Lunar Occultation of Saturn II. The normal reflectances of Rhea,Titan, and Iapetus

An inversion procedure to obtain the reflectance of the central region of a satellite's disk from lunar occultation data is presented. The scheme assumes that the limb darkening of the satellite depends only on the radial distance from the center of the disk. Given this assumption, normal reflectances can be derived that are essentially independent of the limb darkening and the diameter of the satellite. The procedure has been applied to our observations of the March 1974 lunar occultation of Tethys, Dione, Rhea, Titan, and Iapetus. In the V passband we derive the following normal reflectances: Rhea (0.97±0.20), Titan (0.24±0.03), Iapetus, bright face (0.60±0.14). For Tethys and Dione the values derived have large uncertainties, but are consistent with our result for Rhea.     

The 10 February 1977 lunar occultation of Uranus. Radius,limb darkening,and polar brightening at 6900 Å

A photoelectric observation in the near infrared of the 10 February 1977 lunar occultation of Uranus is described and analyzed in terms of planetary radius, limb darkening, and polar brightening. Contact timings, corrected for lunar limb effects, indicate an equatorial radius of 25,700 ± 500km for the visible disk. A modified Minnaert function is used to model limb darkening and polar brightening. Least-squares fits to the observed light curve indicate that Uranus is slightly limb darkened in the passband of the observation (450 ÅA FWHM centered near 6900 ÅA) and that polar brightening is present.     

The Measurement of Solar Diameter and Limb Darkening Function with the Eclipse Observations

The total solar irradiance varies over a solar cycle of 11 years and maybe over cycles of longer periods. Is the solar diameter variable over time too? A discussion of the solar diameter and its variations must be linked to the limb darkening function (LDF). We introduce a new method to perform high-resolution astrometry of the solar diameter from the ground, through the observations of eclipses, using the luminosity evolution of Baily’s bead and the profile of the lunar edge available from satellite data. This approach unifies the definition of the solar limb with the inflection point of LDF for eclipses and drift-scan or heliometric methods. The method proposed is applied for the videos of the eclipse on 15 January 2010 recorded in Uganda and in India. The result suggests reconsidering the evaluations of the historical eclipses observed with the naked eye.     

Lunar occultation of Saturn: IV. Astrometric results from observations of the satellites

The method of determining local lunar limb slopes, and the consequent time scale needed for diameter studies, from accurate occultation timings at two nearby telescope is described. The results for the photoelectric observations made at Mauna Kea Observatory during the occultation of Saturn's satellites on March 30, 1974, are discussed. Analysis of all observations of occultations of Saturn's satellites during 1974 indicates possible errors in the ephemerides of Saturn and its satellites.     

A high speed near infrared photometer for lunar occultation studies

A recently developed near infrared high speed photometer intended for lunar oc-cultation studies is described. The primary scientific objective is to reach milli arc second levels of angular resolution so that circumstellar structure of the occulted sources can be resolved. Near infrared sky brightness close to the lunar limb is also studied. Angular diameter derived from the observed occultation of IRC +20169 is presented and system performance discussed.     

Martian cratering revisited: Implications for early geologic evolution

Improved crater statistics from varied Martian terrains are compared to lunar crater populations. The distribution functions for the average Martian cratered terrain and the average lunar highlands over the diameter range 8–2000 km are quite similar. The Martian population is less dense by approximately 0.70 from 8 to 256 km diameter and diverges to proportionally lower densities at greater diameters. Crater densities on Martian “pure” terra give a lower limit to the Mars/Moon integrated crater flux of 0.75 since the last stabilization of the respective planetary crusts. The crater population >8 km diameter postdating the Martian northern plains is statistically indistinguishable from that population postdating the lunar maria. Monte Carlo simulations were performed to constrain plausible mechanisms of crater obliteration. The models demonstrate that if the crater density difference between the lunar and Martian terra has been due to resurfacing processes, random intercrater plains formation cannot be the sole process. If plains preferentially form in and obliterate larger craters, then the observed Martian distribution retains its “shape” as the crater density decreases. This result is consistent with the morphology of Martian intercrater plains.     

