Comparative analysis of sky quality and meteorological variables during the total lunar eclipse on 14-15 April 2014 and their effect on qualitative measurements of the Bortle scale
MManuscript for
Revista Mexicana de Astronom´ıa y Astrof´ısica (2020)
COMPARATIVE ANALYSIS OF SKY QUALITY ANDMETEOROLOGICAL VARIABLES DURING THETOTAL LUNAR ECLIPSE ON 14-15 APRIL 2014 ANDTHEIR EFFECT ON QUALITATIVEMEASUREMENTS OF THE BORTLE SCALE
C. G´oez Ther´an and S. Vargas Dom´ınguez Draft version: September 18, 2020
RESUMENEs factible que un eclipse total de Luna tenga influencia en la variaci´on depar´ametros f´ısicos en una zona ambiental, espec´ıficamente en el brillo del cielo,humedad y temperatura. Durante el eclipse del 14 y 15 de abril de 2014, estospar´ametros se midieron a trav´es de un fot´ometro y de una estaci´on meteo-rol´ogica. Los resultados obtenidos permiten hacer comparaciones de manerapr´actica sobre las condiciones ´optimas para el trabajo de astronom´ıa observa-cional en el desierto de la Tatacoa (Colombia) y as´ı catalogarlo como un lugarapto para realizar observaciones astron´omicas nocturnas. Esta investigaci´onpermite determinar hasta cierto punto, la idoneidad de este lugar para llevar acabo trabajos astron´omicos e investigaciones dentro del rango ´optico. De estamanera, los cambios registrados durante el fen´omeno astron´omico permitieronla clasificaci´on del cielo con base en la escala de Bortle.ABSTRACTA total lunar eclipse is plausible to have an influence on the variation of someenvironmental physical parameters, specifically on the conditions of the skybrightness, humidity and temperature. During the eclipse on 14 th -15 th April2014, these parameters were measured through a photometer and a weatherstation. The obtained results allow the comparison, practically, of the optimalconditions for observational astronomy work in the Tatacoa desert and there-fore to certify it as suitable perfect place to develop night sky astronomicalobservations. This investigation determined, to some extent, the suitabilityof this place to carry out astronomical work and research within the opti-cal range. Thus, the changes recorded during the astronomical phenomenonallowed the classification of the sky based on the Bortle Scale.
Key Words: atmospheric effects — eclipses — methods: observational — sitetesting CINDES Research Group, Department of Engineering, Universidad Libre. Research Group CENIT, Universidad Nacional de Colombia. Olympiades Office, Astronomy and Astrophysics, Universidad Antonio Nari˜no. Universidad Nacional de Colombia - Sede Bogot´a - Facultad de Ciencias - ObservatorioAstron´omico Nacional. a r X i v : . [ a s t r o - ph . I M ] S e p G ´OEZ THER ´AN & VARGAS DOM´INGUEZ1. INTRODUCTION AND MOTIVATIONThrough the history of our species, the universe populated with stars,galaxies and other celestial objects has been visible in the darkness of thenight sky, inspiring questions about the cosmos and our relationship to it,and therefore star gazing has been crucial not only for astronomy but alsofor literature, arts, philosophy and multiple human activities. Nevertheless,the technological advances of our society added to world population growthresulted in light pollution in most living areas where people do not have theopportunity to enjoy the night sky. Furthermore it has also been demon-strated issues on human health caused by artificial lighting and impacts onthe behavior of plants and animals. For years now, astronomers have high-lighted the negative consequences of the current situation and are promotingthe conservation of the dark sky through multiple initiatives (Dark Skies An-nual Report 2019). Millions of astronomy enthusiasts will benefit from