• Journal of Astronomy and Space Sciences
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• v.25 no.2
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• pp.71-76
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• 2008

Study of terrestrial impact craters is important not only in the field of the solar system formation and evolution but also of the Galactic astronomy. The terrestrial impact cratering record recently has been examined, providing short- and intermediate-term periodicities, such as, ${\sim}26$ Myrs, ${\sim}37$ Myrs. The existence of such a periodicity has an implication in the Galactic dynamics, since the terrestrial impact cratering is usually interpreted as a result of the environmental variation during solar orbiting in the Galactic plane. The aim of this paper is to search for a long-term periodicity with a novel method since no attempt has been made so far in searching a long-term periodicity in this research field in spite of its great importance. We apply the cepstrum analysis method to the terrestrial impact cratering record for the first time. As a result of the analysis we have found noticeable peaks in the Fourier power spectrum appear ing at periods of ${\sim}300$ Myrs and ${\sim}100$ Myrs, which seem in a simple resonance with the revolution period of the Sun around the Galactic center. Finally we briefly discuss its implications and suggest theoretical study be pursued to explain such a long-term periodicity.

• The Bulletin of The Korean Astronomical Society
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• v.46 no.1
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• pp.54.4-55
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• 2021

The Seoul National University AGN Monitoring Project (SAMP) is a welldesigned long-term AGN reverberation mapping project. SAMP focuses on the luminous AGNs out to z~0.5 with relative long time lags between AGN continuum and broad emission lines and aims to probe the high-end of the AGN broad line region (BLR) size-luminosity (R-L) relation. The pilot observations started in October 2015 for 100 AGNs to confirm the variability and the H and [O III] emission line strengths. Based on the initial variability test, 48 quasars has been continued spectroscopic monitoring since Feb. 2016 with Lick 3m and MDM 2.4m telescopes with a cadence of ~20 days. Supporting photometric monitoring in B and V band was conducted at multiple facilities including the MDM 1.3m, LOAO, and DOAO telescopes with a cadence of ~10 days. By the time of Feb. 2021, we have obtained five years spectroscopic and photometric data. More than 30 AGNs shows significant variability in five-year baseline and 16 of them show well detected lags between B-band and H. Here, we report some examples of SAMP light curves and lag detections using the first five-year data as well as the location of our 16 targets in the AGN BLR R-L relation. These measurements are consistent with the existing R-L relation and located at the high-end. With the coming data, SAMP are hopefully to report more AGNs with well detected lags. Our results demonstrate the general feasibility and potential of long-term reverberation project with medium cadence for luminous AGNs.

• Journal of Astronomy and Space Sciences
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• v.34 no.2
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• pp.161-170
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• 2017

In this study, the characteristics of a horizontal sundial from the Joseon Dynasty were investigated. Korea's Treasure No. 840 (T840) is a Western-style horizontal sundial where hour-lines and solar-term-lines are engraved. The inscription of this sundial indicates that the latitude (altitude of the north celestial pole) is $37^{\circ}$ 39', but the gnomon is lost. In the present study, the latitude of the sundial and the length of the gnomon were estimated based only on the hour-lines and solar-term-lines of the horizontal sundial. When statistically calculated from the convergent point obtained by extending the hour-lines, the latitude of this sundial was $37^{\circ}$ $15^{\prime}{\pm}26^{\prime}$, which showed a 24' difference from the record of the inscription. When it was also assumed that a convergent point is changeable, the estimation of the sundial's latitude was found to be sensitive to the variation of this point. This study found that T840 used a vertical gnomon, that is, perpendicular to the horizontal plane, rather than an inclined triangular gnomon, and a horn-shaped mark like a vertical gnomon is cut on its surface. The length of the gnomon engraved on the artifact was 43.1 mm, and in the present study was statistically calculated as $43.7{\pm}0.7mm$. In addition, the position of the gnomon according to the original inscription and our calculation showed an error of 0.3 mm.

• Journal of Astronomy and Space Sciences
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• v.9 no.1
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• pp.89-96
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• 1992

UBV observations of VV Cep were made in 1988-1992 as a part of the Ten-year Observing Program for Long Period Eclipsing Binary Stars(1982-1992) at Yonsei University Observatory. In addition to these the observations in the longer passbands in R and I are also made in the 1991-1992 season at the same observatory. Atmospheric extinction coefficinets determined by a comparison star 20 Cap for B and V each night have been deduced a linear relation, $K_B$=0.159+1.066kV. In this paper, light curves of this star in BVRI passbands for the 1991-1992 season only are presented. Two periodic light variations of both long-term and short-term are found as for 90 days and 20 days, respectively.