Developing an Optical Lunar Occultation Measurement Reduction System for Observations at Kaau Observatory

We present here a detailed explanation of the reduction method that we use to determine the angular diameters of the stars occulted by the dark limb of the moon. This is a main part of the lunar occultation observation program running at King Abdul Aziz University observatory since late 1993. The process is based on the least square model fitting method of analyzing occultation data, first introduced by Nather et al. (Astron. J. 75:963, 1970).     

Comment on: Constraints on the depth and variability of the lunar regolith,by B. B. Wilcox,M. S. Robinson,P. C. Thomas,and B. R. Hawke

Abstract— Any permanent presence on the Moon will require use of materials from the lunar regolith, the surface soil layer on the Moon. Thus, knowledge of the thickness of the lunar regolith is essential. It has been proposed that crater counts obtained from high Sun angle photography give larger estimates of impact crater equilibrium diameters than for low Sun angle photography, and thus deeper estimates of lunar surface regolith than were previously made using crater morphology, size of blocky rimmed craters, and equilibrium diameters determined on low Sun angle images. The purpose of this comment is to evaluate this result as a means of resolving this important question before planning for future lunar missions is undertaken     

The occultation of AG+29°398 by 93 Minerva

The occultation of AG+29°398 by 93 Minerva on 22 November 1982 was successflly observed at 10 sites. The data are best fitted by a circular limb profile having a diameter of 170.8 ± 1.4 km, a value that agrees well with the published radiometric diameter for this asteriod. However, evidence of significant departure from a spherical shape is found in the occultation observations and in photometric measurements of Minerva made at Lowell Observatory over several months. Additional observations are needed to specify definitively the three-dimensional figure of Minerva.     

On the possibility of investigating the galactic center region with the integral observatory by the method of lunar and terrestrial occultations

The possibility of investigating the sky region near the Galactic center with instruments of the INTEGRAL orbital astrophysical gamma-ray observatory by the method of its occultation by the Earth and the Moon is considered. Existing engineering constraints on the observing conditions, such as the admissible orientation of the INTEGRAL satellite relative to the direction to the Sun and the performance of measurements only outside the Earth??s radiation belts, are taken into account. Long time intervals during which the lunar occultation center passes at angular distances of less than 2° from the Galactic center have been found. Such events occur under the adopted constraints two or three times per year without any correction of the INTEGRAL satellite orbit. The orbit can be corrected to reduce the angular distance between the Moon and the Galactic center in occultation events. The required velocity impulses do not exceed several meters per second. The possibility of the Galactic center being occulted by the Earth has been analyzed. In this case, to perform measurements, the admissible (in radiation exposure) height of the working segment of the orbit should be reduced to 25 000 km, which can be problematic. At the same time, part of the Galaxy??s equatorial region is shadowed by the Earth for a time long enough to carry out the corresponding experiments.     

Tektites: Size estimates of their source craters and implications for their origin

From estimates of the total masses of tektites in three strewnfields, calculations by Orphal et al. (1980) of the amount of melt that could be ejected from impact craters, and equations relating kinetic energy of impact to crater diameter, it is possible to calculate minimum diameters of lunar craters capable of ejecting the liquid masses that could have formed the various tektite strewnfields. No lunar craters of the requisite sizes have been found that are young enough to correlate with the dates of formations of the strewnfields and it seems clear that the Moon must be eliminated as a source of tektites on the Earth. It is concluded that the associations of the Ivory Coast tektites with the Bosumtwi crater and the moldavites with the Rieskessel are real and the tektites are of terrestrial origin. It follows that if the Ivory Coast tektites came from the 10.5-km-wide Bosumtwi crater, the larger masses in the Australasian and North American strewnfields came from craters 17 km in diameter and between 33 and 65 km in diameter, respectively. No crater has yet been proven to be the parent of the Australisian tektites. The large crater that formed the North American tektites may not yet have been found, although the Mistastin Lake Crater may eventually be proven to be the source.     

Angular diameter and effective temperature of a sample of 15 M giants at 2.2 μm from lunar occultation observations

High angular resolution measurements of a sample of 15 M giants at 2.2 μm by the technique of lunar occultation are presented in this paper. We obtain angular diameters for 11 sources of which five are the first diameter measurements. For these resolved sources we have estimated the effective temperatures, which are consistent with previous calibrations. For the other four sources we put the first upper limits on their angular sizes to be 2 mas. Two sources, namely IRC+20090 and IRC+20067, yield appreciably low temperatures, which could point to their possible Mira nature. For sources with Hipparcos parallax measurements, we have calculated the linear radii.     

