Lunar Occultation Observations Research Articles

Observation of a Lunar Eclipse at 89, 150, and 183 GHz

We describe the measurement of the brightness temperature of the Moon from space during a total lunar eclipse by using a microwave sounder aboard a weather satellite. Previous observations of lunar eclipses were inconsistent and did not cover the frequency range between 100 and 200 GHz. In this work, we seek to establish a reliable relationship between frequency and drop in brightness temperature during a total eclipse for millimeter wavelengths. For this purpose, we chose the eclipse on 2004 October 28, because it coincided with appearances of the Moon in the deep space view of the Advanced Microwave Sounding Unit-B on NOAA-15. It was therefore possible to measure its disk-integrated radiance at 89, 150, and 183 GHz at 100 minutes intervals. Our observations are, to the best of our knowledge, the only ones between 100 and 200 GHz, and demonstrate the nearly linear dependency on frequency of the maximum relative drop in effective temperature during an eclipse. The slope of this function is m = 0.00114 ± 0.00017 GHz−1 in the range 88–300 GHz. The good agreement between the variations of the effective lunar temperature and a new radiative-transfer model suggests that the Moon is suitable as a flux standard for microwave observations with beam sizes larger than 0.5°.

The first results of observations of lunar occultations in different spectral ranges at the 6-m telescope of the SAO RAS

Observations by the method of lunar occultations at the 6-m telescope of the SAO RAS reach a resolution limit better than 1 mas. We have devised a new method of observation, which allows obtaining the curves of lunar occultations simultaneously in different ranges of the visible part of the spectrum, and conducted the first observations using a modified optical system. In January 2019, applying the new method, the magnitude differences of the components of the binary star HD 36524 in the R and I bands were measured, which were 1.5 and 1.3 mag, respectively, in good agreement with values calculated from the Gaia mission. According to the obtained data, it was determined that with this method it is possible to reach a signal-to-noise ratio of about 5–10 for stars of 10th magnitude. We discuss the applicability of the proposed optical scheme for the study of binary stars.

First lunar occultations results from the Trebur observatory

A program of routine observations of lunar occultations has been initiated at the Michael Adrian Observatorium using a 1.2 m telescope equipped with a commercial CMOS camera. The goal is mainly the direct measurement of the angular diameter of late-type stars, as well as of binary stars with small separations. Among the results reported in this first paper, we discuss a measurement of Aldebaran (α Tau), of the two cool, chemically peculiar giant stars R Gem and T Cnc, and of the two newly detected binary stars SAO 139528 and SAO 119489.This work demonstrates the capability to carry out milliarcsecond-level investigations at this relatively small facility on sources down to ≈ 10 mag. We plan to continue this routine program in the coming years, also in coordination with other similar efforts at various observatories with comparable instrumentation.

The Hubble Space Telescope's Near-UV and Optical Transmission Spectrum of Earth as an Exoplanet

Abstract We observed the 2019 January total lunar eclipse with the Hubble Space Telescope’s STIS spectrograph to obtain the first near-UV (1700–3200 Å) observation of Earth as a transiting exoplanet. The observatories and instruments that will be able to perform transmission spectroscopy of exo-Earths are beginning to be planned, and characterizing the transmission spectrum of Earth is vital to ensuring that key spectral features (e.g., ozone, or O3) are appropriately captured in mission concept studies. O3 is photochemically produced from O2, a product of the dominant metabolism on Earth today, and it will be sought in future observations as critical evidence for life on exoplanets. Ground-based observations of lunar eclipses have provided the Earth’s transmission spectrum at optical and near-IR wavelengths, but the strongest O3 signatures are in the near-UV. We describe the observations and methods used to extract a transmission spectrum from Hubble lunar eclipse spectra, and identify spectral features of O3 and Rayleigh scattering in the 3000–5500 Å region in Earth’s transmission spectrum by comparing to Earth models that include refraction effects in the terrestrial atmosphere during a lunar eclipse. Our near-UV spectra are featureless, a consequence of missing the narrow time span during the eclipse when near-UV sunlight is not completely attenuated through Earth’s atmosphere due to extremely strong O3 absorption and when sunlight is transmitted to the lunar surface at altitudes where it passes through the O3 layer rather than above it.