theseefforts that were not an issue for our ancestors.In Colombia, the interest in astronomy dates back to the end of the 17thcentury according to some recent evidence found in a historical manuscriptdescribing some ideas about the universe from Antonio S´anchez de Cozar, ahumble priest whose work, drafted between 1676 and 1696, can be consid-ered as the first original study of astronomy written in Colombia (Portilla &Moreno 2019). Some time later, Jos´e Celestino Mutis, a big pioneer of scienceand knowledge, leaded a botanic expedition and founded an astronomical ob-servatory as part of his initiative to spread science in the region. Erected in1803, the National Astronomical Observatory of Colombia is the first astro-nomical observatory that was built in the Americas. The work by Arias DeGreiff (1987) details the different historical and cultural aspects related to thedevelopment of astronomy in Colombia until the 20th century.Even though the sky conditions for astronomical observations in the op-tical range are not very suitable in Colombia, there is a great interest fromamateur astronomers, and still quite a few places in which people can enjoythe wonders of the night sky. Many careers of professional astronomers arethe result of their youth interest in astronomy while being part of amateurgroups. This was the case of the geographer, meteorologist and astrometryspecialist, William Cepeda Pe˜na, an important disseminator of astronomysince 1965. In the 70s he began astronomical studies in Colombia, such as astar occultation by the Moon (1988-1995), the movement of the Sun using theHipparcos catalog (2002) and, in particular, related to eclipses observations,e.g., annular solar eclipse (1995), annular eclipse (1980), total solar eclipse(1998), works grouped in Cepeda (2006).With a significant valuable legacy behind, we continue investigating oneclipses observed from Colombia. In this work, we report on information col-lected, analyzed and interpreted during the total lunar eclipse on April 14 th and 15 th in 2014, one of the four total lunar eclipses visible during 2014 and2015 from Colombia. Particularly, we complement the work by G´oez Ther´an& Vargas Dom´ınguez (2016) doing a deeper analysis of the data acquired fromKY QUALITY DURING A TOTAL LUNAR ECLIPSE 3the Tatacoa Desert, a valuable natural location in the country, by measuringmeteorological variables and the sly brightness to study the variation of theseparameters during the occurrence of the eclipse. This work presents a sta-tistical study and comparative analysis of sky quality to establish qualitativemeasurements of the Bortle scale.2. METHODOLOGYAn eclipse takes place when three celestial bodies, i.e. the Earth, the Sunand the Moon, line up or get closer to be. A total lunar eclipse is a well-known astronomic event that occurs when planet Earth stands in the waybetween the Sun and the Moon, allowing our natural satellite to enter intothe cone of shadow that casts the Earth, thus, getting darker and turning intoa characteristic russet color during the total occultation. At the totality phase,the Earth obstructs most of the solar rays that arrive at the Moon, which hasto happen during full-moon phase, but some portion (mainly the red part ofthe visible light spectrum) is deflected by the Earth’s atmosphere and hits thelunar surface making the moon getting a brownish-red color from the typicalyellow light. The effect and dye depends also on the atmospheric conditions ofour planet (clouds, dust particles, clouds of gas due to the volcanic eruptions,fires and other gas emissions close to the location where the eclipse observationis carried out), and on the distance between the Moon and the center of theumbra. Figure 