Early latitude observations at the Pulkovo observatory

Pulkovo astrometric observations began in the 1840s using the Repsold transit instrument in the prime vertical and Ertel vertical circle. The first observers on these instruments were W.I. Struve, 1840–1856, and Kh.I. Peters, 1842–1849. In the present work, we collected and analyzed different series of latitude variations from observations made by M.O. Nuren, B. Wanach, A.A. Ivanov, I.N. Bonsdorf, and A.Ya. Orlov. In addition, results are given of investigations of a specific behavior of the Chandler polar motion in this interval, obtained by C. Chandler, Ivanov, Kh. Kimura, Orlov, and N. Sekiguchi. The aim of this paper is to search for and analyze the earliest series of Pulkovo latitudes, in order to evaluate the possibility of their use to study the motion of the pole at the maximum available range of observations. Different methods were used to isolate and analyze the sum of Chandler and annual latitude variations. The annex provides a series of Pulkovo latitude variations for 1840–1848, which may be used to extend latitude variation back to 1840.     

On the Chandler wobble. I. Theory

This paper introduces the idea that there exists a nutation separation layer in the superficial zone of the Earth, thereby sets up a new model of nutation. The equations of motion are derived and solved. It is proved that the separation layer is “soft”, and is probably located at a depth of 300 km from the surface. Its allowable minimum thickness is less than 1 km and its viscosity is on the order of 10₁₀ poise, like pitch at 15 °C. The results show that the main peak of the Chandler wobble (CW) is double, one being the nutational frequency of the inner main body, the other , the outer component. The beat between the two frequencies produces the 40-yr period in the amplitude of the CW. The two amplitudes interact so they do not differ greatly in size, their approximate ratio being 1 to 0.7. The famous amplitude-period relation of the CW is a direct result of this paper. The sudden changes in the polar trace and the 180° phase change in 1924–1926 are essentially the same phenomenon determined by relative changes in the two frequencies. The value of Q for the CW of between 30 and 60 deduced by previous workers from observations is a false Q, the true Q is around 200, in agreement with the value from seismic waves. This paper also predicts that apart from the two main frequencies, there is a series (more than one pair) of secondary frequencies symmetrically located on either side. The time constant of the CW is about 70 yr and it can be approximately maintained by excitation by earthquakes. Its unsmooth motion is one that zigzags about the mean trace.     

Observing programs for the Pulkovo Horizontal meridian circle

Observing programs are discussed for the Pulkovo automatic horizontal meridian circle. The instrument is highly efficient and yields precise and accurate positions. We prepare an observing program of faint FK5 stars with the aim of improving on the system of this catalogue and also prepare a program of observations of IRS reference catalogue stars.     

Determination and analysis of local seasonal terms of latitude variations

Local seasonal terms of latitude variations were determined on the basis of data of 17 BIH/IPMS stations andx, y, z data of the BIH global solution during the years 1967–1978. Some models for local correctionsR to the latitude introduced by the BIH are created. The correctionsR obtained from the least squares adjustment from one year data appear to be the best. The present study shows that it is necessary to determine these more frequently than once a year or to compute them for all stations simultaneously with the ERP. This will be particularly important in view of the future re-reduction of past astrometric data.     

Characteristics of the chandler wobble

We carried out a spectral analysis (by FFT and periodograms) on a homogenized set of data of coordinates of the Earth's pole in the period 1900–1969.9, and found the Chandler wobble to have 4 peaks at periods of 1.142, 1.169, 1.199 and 1.230 years. The two main peaks at 1.169 and 1.199 years have equal amplitudes (TABLE 4 and Fig. 1). According to Fedrov and Yatskiv's (1964) interpretation of sudden phase changes, our data would give a natural period of 1.184 years, but the observed asymmetry between the two secondary peaks is a difficulty for this interpretation. The observed multiplicity of peaks can also be explained by the theory of amplitude modulation, if we suppose there is a modulating oscillation with a period around 48 years. In this case, the natural period of the Chandler wobble would be 1.199 years; but this interpretation also has some difficulties.     

Astrometric observations of Uranus with the Pulkovo Normal astrograph

The positions of Uranus were observed astrometrically with a CCD detector attached to the Pulkovo Normal astrograph (D/F = 0.33 m/3.5 m, S2C CCD, FOV 18′ × 16′). We provide the positions in the time interval from 2006 to 2011. Reduction of the CCD images was made with reference to the UCAC3 catalogue. The (O-C) values were calculated using the “Natural Satellites Service”. The results were compared with two contemporary theories of Uranus’s motion: INPOP10 and DE414/LE414. The obtained equatorial coordinates correspond well to both theories. On average, (O-C) over both coordinates relative to both theories are 0.1″.     