Constraining ΔT from Babylonian lunar appulse and occultation observations

We examine in detail 15 Babylonian observations of lunar appulses and occultations made between 80 and 419 BC for the purpose of setting useful limits on Earth's clock error, as quantified by $\Delta$T, the difference between Terrestrial Time and Universal time. Our results are generally in agreement with reconstructions of $\Delta$T using untimed solar eclipse observations from the same period. We suggest a revised version of the simple quadratic fit to $\Delta$T in light of the new results.

LUNAR OCCULTATIONS OF 18 STELLAR SOURCES FROM THE 2.4 m THAI NATIONAL TELESCOPE

ABSTRACT We report further results from the program of lunar occultation (LO) observations started at the 2.4 m Thai National Telescope (TNT) in 2014. We have recorded LO events of 18 stellar sources, leading to the detection of four angular diameters and two binary stars. With two exceptions, these are first-time determinations. We could resolve angular diameters as small as 2 milliarcseconds (mas) and projected separations as small as 4 mas. We discuss the individual results, in the context of previous observations, when available. The first-time angular diameters for o Psc, HR 6196 and 75 Leo are in good agreement with expected values, while that of π Leo agrees with the average of previous determinations but has a higher accuracy. We find a new secondary in o Psc, as previously suspected from Hipparcos data. We also obtain an accurate measurement of the companion in 31 Ari, revealing inconsistencies in the currently available orbital parameters. The TNT, equipped with the fast ULTRASPEC imager, is the leading facility in Southeast Asia for high time resolution observations. The LO technique at this telescope achieves a sensitivity of i′ ≈ 10 mag, with a potential to detect several hundreds of LO events per year.

The centre-to-limb variations of solar Fraunhofer lines imprinted upon lunar eclipse spectra

The atmospheres of exoplanets are commonly studied by observing the transit of the planet passing in front of its parent star. The obscuration of part of the stellar disk during a transit will reveal aspects of its surface structure resulting from general centre-to-limb variations (CLVs). These become apparent when forming the ratio between the stellar light in and out of transit. These phenomena can be seen particularly clearly during the progress of a penumbral lunar eclipse, where the Earth transits the solar disk and masks different regions of the solar disk as the eclipse progresses. When inferring the properties of the planetary atmosphere, it is essential that this effect originating at the star is properly accounted for. Using the data observed from the 2014-April-15 lunar eclipse with the ESPaDOnS spectrograph mounted on the Canada France Hawaii Telescope (CFHT), we have obtained for the first time a time sequence of the penumbral spectra. These penumbral spectra enable us to study the centre-to-limb variations of solar Fraunhofer lines when the Earth is transiting Sun. The Na i and Ca ii absorption features reported from previous lunar eclipse observations are demonstrated to be CLV features, which dominate the corresponding line profiles and mask possible planetary signal. Detecting atmospheric species in exoplanets via transit spectroscopy must account for the CLV effect.

THE DISCOVERY OF QUASARS AND ITS AFTERMATH

Although the extragalactic nature of quasars was discussed as early as 1960 by John Bolton and others it was rejected largely because of preconceived ideas about what appeared to be an unrealistically-high radio and optical luminosity. Following the 1962 observation of the occultations of the strong radio source 3C 273 with the Parkes Radio Telescope and the subsequent identification by Maarten Schmidt of an apparent stellar object, Schmidt recognized that the simple hydrogen line Balmer series spectrum implied a redshift of 0.16. Successive radio and optical measurements quickly led to the identification of other quasars with increasingly-large redshifts and the general, although for some decades not universal, acceptance of quasars as being by far the most distant and the most luminous objects in the Universe. However, due to an error in the calculation of the radio position, it appears that the occultation position played no direct role in the identification of 3C 273, although it was the existence of a claimed accurate occultation position that motivated Schmidt's 200-in Palomar telescope investi-gation and his determination of the redshift. Curiously, 3C 273, which is one of the strongest extragalactic sources in the sky, was first catalogued in 1959, and the 13^th magnitude optical counterpart was observed at least as early as 1887. Since 1960, much fainter optical counterparts were being routinely identified, using accurate radio interferometer positions which were measured primarily at the Caltech Owens Valley Radio Observatory. However, 3C 273 eluded identification until the series of lunar occultation observations led by Cyril Hazard. Although an accurate radio position had been obtained earlier with the Owens Valley Interferometer, inexplicably 3C 273 was misidentified with a faint galaxy located about one arc minute away from the true position. It appears that the Parkes occultation position played only an indirect role in the identification of the previously-suspected galactic star, which was only recognized as the optical counterpart after Schmidt's 200-in observations showed it to have a peculiar spectrum corresponding to a surprisingly-large redshift.