1 shows the visibility map for the total lunar eclipse on 14 th -15 th April 2014 and the different phases of this type of eclipse, which is farmore common than a total solar eclipse. The total lunar eclipse was followedfrom the astronomical observatory of Tatacoa and other places in Colombia,with monitoring that depended terribly on the local meteorological conditionsfrom the observing points.In this work we aim at measuring the variations of sky quality (Cinzano2005) and main meteorological variables during the total lunar eclipse onApril 14 th and 15 th of 2014 and their effect on qualitative measurementsof the Bortle scale. Tracking changes in environmental parameters and skybrightness prior to and during the eclipse occurrence are used to classify thesky according to the official global scales.3. OBSERVATION AND MEASUREMENTS3.1 . Location The observation of the total lunar eclipse on 14 th -15 th April, 2014 wascarried out from the astronomical observatory of the Tatacoa, near the townof Villavieja in the Department of Huila, as shown in figure 2. The geographiccoordinates of this location are 3 ◦ (cid:48) North and 75 ◦ (cid:48) West.The Tatacoa Desert is one of the most exotics landscape in Colombiangeography with an area of 370 km . This dry tropical forest is the secondlargest arid zone in the country after the Guajira Peninsula, with geomorphic G ´OEZ THER ´AN & VARGAS DOM´INGUEZ Fig. 1. Visibility map of the total lunar eclipse on 14 th -15 th April 2014 (left panel).The red X marks the location in Colombia where the eclipse was observed to carryout this research work. All different stages of a total lunar eclipse are also shown inthe figure (right panel). Modified from NASA Reference Publication 1178.Fig. 2. Map of Colombia with a colored reddish area highlighting the Departmentof Huila (left panel) and a night sky satellite image (right panel) showing someilluminated areas in the same Department corresponding to small towns and thecity of Neiva (white arrow). The location of Tatacoa Desert is encircled in red. principally of estoraques and cavarcas , among others. We decided to selectthis place for the follow up of the eclipse, motivated by the success of previousvisits pursuing astronomical observations of the Milky Way, globular clusters,nebulae and meteor showers, besides the reasonably good average conditionsof the location in terms of clear nights, low clouds and water vapor (G´oezTher´an 2015; G´oez Ther´an & Vargas Dom´ınguez 2016; Pinzon 2016).3.2 . Calibration
Before the acquisition of scientific data, the equipment was tested andprepared. The installation, assembling and calibration process of the differentdevices and sensors began at 00:40 UTC, i.e., photometers, weather stations,KY QUALITY DURING A TOTAL LUNAR ECLIPSE 5TABLE 1STAGES OF THE TOTAL LUNAR ECLIPSESTAGE Time (UTC)P1 04:53:37U1 05:58:19U2 07:06:47 (Beginning of Totality Phase)MAX 07:45:40 (Totality)U3 08:24:35 (Completion of Totality PhaseU4 09:33:04P4 10:37:37telescopes, computers and CCD cameras, in order to ensure an optimal andreliable data collection, before the eclipse begin. Figure 3 displays the maincomponents of the equipment. 3.3 . Data