• Journal of The Korean Astronomical Society
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• v.53 no.6
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• pp.125-138
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• 2020

We determine the observing sites for eclipses of large magnitude recorded in ancient Chinese chronicles from 200 BCE to 900 CE, by adopting the difference between terrestrial time and universal time, ΔT, given by Morrison & Stephenson (2004). The records of solar eclipses with large magnitude are divided into four groups in accordance with the historical variations of the capital cities of ancient Chinese dynasties. We determine areas in which all the eclipses in each group, with an eclipse magnitude larger than a certain threshold value, could be observed. We find that these areas coincide with the historical capitals, which agrees with the general idea that the solar eclipses were observed at the capital of each dynasty. This result also verifies the ΔT values during the period from 100 BCE to 400 CE, during which historical records of eclipses are so rare that the ΔT values can only be obtained by interpolating the long-term data. Moreover, we show that the eclipses described by the term Ji in East-Asian history are not all total eclipses; their mean magnitude is 0.96 ± 0.04. We find that complementary expressions, such as dark daytime and appearance of stars during the eclipse, strengthen the possibility that eclipses described by the term Ji were total. We also provide quantitative definitions for expressions such as 'being not complete and like a hook', 'being almost complete', 'visibility of stars during the eclipse', and 'darkness during an eclipse.' The literal meanings of these expressions are in agreement with the recent physical modeling of sky brightness during total eclipses provided by Können & Hinz (2008).

• The Bulletin of The Korean Astronomical Society
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• v.46 no.2
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• pp.49.2-49.2
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• 2021

We present the results of long-term simultaneous monitoring observations (~ 12 years) of H²O (22 GHz) maser and several vibrationally excited lines of SiO J = 1-0, 2-1, 3-2 masers (43, 86, 129 GHz) carried out with the 21-m antennas of the Korean VLBI Network (KVN) toward a sample of three AGB stars (RT Vir, RR Aql, IRC-10151) that are believed to be semiregular variable star, Mira variable star, and OH/IR star, respectively, according to a sequential evolutionary phase of AGB star. A total 10 transitions were observed, of which we detected H²O, SiO 𝝊 = 1 and 2, J = 1-0, SiO 𝝊 = 1, J = 2-1 and J = 3-2 maser lines in all three target objects, depending on the observational epochs. In this study, we scrutinize the evolutionary traits of each target object based on the maser line profiles, flux/velocity variations, and phase lags with the optical light curves. The IRAS two color diagram and the infrared spectral energy distributions (SEDs) in the wavelength range from 1.2 to 240 ㎛ of three observed sources were also analyzed.

• Bulletin of the Korean Space Science Society
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• 2003.05b
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• pp.60-60
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• 2003

No Abstract, See Full Text

• The Bulletin of The Korean Astronomical Society
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• v.33 no.2
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• pp.34.1-34.1
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• 2008

• The Bulletin of The Korean Astronomical Society
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• v.37 no.2
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• pp.115.2-115.2
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• 2012

In this study we have used the data of various instruments onboard the THEMIS spacecraft to study the characteristics of the outer radiation belt during the ascending phase of solar cycle 24. The most astonishing result is that we discovered four long-term (a month or so) periods during which the belt has nearly disappeared. The first disappearance started late 2008, followed by reappearance in ~a month, and three more similar events repeated until early 2010 when the belt has reappeared. This is well revealed at 719 keV electrons, which is the currently available uppermost energy channel from the THEMIS SST observation, but also seen at even lower energies. Overall consistent features were confirmed using the NOAA-POES observations. The vanished belt periods are associated with extremely weak solar wind conditions, low geomagnetic disturbances (in terms of Kp and AE/AL), greatly suppressed wave (ULF and chorus) activities, greatly reduced storm and substorm activities (little source particle supply), and expanded plasmapause locations. The direct observations of such events shed light on the fundamental question of the origin of the radiation belt, which is the main focus of our presentation.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.116.2-116.2
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• 2014

As Korean engineering contribution to ALMA enhancement, development of focal plane arrays(FPAs) for the total power array in ALMA compact array has been projected mainly to increase mapping speed in interferometric multi-pointing observation(mosaicking). To tackle engineering issues expected in order to be compatible with the existing ALMA receivers, we plan to develop a prototype 300-500 GHz heterodyne FPA system including a software spectrometer using GPU clusters for ASTE(Atacama Submillimeter Telescope Experiment) telescope by 2017.