High-resolution transmission spectrum of the Earth's atmosphere-seeing Earth as an exoplanet using a lunar eclipse

AbstractWith the rapid developments in the exoplanet field, more and more terrestrial exoplanets are being detected. Characterizing their atmospheres using transit observations will become a key datum in the quest for detecting an Earth-like exoplanet. The atmospheric transmission spectrum of our Earth will be an ideal template for comparison with future exo-Earth candidates. By observing a lunar eclipse, which offers a similar configuration to that of an exoplanet transit, we have obtained a high-resolution and high signal-to-noise ratio (SNR) transmission spectrum of the Earth's atmosphere. This observation was performed with the High Resolution Spectrograph at Xinglong Station, China during the total lunar eclipse in December 2011. We compare the observed transmission spectrum with our atmospheric model, and determine the characteristics of the various atmospheric species in detail. In the transmission spectrum, O2, O3, O2 · O2, NO2 and H2O are detected, and their column densities are measured and compared with the satellites data. The visible Chappuis band of ozone produces the most prominent absorption feature, which suggests that ozone is a promising molecule for the future exo-Earth characterization. Due to the high resolution and high SNR of our spectrum, several novel details of the Earth atmosphere's transmission spectrum are presented. The individual O2 lines are resolved and O2 isotopes are clearly detected. Our new observations do not confirm the absorption features of Ca II or Na I which have been reported in previous lunar eclipse observations. However, features in these and some other strong Fraunhofer line positions do occur in the observed spectrum. We propose that these are due to a Raman-scattered component in the forward-scattered sunlight appearing in the lunar umbral spectrum. Water vapour absorption is found to be rather weak in our spectrum because the atmosphere we probed is relatively dry, which prompts us to discuss the detectability of water vapour in Earth-like exoplanet atmospheres.

The road to quasars

AbstractAlthough the extragalactic nature of 3C 48 and other quasi stellar radio sources was discussed as early as 1960 by John Bolton and others, it was rejected largely because of preconceived ideas about what appeared to be unrealistically high radio and optical luminosities. Not until the 1962 occultations of the strong radio source 3C 273 at Parkes, which led Maarten Schmidt to identify 3C 273 with an apparent stellar object at a redshift of 0.16, was the true nature understood. Successive radio and optical measurements quickly led to the identification of other quasars with increasingly large redshifts and the general, although for some decades not universal, acceptance of quasars as the very luminous nuclei of galaxies.Curiously, 3C 273, which is one of the strongest extragalactic sources in the sky, was first cataloged in 1959 and the magnitude 13 optical counterpart was observed at least as early as 1887. Since 1960, much fainter optical counterparts were being routinely identified using accurate radio interferometer positions which were measured primarily at the Caltech Owens Valley Radio Observatory. However, 3C 273 eluded identification until the series of lunar occultation observations led by Cyril Hazard. Although an accurate radio position had been obtained earlier with the OVRO interferometer, inexplicably 3C 273 was initially misidentified with a faint galaxy located about an arc minute away from the true quasar position.

Photoelectric lunar-occultation observations of several close binary stars

The results of photoelectric lunar-occultation observations for several stars are presented. Some of these are obvious close binary systems, while others are suspected close binaries or multiple systems, or show evidence for the presence of complex structure of some other kind. It is expected that new, efficient interferometric systems will enable detailed studies of the structure of many “ordinary” stars with an angular resolutions of 10−4–10−5 arcsec.

Lunar eclipses: Probing the atmosphere of an inhabited planet

The Moon's brightness during a lunar eclipse is indicative of the composition, cloudiness and aerosol loading of the Earth's atmosphere. The idea of using lunar eclipse observations to characterize the Earth's atmosphere is not new, but the interest raised by the prospects of discovering Earth-like exoplanets transiting their host stars has brought renewed attention to the method. We review some recent efforts made in the prediction and interpretation of lunar eclipses. We also comment on the contribution of the lunar eclipse theory to the refractive theory of planetary transits.