Data acquisition for scientific measurements started right after the firstcontact, P1, at 4:53:37 UTC. Contact times and all different phases of theeclipse are listed in Table 1. Measurements of sky brightness were takenpointing the SQM to the zenith in all cases.We use a photometer called SQM-LE to measure the brightness of thesky , a weather station, WMR200 Davis Instruments Pro with wireless sen-sors, as shown in figure 3. Furthermore, three CCD cameras for Celestronastrophotography were employed in order to record all stages of the eclipse(P1, U1, U2, U3 and U4) by using a calibrated Meade ETX-90 telescope(two-star method) and with a permanent monitoring of the Moon to obtainthe alt-azimuth values during the data collection period.The photometer acquires data on a scale of mag.arsec − . Figure 4 ex-emplifies the use of this instrument and the way we refer to the magnitudeas describing the brightness of an object, i.e. the amount of light strikingthe sensor. Sky brightness, humidity, pressure and horizontal coordinates ofthe Moon were registered every 5 minutes and tabulated for the subsequentstatistical analysis using the software package SPSS (Argyrous 2005).Table 2 presents all the collected data during the observing session.4. ANALYSIS AND RESULTSFigure 5 displays the temporal variation of sky brightness as plotted fromall acquired data. The multiple total lunar eclipse stages are identified at thecorresponding times (see Table 1). The plot shows clear periods of stability, G ´OEZ THER ´AN & VARGAS DOM´INGUEZTABLE 2DATA
DATA LOCAL TIME UTC SQM (mag.arcsec − ) TEMPERATURE ( ◦ C) PRESSURE (mbar) MOON HEIGHT ( ◦ ) HUMIDITY (%) STAGE1 19:40 0:40 17.11 32.3 1000 26.6 43 CALIBRATION OF EQUIPMENT AND SENSORS2 19:45 0:45 17.04 32.2 1000 27.8 43 CALIBRATION OF EQUIPMENT AND SENSORS3 19:50 0:50 16.94 32 1000 28.9 43 CALIBRATION OF EQUIPMENT AND SENSORS4 19:55 0:55 16.92 31.9 1000 30.1 43 CALIBRATION OF EQUIPMENT AND SENSORS5 20:00 1:00 16.95 31.8 1000 31.3 43 CALIBRATION OF EQUIPMENT AND SENSORS6 20:05 1:05 16.9 31.8 1000 32.3 43 CALIBRATION OF EQUIPMENT AND SENSORS7 20:10 1:10 16.82 31.7 1001 33.7 43 CALIBRATION OF EQUIPMENT AND SENSORS8 20:15 1:15 17 31.6 1001 34.9 43 CALIBRATION OF EQUIPMENT AND SENSORS9 20:20 1:20 17.2 31.6 1001 36 43 CALIBRATION OF EQUIPMENT AND SENSORS10 20:25 1:25 17.15 31.6 1001 37.2 43 CALIBRATION OF EQUIPMENT AND SENSORS11 20:30 1:30 17.18 31.5 1001 38.4 43 CALIBRATION OF EQUIPMENT AND SENSORS12 20:35 1:35 17.03 31.5 1001 39.6 43 CALIBRATION OF EQUIPMENT AND SENSORS13 20:40 1:40 16.72 31.3 1001 40.8 43 CALIBRATION OF EQUIPMENT AND SENSORS14 20:45 1:45 16.5 31.2 1001 41.9 43 CALIBRATION OF EQUIPMENT AND SENSORS15 20:50 1:50 16.35 31.1 1001 43.1 43 CALIBRATION OF EQUIPMENT AND SENSORS16 20:55 1:55 16.29 31 1002 44.3 43 CALIBRATION OF EQUIPMENT AND SENSORS17 21:00 2:00 16.71 31 1002 45.4 43 CALIBRATION OF EQUIPMENT AND SENSORS18 21:05 2:05 16.76 30.9 1002 46.6 42 CALIBRATION OF EQUIPMENT AND SENSORS19 21:10 2:10 16.73 30.8 1002 47.8 42 CALIBRATION OF EQUIPMENT AND SENSORS20 21:15 2:15 16.6 30.6 1002 48.9 42 CALIBRATION OF EQUIPMENT AND SENSORS21 21:20 2:20 16.51 30.5 1002 50.1 42 CALIBRATION OF EQUIPMENT AND SENSORS22 21:25 2:25 16.49 30.2 1003 51.2 42 CALIBRATION OF EQUIPMENT AND SENSORS23 21:30 2:30 16.39 30 1003 52.4 42 CALIBRATION OF EQUIPMENT AND SENSORS24 21:35 2:35 16.39 29.8 1003 53.5 41 CALIBRATION OF EQUIPMENT AND SENSORS25 21:40 2:40 16.27 29.7 1003 54.7 41 CALIBRATION OF EQUIPMENT AND SENSORS26 21:45 2:45 16.27 29.6 1003 55.8 41 CALIBRATION OF EQUIPMENT AND SENSORS27 21:50 2:50 15.52 29.6 1003 56.9 41 CALIBRATION OF EQUIPMENT AND SENSORS28 21:55 2:55 15.55 29.6 1003 58 41 CALIBRATION OF EQUIPMENT AND SENSORS29 22:00 3:00 15.57 29.6 1003 59.2 41 CALIBRATION OF EQUIPMENT AND SENSORS30 22:05 3:05 15.52 29.7 1003 60.3 41 CALIBRATION OF EQUIPMENT AND SENSORS31 22:10 3:10 15.79 29.7 1003 61.4 41 CALIBRATION OF EQUIPMENT AND SENSORS32 22:15 3:15 15.91 29.8 1003 62.4 41 CALIBRATION OF EQUIPMENT AND SENSORS33 22:20 3:20 15.84 29.8 1003 63.5 41 CALIBRATION OF EQUIPMENT AND SENSORS34 22:25 3:25 15.57 29.9 1003 64.6 41 CALIBRATION OF EQUIPMENT AND SENSORS35 22:30 3:30 15.50 29.9 1003 65.6 41 CALIBRATION OF EQUIPMENT AND SENSORS36 22:35 3:35 15.44 30.0 1003 66.6 41 