Light variations of RT Lac during the last ten years

The RS CVn-type eclipsing binary star RT Lac has been observed inB andV colours, during 1984, 1985, 1986, 1987, and 1988 observing seasons. The orbital period of the system was found to be decreasing with an amount of 62 s century^–1. The brightnesses both at minima and maxima are changing with time. It seems that the variations of the brightnesses at mid-primary and second quarter are similar, but at second first quarters are opposite to each other. We believe that these variations are mainly related with the spots (or spot groups) located on the surface of the component star which is in front during the primary minimum. We also estimate the period of the spot activity cycle to be approximately six years.     

Pulkovo Compilation of Radial Velocities for 35 495 Hipparcos stars in a common system

The Pulkovo Compilation of Radial Velocities (PCRV) has been made to study the stellar kinematics in the local spiral arm. The PCRV contains weighted mean absolute radial velocities for 35 495 Hipparcos stars of various spectral types and luminosity classes over the entire celestial sphere mainly within 500 pc of the Sun. The median accuracy of the radial velocities obtained is 0.7 km s^?1. Results from 203 publications were used in the catalogue. Four of them were used to improve the radial velocities of standard stars from the IAU list. The radial velocities of 155 standard stars turned out to be constant within 0.3 km s^?1. These stars were used to analyze 47 768 mean radial velocities for 37 200 stars from 12 major publications (～80% of all the data used). Zero-point discrepancies and systematic dependences on radial velocity, B-V color index, right ascension, and declination were found in radial velocity differences of the form “publication minus IAU list of standards.” These discrepancies and dependences were approximated and taken into account when calculating the weighted mean radial velocities. 1128 stars whose independent radial-velocity determinations were available at least in three of these publications and agreed within 3 km s^?1 were chosen as the work list of secondary standards. Radial-velocity differences of the form “publication minus list of secondary standards” were used by analogy to correct the zero points and systematic dependences in the radial velocities from 33 more publications (～ 13% of the data used). In addition, the radial velocities from 154 minor publications (～7% of the data used) pertaining to well-known instruments were used without any corrections.     

Astrometric CCD observations of visual double stars at the Pulkovo Observatory

We present the results of 3489 astrometric observations for 361 visual double stars performed in 2003–2007 with the 26-inch refractor of the Pulkovo Observatory. The angular separations between the components (ρ) and the position angles (θ) are given. The errors in these quantities are, on average, 0″.009 for ρ and 0°.40/ρ for θ, where ρ is the separation in arcseconds.     

Study of total electron content variations over equatorial and low latitude ionosphere during extreme solar minimum

In recent years with the advancement in satellite based navigational applications, study of Total Electron Content (TEC) has gained significant importance. It is well known that due to dynamical behaviour of equatorial and low latitude ionosphere, the levels of ionization is relatively high herein. The sustained decrease in solar extreme ultraviolet radiations during the current minimum is greater than any in recent history. This gives us the opportunity to study the observations of global positioning system total electron content (GPS-TEC) dual frequency signals from the GPS satellites continuously recorded at Trivandrum (an equatorial station) and Delhi (a low latitude station) during the extremely low solar activity period from January 2007 to June 2009. This study illustrates the diurnal, seasonal and annual variations of TEC during the extended solar minimum period. This study also investigates the behaviour of daytime ionosphere around spring and autumn equinoxes at low solar activity period. The results clearly reveal the presence of equinoctial asymmetry which is more pronounced at equatorial station Trivandrum. The diurnal variation of TEC shows a short-lived day minimum which occurs between 0500 to 0600 LT at both the stations. Delhi TEC values show its steep increase and reach at its peak value between 1200 and 1400 LT, while at the equator the peak is broad and occurs around 1600 LT. Further, the daily maximum TEC ranges from about 5 to 40 TEC units at Trivandrum and about 10 to 40 TEC units at Delhi, which correspond to range delay variations of about 1 to 8 m at the GPS L1 frequency of 1.575 GHz. The Maximum values of TEC were observed during spring equinox rather than autumn equinox, showing presence of semi annual variation at both the locations. The minimum values of TEC were observed during the summer solstice at Trivandrum indicating the presence of winter anomaly at equatorial region while Delhi TEC values were minimum during winter solstice showing absence of winter anomaly. Also the TEC values at both the locations have been decreasing since 2007 onwards exhibit good positive correlation with solar activity.     

