According to an old but still unproven theory, Viking navigators analysed the skylight polarization with dichroic cordierite or tourmaline, or birefringent calcite sunstones in cloudy/foggy weather. Combining these sunstones with their sun-dial, they could determine the position of the occluded sun, from which the geographical northern direction could be guessed. In psychophysical laboratory experiments, we studied the accuracy of the first step of this sky-polarimetric Viking navigation. We measured the adjustment error e of rotatable cordierite, tourmaline and calcite crystals when the task was to determine the direction of polarization of white light as a function of the degree of linear polarization p. From the obtained error functions e(p), the thresholds p* above which the first step can still function (i.e. when the intensity change seen through the rotating analyser can be sensed) were derived. Cordierite is about twice as reliable as tourmaline. Calcite sunstones have smaller adjustment errors if the navigator looks for that orientation of the crystal where the intensity difference between the two spots seen in the crystal is maximal, rather than minimal. For higher p (greater than pcrit) of incident light, the adjustment errors of calcite are larger than those of the dichroic cordierite (pcrit=20%) and tourmaline (pcrit=45%), while for lower p (less than pcrit) calcite usually has lower adjustment errors than dichroic sunstones. We showed that real calcite crystals are not as ideal sunstones as it was believed earlier, because they usually contain scratches, impurities and crystal defects which increase considerably their adjustment errors. Thus, cordierite and tourmaline can also be at least as good sunstones as calcite. Using the psychophysical e(p) functions and the patterns of the degree of skylight polarization measured by full-sky imaging polarimetry, we computed how accurately the northern direction can be determined with the use of the Viking sun-dial under 10 different sky conditions at 61° latitude, which was one of the main Viking sailing routes. According to our expermiments, under clear skies, using calcite or cordierite or tourmaline sunstones, Viking sailors could navigate with net orientation errors |Σtruemax|≤3∘. Under overcast conditions, their net navigation error depends on the sunstone type: |Σtruemaxfalse(calcitefalse)|≤6∘, |Σtruemaxfalse(cordieritefalse)|≤10∘ and |Σtruemaxfalse(tourmalinefalse)|≤17∘.

In dancing sprite events, sprite entities and groups appear in rapid succession together with a corresponding series of parent lightning strokes. Dancing sprite events, including a case with possible sprite rebrightening, were recorded on video simultaneously from two observation sites above a mesoscale convective system in Central Europe on the night of 6 August 2013. Joint analysis of triangulated locations of sprite elements, position, type, and peak current of lightning strokes from the LINET lightning detection network database and current moment waveforms deduced at the Hylaty station, Poland, showed that subsequent sprite-parent lightning strokes occurred no further than 21 km from the closest preceding sprite entity in the cases analyzed in this study. Additionally, it was found that longer sprite delay times tend to correspond to larger sprite location offsets from the parent +CG stroke. These observations, the occurrence of +CG lightning stroke and sprite sequences, as well as sprite-sprite delay times and displacements can be explained if +CG strokes are part of one extended lightning flash. A corresponding production mechanism based on previous findings on the formation of sprite-producing and general +CG lightning discharges is suggested.Plain Language Summary In dancing sprite events, sprites appear in rapid succession together with a corresponding series of parent lightning strokes. Dancing sprite events were recorded on video simultaneously from two observation sites above an extended thunderstorm system in Central Europe on the night of 6 August 2013. Comparison of triangulated sprite locations and the locations of corresponding lightning activity revealed that subsequent sprite-parent lightning strokes occurred below or relatively near the location of the closest preceding sprite entity in the examined cases. In this paper, a mechanism is suggested for the development of lightning flashes that produce dancing sprites. The proposed mechanism accounts for prompt and delayed sprites relative to their parent lightning stroke, highlights the role of the variation of lightning current in sprite production, and can explain the observed closeness of the next +CG lightning stroke to the area of previous sprites.

It is shown that the well-known W Ursae Majoris-type eclipsing binary system UZ Leonis has an appreciable period increase. This conclusion is obtained by the analysis of 61 earlier times of minimum (available from the literature) and 13 newer ones (from photographic plates and photoelectric measurements). The period increase implies that the primary is accreting mass at a rate of M/M=5.4X 10" 8 yr" 1 .

ABSTRACT We present observations and analysis of PSN J17292918+7542390, a low-luminosity Type II-P supernova (LL SN IIP). The observed sample of such events is still low, and their nature is still under debate. Such SNe are similar to SN 2005cs, a well-observed LL Type II-P event, having low expansion velocities, and small ejected 56Ni mass. We have developed a robust and relatively fast Monte Carlo code that fits semi-analytic models to light curves of core-collapse SNe. This allows the estimation of the most important physical parameters, like the radius of the progenitor star, the mass of the ejected envelope, the mass of the radioactive nickel synthesized during the explosion, among others. PSN J17292918+7542390 has $R_0 = 91_{-70}^{+119} \times 10^{11} \, \text{cm}$, $M_\text{ej} = 9.89_{-1.00}^{+2.10} \, \mathrm{ M}_{\odot }$, $E_{\mbox{kin}} = 0.65_{-0.18}^{+0.19} \, \text{foe}$, and $v_{\mbox{exp}} = 3332_{-347}^{+216}$ km s−1, for its progenitor radius, ejecta mass, kinetic energy, and expansion velocity, respectively. The initial nickel mass of the PSN J17292918+7542390 turned out to be $1.55_{-0.70}^{+0.75} \times 10^{-3} \, \mathrm{M}_{\odot }$. The measured photospheric velocity at the earliest observed phase is 7000 km s−1. As far as we can tell based on the small population of observed LL SNe IIP, the determined values are typical for these events.

Here we present the analysis of the distribution of rotation periods and light curve amplitudes based on 2859 family asteroids in 16 Main Belt families based on 9912 TESS asteroid light curves in the TSSYS-DR1 asteroid light curve database. We found that the distribution of the light curve properties follow a family-specific character in some asteroid families, including the Hungaria, Maria, Juno, Eos, Eucharis, and Alauda families. While in other large families, these distributions are in general very similar to each other. We confirm that older families tend to contain a larger fraction of more spheroidal, low-amplitude asteroids. We found that rotation period distributions are different in the cores and outskirts of the Flora and Maria families, while the Vesta, Eos, and Eunomia families lack this feature. We also confirm that very fast spinning asteroids are close to spherical (or spinning top shapes), and minor planets rotating slower than ≈11 h are also more spherical than asteroids in the 4–8 h period range and this group is expected to contain the most elongated bodies.