CALIBRATION OF EQUIPMENT AND SENSORS37 22:40 3:40 15.38 30.0 1003 67.6 42 CALIBRATION OF EQUIPMENT AND SENSORS38 22:45 3:45 15.32 30.0 1003 68.6 42 CALIBRATION OF EQUIPMENT AND SENSORS39 22:50 3:50 15.26 30.1 1003 69.6 42 CALIBRATION OF EQUIPMENT AND SENSORS40 22:55 3:55 15.20 30.1 1003 70.5 42 CALIBRATION OF EQUIPMENT AND SENSORS41 23:00 4:00 15.14 30.2 1003 71.4 41 CALIBRATION OF EQUIPMENT AND SENSORS42 23:05 4:05 15.08 30.2 1003 72.2 42 CALIBRATION OF EQUIPMENT AND SENSORS43 23:10 4:10 15.02 30.3 1003 73.1 42 CALIBRATION OF EQUIPMENT AND SENSORS44 23:15 4:15 14.95 30.3 1003 73.8 41 CALIBRATION OF EQUIPMENT AND SENSORS45 23:20 4:20 14.89 30.4 1003 74.5 41 CALIBRATION OF EQUIPMENT AND SENSORS46 23:25 4:25 14.83 30.4 1003 75.1 42 CALIBRATION OF EQUIPMENT AND SENSORS47 23:30 4:30 14.77 30.5 1003 75.7 42 CALIBRATION OF EQUIPMENT AND SENSORS48 23:35 4:35 14.71 30.5 1003 76.1 42 CALIBRATION OF EQUIPMENT AND SENSORS49 23:40 4:40 14.65 30.5 1003 76.5 43 CALIBRATION OF EQUIPMENT AND SENSORS50 23:45 4:45 14.59 30.6 1003 76.8 43 CALIBRATION OF EQUIPMENT AND SENSORS51 23:50 4:50 14.53 30.6 1003 76.9 43 P152 23:55 4:55 14.47 30.7 1003 76.9 43 P153 0:00 5:00 14.40 30.7 1003 76.9 43 P154 0:05 5:05 14.34 30.8 1003 76.7 43 P155 0:10 5:10 14.28 30.8 1003 76.4 43 P156 0:15 5:15 14.22 30.9 1003 76 43 P157 0:20 5:20 14.16 30.9 1003 75.5 43 P158 0:25 5:25 14.10 31.0 1003 74.9 43 P159 0:30 5:30 14.04 31.0 1003 74.3 43 P160 0:35 5:35 13.98 31.0 1003 73.6 43 P161 0:40 5:40 13.91 31.1 1003 72.8 43 P162 0:45 5:45 13.85 31.1 1003 72 43 P163 0:50 5:50 14.28 28.1 1003 71.1 43 U164 0:55 5:55 14.36 28.3 1003 70.2 43 U165 1:00 6:00 14.42 28.3 1003 69.3 43 U166 1:05 6:05 14.58 28.5 1003 68.3 43 U167 1:10 6:10 14.62 28.6 1003 67.3 43 U168 1:15 6:15 14.97 28.8 1003 66.3 43 U169 1:20 6:20 15.33 29 1003 65.3 42 U170 1:25 6:25 16.69 28.9 1003 64.2 42 U171 1:30 6:30 18.33 28.8 1003 63.2 42 U172 1:35 6:35 18.51 28.8 1003 62.1 40 U173 1:40 6:40 18.74 30.2 1003 61 40 U174 1:45 6:45 18.81 32.2 1003 59.9 40 U175 1:50 6:50 19.56 32.2 1003 58.8 40 U176 1:55 6:55 19.73 30.2 1003 57.7 40 U177 2:00 7:00 19.85 30.2 1003 56.6 40 U178 2:05 7:05 20.95 30.3 1003 55.4 40 U179 2:10 7:10 21.03 30.4 1002 54.3 39 U280 2:15 7:15 21.11 29.8 1002 53.2 40 U281 2:20 7:20 21.13 29.8 1002 52 40 U282 2:25 7:25 21.11 29.7 1002 50.9 40 U283 2:30 7:30 21.2 29.3 1002 49.7 40 U284 2:35 7:35 21.14 29.6 1002 48.6 41 U285 2:40 7:40 21.15 29.8 1002 47.4 40 U286 2:45 7:45 21.06 29.7 1002 46.2 41 MAX87 2:50 7:50 21.03 29.4 1002 45.1 41 U388 2:55 7:55 21.19 29.2 1002 43.9 42 U389 3:00 8:00 21.24 29 1002 42.7 43 U390 3:05 8:05 21.22 29.1 1002 41.6 45 U391 3:10 8:10 21.15 28.6 1003 40.4 45 U392 3:15 8:15 21.22 28.4 1003 39.2 46 U393 3:20 8:20 21.22 28.3 1003 38.1 46 U394 3:25 8:25 21.26 28.1 1002 36.9 44 U395 3:30 8:30 21.19 28.4 1002 35.7 46 U496 3:35 8:35 21.12 28.7 1002 34.5 47 U497 3:40 8:40 20.59 28.9 1001 33.3 46 U498 3:45 8:45 20.01 29.3 1001 32.2 47 U499 3:50 8:50 19.39 29.2 1001 31 47 U4100 3:55 8:55 18.9 29.2 1001 29.8 47 U4101 4:00 9:00 18.74 29.2 1001 28.6 47 U4102 4:05 9:05 18.34 29.1 1001 27.4 41 U4103 4:10 9:10 18.04 29.1 1002 26.2 42 U4104 4:15 9:15 17.88 28.7 1002 25.1 42 U4105 4:20 9:20 17.79 28.7 1002 23.9 42 U4106 4:25 9:25 17.71 28.5 1004 22.7 42 U4107 4:30 9:30 17.67 28.5 1004 21.5 42 U4108 4:35 9:35 17.63 28.3 1004 20.3 42 U4 KY QUALITY DURING A TOTAL LUNAR ECLIPSE 7