Sunspot group areas and the latitude distance from the average latitude of activity

Digitized Mount Wilson sunspot data from 1917 to 1985 are analyzed to examine group areas as a function of latitude distance () from the central latitude of activity in each hemisphere. On average these group areas are larger for the smallest values of ¦¦. The effect is similar to that seen for the magnetic fields of active regions (Howard, 1991). It is concluded that this is fundamentally a dependence, and not a latitude dependence. The suggestion is made that the cause of this effect is the influence of large-scale convective motions on the rising flux tubes that make up the active regions. The smaller flux tubes (spot groups) are more easily displaced in latitude during their ascent to the surface by this velocity field than are the larger flux tubes.Operated by the Association of Universities for Research in Astronomy, Inc., under Cooperative Agreement with the National Science Foundation.     

Meridian observations of solar system bodies with the struveertel instruments of the Pulkovo Observatory

Two Struve-Ertel instruments were used for the daytime observations of the Sun, Mercury, Venus and Mars at Pulkovo from 1956 to 1976. The FK4 equinox and equator corrections were derived. Both the instruments were installed in 1983–1986 at the Kislovodsk Station of the Pulkovo Observatory. The atmospheric dispersion and lateral refraction have been estimated at the Station.     

On the accuracy of the Refraction Tables of Pulkovo Observatory,fifth edition

A systematic deviation has been found of the refraction given in Pulkovo Tables, fifth edition, from the standard refraction calculated for the same ground level meteorological conditions by the procedure described in one of our previous papers. This deviation, negative for zenith distances smaller than 75° and positive for zenith distances larger than 80°, is considerably greater than the error proven to be admissible for the pure refraction determined in the latter way. The value of deviation is of the order of 0″.03 already at z₀ = 45°. For zenith distances smaller than 75° the deviation originates almost exclusively from unsufficiently accurate corrections for the air humidity. For larger zenith distances the deviation originates mainly from the correction for the air temperature and contributions of the remaining corrections can partly compensate for it. In conclusion a proposition of a new unique standard of the pure refraction is stated.     

Estimating masses of selected stars in the Pulkovo program by means of astrometry methods

Stars in the Pulkovo Observatory program are observed with a 65-cm refractor during many years to study their positions and movements. We present examples of two visual binary stars, for which orbits and masses of components were determined, and two astrometric stars, for which masses of their unseen companions were estimated. The first two stars are ADS 14636(61 Cygni) and ADS 7251,and the others are Gliese 623 and ADS 8035(Alpha UMa). Direct astrometric methods are used for estimation of mass-ratio and masses.     

Brightness variations of the white light corona during the years 1964–67

Observations of the white light corona were made on over 900 days during the years 1964–67 at heights between 1.125 and 2.0 R with the K-coronameter at Mount Haleakala and Mauna Loa, Hawaii. The brightness distribution of the minimum corona was elliptical with average equatorial intensities three times the polar. Coronal features of the new cycle at 1.125 R occurred predominantly in the sunspot zones at 25–30° latitude and in a high latitude zone which migrated toward the North pole before solar maximum. The brightness of the inner corona doubled over this period and a close association is found between the average corona and 10.7-cm solar radio flux. Electron densities in the equatorial regions were nearly twice those of Van de Hulst's model corona, in agreement with the results of recent eclipse observations.At Hawaii Institute of Geophysics.     

Horizontal electric fields in the middle latitude

Three dimensional electric fields were measured at the altitude of about 27 km in the stratosphere over the Pacific Ocean about 200–400 km away from the Sanriku coast of Honsyu Island (L = 1·4) on 16–17 October 1973, which was magnetically disturbed. The average horizontal electric field thus measured is about 10 mV/m, and the electric field vectors made clockwise semidiurnal rotations rather than diurnal. Daily variation of this electric field was compared with data at L = 2·7–3·5 published by Mozer (1973) and was found to be very similar. This suggests that these electric fields are of common origin in the plasmasphere. From their mean daily variation it is estimated that the plasmaspheric convection is decreased in the night side and is increased in the day side by 200–300 m/sec, and there is an outward flow in the first half of the afternoon and an inward flow in the plasma bulge region of about 500 m/sec.     

Determinations of the quality factor Q of the chandler wobble

The polar coordinates series of ILS (H), IPMS and BIH are analysed for the Chandler wobble using the Okamoto-Kikuchi method [3] and the Chao-Gilbert (ARH) method [4]. More accordant results for the quality factor Q than in Ref. [5] are obtained.