodal motion or presence of a third body. It is more probable that an actual change in the true period took place.
Mechanisms currently under consideration for the evolution of cometary nuclei are reviewed. Attention is paid to processes that shouid have occurred in the past history of observed comets,
related to their origin or storage in the Oort cloud, but emphasis is placed on presently observed evolutionary effects and in particular the late stages characteristic of short-period comets. Evidence from brightnesses and nongravitational forces is discussed and support for a scenario of dust coverage coupled to the evolution of perihelion distance is found. However, for the cases of comets P/Halley and P/Encke where the perihelion distance is unusually small an ultimate fate of complete mass loss or disintegration can not be excluded.
In the paper a general description of operation of a Doppler receiver DOG-2, produced in Poland is presented as well as the actual results of determination of the Borowiec geocentric coordinates obtained from observations made by the use of that receiver. Processing of data was performed with program GEODOP.
In 1983 a special campaign was organized in the framework of program Photodoppler, with observations of satellite NOVA. About 400 directions were measured at station Riga. The data were processed and Riga coordinates obtained with an accuracy similar to those of Single Point Positioning. In this method the photographic camera can be considered as a stationary Doppler receiver.
The possible formation of a new stellar generation may be seen in the bright rimmed dark clouds that surround the giant HII region IC 1396. A well.defined ring of IRAS point sources is projected near the ionisation front indicated by the bright rims. We sugest that some of these sources are young stellar objects, and the majority represents density enhancements in the shocked neutral gas layer preceding the ionisation front which might eventually become stars.
A new method to investigate periods and their variations is proposed. The phase curve is approximated by the cubic
spline-functíon with fixed number of equidistantly distributed in phasev characteristic points. Because such a smoothing function depends on the value of the Inltial epoch it is proposed to use the mean of some spline-curves corresponding to various initial epochs for the approximation. The mean guadratic deviation of observatlons from this best fit curve serves as the test function. The best fit phase deviations of the 'season sets’ of observations are used to study period variations.
Solar radio spectra in the range of 2.0 - 4.5 GHz observed by using a digital radio spectrometer at the Astronomical Observatory Ondřejov are presented. A brief description of the observational equipment is given in Part I. In Part II are presented solar radio spectra observed in the period April 10 - December 29, 1989, including also the First Flares 22/MAX´91 Campaign No. 1.