GLANCING VIEWS OF THE EARTH: FROM A LUNAR ECLIPSE TO AN EXOPLANETARY TRANSIT

It has been posited that lunar eclipse observations may help predict the in-transit signature of Earth-like extrasolar planets. However, a comparative analysis of the two phenomena addressing in detail the transport of stellar light through the planet's atmosphere has not yet been presented. Here, we proceed with the investigation of both phenomena by making use of a common formulation. Our starting point is a set of previously unpublished near-infrared spectra collected at various phases during the August 2008 lunar eclipse. We then take the formulation to the limit of an infinitely distant observer in order to investigate the in-transit signature of the Earth-Sun system as being observed from outside our Solar System. The refraction-bending of sunlight rays that pass through the Earth's atmosphere is a critical factor in the illumination of the eclipsed Moon. Likewise, refraction will have an impact on the in-transit transmission spectrum for specific planet-star systems depending on the refractive properties of the planet's atmosphere, the stellar size and the planet's orbital distance. For the Earth-Sun system, at mid-transit, refraction prevents the remote observer's access to the lower ~12-14 km of the atmosphere and, thus, also to the bulk of the spectroscopically-active atmospheric gases. We demonstrate that the effective optical radius of the Earth in transit is modulated by refraction and varies by ~12 km from mid-transit to 2nd contact. The refractive nature of atmospheres, a property which is rarely accounted for in published investigations, will pose additional challenges to the characterization of Earth-like extrasolar planets. Refraction may have a lesser impact for Earth-like extrasolar planets within the habitable zone of some M-type stars.

Near-infrared angular diameters of a few asymptotic giant branch variables by lunar occultations

The uniform disk (UD) angular diameter measurements of two oxygen-rich Mira variables (AW Aur and BS Aur) and three semiregular (SRb) variables (GP Tau, RS Cap, RT Cap), in near Infrared K-band (2.2 micron) by lunar occultation observations are reported. UD angular diameters of the two Miras and one SRV are first time measurements. In addition a method of predicting angular diameters from (V-K) colour is discussed and applied to the five sources. The effect of mass-loss enhancing measured K-band diameters is examined for Miras using (K-[12]) colour excess as an index. In our sample the measured angular diameter of one of the Miras (BS Aur) is found enhanced by nearly 40% compared to its expected value, possibly due to mass loss effects leading to formation of a circumstellar shell.

New high-sensitivity, milliarcsecond resolution results from routine observations of lunar occultations at the ESO VLT

(Abridged) Lunar occultations (LO) are a very efficient and powerful technique, that achieves the best combination of high angular resolution and sensitivity possible today at near-infrared wavelengths. Given that the events are fixed in time, that the sources are occulted randomly, and that the telescope use is minimal, the technique is very well suited for service mode observations. We have established a program of routine LO observations at the VLT observatory, especially designed to take advantage of short breaks available in-between other programs. We have used the ISAAC instrument in burst mode, capable of producing continuous read-outs at millisecond rates on a suitable subwindow. Given the random nature of the source selection, our aim has been primarily the investigation of a large number of stellar sources at the highest angular resolution in order to detect new binaries. Serendipitous results such as resolved sources and detection of circumstellar components were also anticipated. We have recorded the signal from background stars for a few seconds, around the predicted time of occultation by the Moon's dark limb. At millisecond time resolution, a characteristic diffraction pattern can be observed. Patterns for two or more sources superimpose linearly, and this property is used for the detection of binary stars. The detailed analysis of the diffraction fringes can be used to measure specific properties such as the stellar angular size and the presence of extended light sources such as a circumstellar shell. We present a list of 191 stars for which LO data could be recorded and analyzed. Results include the detection of 16 binary and 2 triple stars, all but one of which were previously unknown. The projected angular separations are as small as 4 milliarcseconds and magnitude differences as high as ?K=5.8 mag...

Observations and modelling of earth's transmission spectrum through lunar eclipses: A window to transiting exoplanet characterization

AbstractRecently we were able to retrieve the Earth's transmission spectrum through lunar eclipse observations. This spectrum showed that the depth of most molecular species was stronger than models had anticipated. The presence of other atmospheric signatures, such as atmospheric dimers, were also present in the spectrum. We have been developing a radiative transfer code able to reproduce the Earth's transmission spectra at different depths into the penumbra and umbra, and taking into account transmission, refraction, and multiple scattering. Here we discuss the results to date and the work ahead.