Optimization of lunar occultations for determining the positions of point-like X-ray sources

In view of the scheduled satellite mission EXOSAT (European X-Ray Observatory Satellite) of ESA (European Space Agency) the lunar occultation technique to determine the position of point-like X-ray sources is investigated. An error analysis for the source coordinates resulting from this technique is presented and an occultation strategy is proposed to achieve optimum lunar occultations. The analysis takes into account the errors of the space coordinates of the satellite and the Moon, the unevenness of the lunar surface, the intensities of source and background, the apparent angular velocity of the Moon as seen from the satellite, the finite sizes of the preoccultation position error boxes of the X-ray sources and the inaccuracies in the satellite orbit correction manoeuvres necessary to achieve the occultations.     

Electron content near the lunar surface using dual-frequency VLBI tracking data in a single lunar orbiter mission

In VLBI observations of Vstar, a subsatellite of the Japanese lunar mission SELENE, there were opportunities for lunar grazing occultation when Vstar was very close to the limb of the Moon. This kind of chance made it possible to probe the thin plasma layer above the Moon's surface as a meaningful by-product of VLBI,by using the radio occultation method with coherent radio waves from the S/X bands.The dual-frequency measurements were carried out at Earth-based VLBI stations. In the line-of-sight direction between the satellite and the ground-based tracking station where VLBI measurements were made, the effects of the terrestrial ionosphere, interplanetary plasma and the thin lunar ionosphere mixed together in the combined observables of dual-frequency Doppler shift and phase shift. To separate the variation of the ionospheric total electron content(TEC) near the surface of the Moon from the mixed signal, the influences of the terrestrial ionosphere and interplanetary plasma have been removed by using an extrapolation method based on a short-term trend. The lunar TEC is estimated from the dual-frequency observation for Vstar from UT 22:18to UT 22:20 on 2008 June 28 at several tracking stations. The TEC results obtained from VLBI sites are identical, however, they are not as remarkable as the result obtained at the Usuda deep space tracking station.     

The McDonald photoelectric occultation observation program

Since 1968 an assiduous program of photoelectric observation of occultations of stars by the Moon has been pursued at McDonald Observatory. A total of about 600 events has been observed of which 254 have been published and a second list is in preparation. Timings derived from the reductions have errors of the order of 1 ms corresponding to a positional uncertainty in the lunar limb of typically 80 cm. Results are routinely communicated to the ephemerides offices.In about 140 events the fringe pattern due to diffraction by the lunar limb is sufficiently well defined to permit a determination of the slope near the point of occultation. A statistical discussion of these data is given. In all except a few cases, the slopes are numerically less than 15°, though 7 cases with slopes between 30° and 40° have been found. The relations between numbers of observations and contact angle, and between errors of slope determination and contact angle are discussed. The distribution of slope data with contact angle seems adequately explained. When slope data are collected by position angle consistently large and consistently small values show a tendency to group in a pattern suggesting a connection with observed large scale features on the lunar limb.The influence of lunar limb irregularities of a scale of a few meters on observed diffraction patterns and inferred timings is discussed. Multichannel observations should be of value in removing ambiguities. The use of occultation observations for the discovery of multiple stars and for the measurement of angular diameters of stars is mentioned. Future developments proposed for the project are considered.The project has involved contributions by a considerable number of individuals from the staff and student body at Austin, Texas, from the staff at McDonald Observatory, from visiting scientists and from the Laser Ranging Group. These contributions are acknowledged in the paper. The work has been supported by NSF Grants GP-21204 and GP-32263X.Communication presented at the conference on Lunar Dynamics and Observational Coordinate Systems held January 15–17, 1973 at the Lunar Science Institute, Houston, Tex., U.S.A.     