Fig. 3. Main equipment and sensors that were used to measure sky quality andmeteorological variables. See the text for details.Fig. 4. Sketch showing the use of a sky quality meter (SQM) to measure the amountof light striking the sensor. Numbers in every panel correspond to the value of skybrightness measured in units of mag. arcsec − increase and decrease in the sky brightness, as follows: from P1 to U1 thereare not many variations, but starting from U1 there is a continuous incrementending at U2. This moment marks the beginning of the totality phase, inwhich the sky brightness remains steady for all the period, that gets extendeduntil U3 is reached. Soon after, the trend of the plot reveals, for the first time,a diminishing of sky brightness.Detailed analysis of figure 5 allows to establish an average SQM valueof 21.15 mag.arcsec − during the totality phase of the eclipse that can beassociated to a Bortle Class 4 of sky quality, as itemized in Table 3 with theinternational measurement scale. Up to this moment, and from the beginning G ´OEZ THER ´AN & VARGAS DOM´INGUEZ Fig. 5. Plot displaying the time evolution of the sky brightness during the totallunar eclipse observed in Colombia on 14-15 April 2014. Upper image shows thedifferent stages as listed in Table 1). Note that times are shown in UTC. of the eclipse, SQM values increased and therefore the sky quality improved(Rabaza et al. 2010), meaning that during this period, the sky can be classifiedin a Bortle class ranging from 1 to 8.9.Figure 6 plots the temporal evolution of different physical parameters. Thevalue of Moon height and sky brightness measurements are also included. Inparticular we are interested in the variation of pressure, humidity and tem-perature during the occurrence of the eclipse. As expected, there are notsignificant or abnormal changes in these variables for about 5 hours (from00:40 to 05:40 UTC) before phase of the eclipse (U1). There is a slight de-crease in the sky quality (SQM) associated with the rising of the Moon in thesky (from ∼ ◦ to ∼ ◦ ). From U1 phase until U2 is reached (07:10 UTC),temperature and humidity exhibit substantial changes, that stay up to phaseU3 (08:35 UTC), corresponding to the finishing time for totality. Humidityevidences a peaking value soon after, while maximum temperature of 32.2 ◦ isreached closer to U2. On the other hand, minimum temperature occurs in U3,about 40 minutes after the maximum (MAX) was observed. After totality,from U3 to U4, changes of these variables are still present, with an overallKY QUALITY DURING A TOTAL LUNAR ECLIPSE 9TABLE 3SCALE MEASUREMENTS OF THE SKY QUALITYSky brightness (mag.arcsec − ) Bortle Class > < ◦ and the azimuthwas 251 ◦ from the observation point.Statistical analysis was carried out in order to study the behavior of alldifferent measurements before and during the eclipse, as shown in figure 7from the values listed in table 4. It also includes de statistics for all theacquired data. 5. CONCLUSIONSThe expedition to the Tatacoa Desert, in Colombia, to observe the totallunar eclipse on 14 th -15 th April 2014, was successfully accomplished. Theteam managed to complete 9 hours of continuous observation and registrationof meteorological variables and sky brightness. The month of April is char-acterized by the occurrence of high rainfalls in the Colombia. Fortunatelythe conditions presented during the total lunar eclipse in the selected loca-tion allow the expedition team to observe and to record, that was not thecase for some other observing locations with failed attempts. Nevertheless,from 09:35 UTC onwards, cloudiness presented at the observation area in theTatacoa did not allowed to take measurements during the last part of the as-tronomical event, in the final penumbral phase. These conditions might haveinfluenced the humidity and temperature values acquired close to the ending0 G ´OEZ THER ´AN & VARGAS DOM´INGUEZ