Earth’s transmission spectrum from lunar eclipse observations

Of the 342 planets so far discovered orbiting other stars, 58 'transit' the stellar disk, meaning that they can be detected through a periodic decrease in the flux of starlight. The light from the star passes through the atmosphere of the planet, and in a few cases the basic atmospheric composition of the planet can be estimated. As we get closer to finding analogues of Earth, an important consideration for the characterization of extrasolar planetary atmospheres is what the transmission spectrum of our planet looks like. Here we report the optical and near-infrared transmission spectrum of the Earth, obtained during a lunar eclipse. Some biologically relevant atmospheric features that are weak in the reflection spectrum (such as ozone, molecular oxygen, water, carbon dioxide and methane) are much stronger in the transmission spectrum, and indeed stronger than predicted by modelling. We also find the 'fingerprints' of the Earth's ionosphere and of the major atmospheric constituent, molecular nitrogen (N(2)), which are missing in the reflection spectrum.

Remote sensing of the Earth’s atmosphere based on lunar eclipse observations

The near-infrared photometric measurements of lunar eclipses in 2004–2008 are analyzed. The obtained distributions of the degree of lunar disk darkening inside the Earth’s umbra are used to reconstruct the distribution of atmospheric aerosol and water vapor in the upper troposphere along the Earth’s limb. The characteristic features of this distribution in various latitude regions and the correlation between aerosol extinction and total ozone content are pointed out. Data on the absorption of radiation by water vapor are used to calculate the characteristic vertical scale of its distribution in the atmosphere.

Milliarcsecond angular resolution of reddened stellar sources in the vicinity of the Galactic center

We present lunar occultation observations obtained in August 2006 with the recently demonstrated burst mode of the ISAAC instrument at the ESO VLT. The results presented here follow the previously reported observations carried out in March 2006 on a similar but unrelated set of sources. Interstellar extinction in the inner regions of the galactic bulge amounts to tens of magnitudes at visual wavelengths. As a consequence, the majority of sources in that area are poorly studied and large numbers of potentially interesting sources such as late-type giants with circumstellar shells, stellar masers, infrared stars, remain excluded from the typical investigations which are carried out in less problematic regions. Also undetected are a large numbers of binaries. By observing LO events in this region, we gain the means to investigate at least a selected number of sources with an unprecedented combination of sensitivity and angular resolution. The LO technique permits to achieve mas resolution with a sensitivity of K~12mag at a very large telescope. We have used the opportunity of a favorable passage of the Moon over a crowded region in the general direction of the Galactic Center to observe 78 LO events of heavily reddened stellar sources. We have detected six new binary and one triple star, with typical projected separation of ~0.01. We have also detected the compact circumstellar emission around one maser and one central star of a planetary nebula. Additionally we have measured the diameter and/or circumstellar shell of two carbon stars and other IR sources. In agreement with our previous result, we conclude that lunar occultations in fast read-out mode on a detector subwindow at an 8m-class telescope can achieve an angular resolution close to 0.001 with a sensitivity K~12mag.

Milliarcsecond angular resolution of reddened stellar sources in the vicinity of the Galactic center

Context. We present lunar occultation (LO) observations obtained in August 2006 with the recently demonstrated burst mode of the ISAAC instrument at the ESO VLT. The results presented here follow the previously reported observations carried out in March 2006 on a similar but unrelated set of sources. Aims. Interstellar extinction in the inner regions of the galacti c bulge amounts to tens of magnitudes at visual wavelengths. As a consequence, the majority of sources in that area are poorly studied and large numbers of potentially interesting sources such as latetype giants with circumstellar shells, stellar masers, inf rared stars, remain excluded from the typical investigatio ns which are carried out in less problematic regions. Also undetected are a large numbers of binaries. By observing LO events in this region, we gain the means to investigate at least a selected number of sources with an unprecedented combination of sensitivity and angular resolution. Methods. The LO technique permits to achieve milliarcsecond resolution with a sensitivity of K≈12 mag at a very large telescope. We have used the opportunity of a favorable passage of the Moon over a crowded region in the general direction of the Galactic Center to observe 78 LO events of heavily reddened stellar sources. Results. We have detected six new binary and one triple star, with typical projected separation of≈ 0. 01. We have also detected the compact circumstellar emission around one maser and one central star of a planetary nebula. Additionally we have measured the diameter and/or circumstellar shell of two carbon stars and other IR sources. Conclusions. We have used the upper limits on the size of about 60 unresolved or marginally resolved sources to establish the performance of the method. In agreement with our previous result, we conclude that lunar occultations in fast read-out m ode on a detector subwindow at an 8 m-class telescope can achieve an angular resolution close to 0. ′′ 001 with a sensitivity K≈ 12 mag.