Probing Titan’s lower atmosphere with acousto-optic tuning

Narrow-band images of Titan were obtained in November 1999 with the NASA/GSFC- built acousto-optic imaging spectrometer (AImS) camera. This instrument utilizes a tunable filter element that was used within the 500- to 1050-nm range, coupled to a CCD camera system. The images were taken with the Mount Wilson 2.54-m (100 in.) Hooker telescope, which is equipped with a natural guide star adaptive optics system. We observed Titan at 830 and 890 nm and at a series of wavelengths across the 940-nm window in Titan’s atmosphere where the methane opacity is relatively low. We determined the absolute reflectivity (I/F) of Titan and fit the values at 940 nm to a Minnaert function at Titan’s equator and at −30° latitude (closer to the subsolar point) and obtained average values for the Minnaert limb-darkening slope, k, of 0.661 ± 0.007 and 0.775 ± 0.018, respectively. Comparison with models suggests that the equatorial value of k is consistent with rain removal of haze in the lower atmosphere. The higher value of k at −30° is consistent with the southern hemisphere being brighter than the equator. However, the fits are not unique. The data and models at 890 are consistent with no limb brightening or darkening at this wavelength either at the equator or at −30°.     

Jovian satellite-satellite eclipses and occultations

Toward the end of 1973 and in the first part of 1974, when the planes of the orbits of the four large Galilean satellites cross the Sun and the Earth, satellite-satellite eclipses and occultations will occur. Calculations indicate that during this period 350 such events will occur, most of them potentially observable. From observation of a few of these events the ephemerides of the satellites can be improved, radii and limb darkening curves determined, and crude information about the degree and extent of albedo fluctuations deduced. If a concerted effort were made and a large fraction of these events carefully observed it would be possible to invert the light curves to obtain albedo maps of most of the surface area at a typical resolution of about 100km for JI and JII and somewhat poorer for JIII and JIV. In 1979—the next opportunity—there will be considerably fewer events, and only a relative few of them will be observable at night.     

Solar limb brightening at the extreme limb from photoelectric eclipse observations

Photoelectric observations of the light intensity from the solar crescent before and after totality were made during the eclipses of 7 March, 1970 and 26 February, 1979. Effective wavelengths were determined by interference filters of 20 nm bandwidth. To obtain the limb darkening function, the resulting intensity curves were analyzed by an extension of the method of Julius in which we take into account the actual lunar limb profile. The limb darkening function at 433 nm was obtained for the region 0.937 < sin < 0.9999. Our results are in good agreement with existing center-to-limb measurements in the region of overlap which extends to sin = 0.99. Additional data were obtained at 600 nm for 0.994 < sin < 0.9999. The curves at both wavelengths show a distinctive limb brightening effect at sin = 0.999.     

火星电离层无线电掩星探测仿真研究

对中俄联合火星星-星电离层掩星技术体制进行了分析和介绍,采用三维射线追踪方法对电离层掩星事件的电波观测值进行了模拟计算,并利用模拟的掩星观测数据进行了电子密度廓线反演,结果说明仿真算法可靠.利用仿真的方法,分别对掩星电波相位观测误差和卫星轨道误差等带来的反演误差进行了个例计算和分析,结果得到:5%周的相位测量误差对白天电离层掩星探测结果的影响可以忽略,而夜间电子密度测量的绝对误差小于4×108 m-3;卫星轨道误差对掩星的主要影响是导致电离层高度抬升或下降.结果表明,中俄联合火星电离层掩星探测技术体制先进,可望获得高精度的电子密度廓线;其技术体制也可以用于月球电离层环境的探测.     

A refined “standard” thermal model for asteroids based on observations of 1 Ceres and 2 Pallas

We present ground-based thermal infrared observations of asteroids 1 Ceres and 2 Pallas made over a period of 2 years. By analysing these data in light of the recently determined occultation diameter of Ceres (933–945 km) and Pallas (538 km) and their known small-amplitude lightcurves, we have determined a new value for the infrared beaming parameter used in the “standard” thermal emission model for asteroids. The new value is significantly lower than that previously used, and when applied in the reduction of thermal infrared observations of other asteroids, should yield model diameters that are closer to actual diameters. In our formulation, we also incorporate the recently adopted IAU magnitude convention for asteroids, which uses the zero-phase magnitudes (including the opposition effect) the same as is used for satellites.