Fig. 6. Temporal evolution of physical parameters (Moon height, SQM, temperature,humidity, pressure) during the total lunar eclipse observed in Colombia on 14 th -15 th April 2014. The red arrows on top stand as a reference of the times pointed by thebottom arrows, for stages P1, U1, U2, MAX, U3 and U4, respectively. Note thattimes are shown in UTC. phase of the partiality, and future work will have to manage to registered thisdata for the whole duration of the eclipse after P4 was reached.As explained in Section 4, it was possible to evidence changes in thesevariables during the occurrence of the eclipse, except pressure measurementswhich were very stable for all recorded data. Particularly for the totalityKY QUALITY DURING A TOTAL LUNAR ECLIPSE 11
Fig. 7. Statistical box plots for several physical parameters before the occurrenceof the eclipse (BEFORE), during the eclipse (ECLIPSE) and for all the acquireddata (ALL) on 14 th -15 th April 2014. Every plot shows the median value (verticalsolid line) , ± σ and total range. Atypical values for humidity are marked by smallcircles. The mean values and standard deviations are listed in Table 4. phase, temperature and humidity experiment significant variations as seen inFigure 6. The corresponding average of sky brightness during totality gives anSQM value of 21.15 mag.arcsec − and conditions are such as to classify the skyof the Tatacoa Desert according to the Bortle scale as 4. The maximum totallunar eclipse took place at 07:45:40 UTC on 15 th of April, 2014, the moment2 G ´OEZ THER ´AN & VARGAS DOM´INGUEZTABLE 4DESCRIPTIVE STATISTICS N MINIMUM MAXIMUM MEAN STANDARD DEVIATIONValue Standard errorALL DATA (00:40 - 09:35 UT)SQM 58 13.85 21.26 18.0598 0.37640 2.86654Temperature 58 28.1 32.2 29.552 0.1383 1.0532Pressure 58 1001 1004 1002.53 0.099 0.754Humidity 58 39 47 42.64 0.284 2.166Height 58 20.37 76.92 52.4621 2.36154 17.98499BEFORE THE ECLIPSE (00:40 - 04:45 UT)SQM 50 14.59 17.20 16.0244 0.11687 0.82636Temperature 50 29.5 32.3 30.618 0.1132 0.8004Pressure 50 1000 1003 1002.16 0.157 1.113Humidity 50 41 43 42.06 0.119 0.843Height 50 26.55 76.73 54.2426 2.21850 15.68714DURING THE ECLIPSE (04:50 - 09:35 UT)SQM 108 13.85 21.26 17.1175 0.23033 2.39368Temperature 108 28.1 32.3 30.045 0.1041 1.0815Pressure 108 1000 1004 1002.36 0.092 0.952Humidity 108 39 47 42.37 0.164 1.705Height 108 20.37 76.92 53.2864 1.62674 16.90561 where the measured temperature was 29.7 ◦ . At the initial phase of the lunareclipse, from P1 to U1, the brightness of the sky slightly increased, thereforereaching a minimum SQM value of 13.85 mag.arcsec − . Once entering theumbral phase, the brightness of the sky begins to decrease considerably asexpected from a reduction in the illumination of the lunar disc. It can beevidenced by a clear trend as shown in Figure 5. At the precise momentwhen the eclipse sequence surpasses to the umbral phase, SQM values start toincrease. From U2 to U3 the values reached a plateau that remains stable foralmost 1.5 hours. It is observed that the lowest value recorded by the SQMduring the entire eclipse (13.85 mag.arcsec − ) corresponds to 8.9 according tothe Bortle scale, whereas the highest value was 4. Sources of error in the skybrightness come from the error introduced by the SQM measurements, whichtypical value is 0.2302, indicating that the samples were taken as accurately aspossible based on the previous calibration of the equipment. Other externalfactors that can influence the local measurements and that should be bearedin mind, are cloudiness, latitude from where it is observed, the environment,vegetation, and the time of year.Close to the moment of maximum altitude registered by the Moon (77 ◦ ),the humidity and pressure do not vary significantly. Humidity is about 43%with a pressure of 1003 mbar. Pressure values keep very stable throughoutthe whole observing session and ∼