Radar and photometric observations and shape modeling of contact binary near-Earth Asteroid (8567) 1996 HW1

We observed near-Earth Asteroid (8567) 1996 HW1 at the Arecibo Observatory on six dates in September 2008, obtaining radar images and spectra. By combining these data with an extensive set of new lightcurves taken during 2008-2009 and with previously published lightcurves from 2005, we were able to reconstruct the object’s shape and spin state. 1996 HW1 is an elongated, bifurcated object with maximum diameters of 3.8 × 1.6 × 1.5 km and a contact-binary shape. It is the most bifurcated near-Earth asteroid yet studied and one of the most elongated as well. The sidereal rotation period is 8.76243 ± 0.00004 h and the pole direction is within 5° of ecliptic longitude and latitude (281°, −31°). Radar astrometry has reduced the orbital element uncertainties by 27% relative to the a priori orbit solution that was based on a half-century of optical data. Simple dynamical arguments are used to demonstrate that this asteroid could have originated as a binary system that tidally decayed and merged.     

Transits of Mercury: Results of Belgrade Observations in 1970 and 1973

Photographic observations of the transits of Mercury over the solar disk in 1970 and 1973 at the Belgrade Observatory were used not only in order to determine the classical parameters, as the times of contacts and least distances to determine the apparent radii of the Sun and Mercury. The mean value of the Sun's semi-diameter reduced to the distance unit differs from AUWERS value by about 1.3 while the Mercury's radius is in good agreement with LE VERRIER 's value. On the basis of quite homogenous observational material and precise observing data it was possible to derive the parallax of the Sun by means of the angular distance changes due to the parallactic effect in the course of transit. The values are determined as: 8.800 ± 0.008 (1970) and 8.788 ± 0.032 (1973). The mean value resulting from the two transits is πs = 8.794 ± 0.013, an amount in striking agreement with that currently used.