Zenith sky brightness, stellar and planets visibility during lunar eclipses at Kottamia Observatory

doi:10.21203/rs.3.rs-3248504/v1

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Zenith sky brightness, stellar and planets visibility during lunar eclipses at Kottamia Observatory

https://doi.org/10.21203/rs.3.rs-3248504/v1

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Observations of sky brightness at zenith during the total lunar eclipse of July 27^th, 2018 and partial lunar eclipse of July 16^th, 2019 were carried out at Kottamia Observatory site in Egypt using a Sky Quality Meter photometer. A variation of the zenith sky brightness with time was observed with average values of 20.3 and 19.44 mag_SQM/arcsec² at the maximum eclipse of the total and partial lunar eclipses, respectively. The visibility of stars and planets during different phases of the two eclipses are discussed.

The present results of the total lunar eclipse are compared with those obtained during the total lunar eclipse of April 14^th, 2014, the total solar eclipse of March 9^th, 2016 and the total lunar eclipse of January 2019. We found similarities in the variation in the sky brightness during the different phases of the eclipses with respect to the different sites and atmospheric conditions.

moon

eclipses

methods: observational

During the lunar eclipse, the sky brightness varies with the different phases of the eclipse. Observations of the sky brightness during the lunar eclipse have been carried out by only few astronomers. The first observations of the night sky brightness (NSB) were carried out by Morton (1983) at Lowell Observatory in Arizona, USA in which he measured the NSB and the color indices during the total lunar eclipse of July 1982 using a reflector telescope aimed at 20° north of the moon. Dvorak (2005) measured the NSB during the total lunar eclipse of October 2004 while observing an eclipsing binary using CCD camera at the Rolling Hills Observatory in Florida, USA. After that, Theran and Dominguez (2016) measured the variation in the zenith sky brightness (ZSB) during the total lunar eclipse of April 2014 using a Sky Quality Meter (SQM) at the Astronomical Observatory of Tatacoa in Colombia. More recently, Birriel and Adkins (2019) measured the sky brightness during the total lunar eclipse of January 2019 using SQM in Morehead, Kentucky, USA. Only a few studies have been conducted to study the variation of the sky brightness during the total eclipse and none were performed to study the effect of the partial lunar eclipse on the NSB.

For the first time, the total lunar eclipse of July 2018 and the partial lunar eclipse of July 2019, have been observed at Kottamia Astronomical Observatory (KAO) in Egypt, using the SQM photometer. Since the effects of the lunar eclipses on the sky brightness are rarely studied, we were motivated to study these effects by comparing the results of our observations to other available observational studies from literature.

The remainder of the paper is organized as follows; the instrument and observation technique are described in Section 2. The results are discussed in Section 3. While in section 4 a comparison between our results and previous studies is displayed. Finally, we give a summary and drew our conclusion in section 5.

2.1 SQM-LU-DL device

The SQM photometer is a silicon photodiode (TAOS TSL237S) covered by HOYA CM-500 IR filter, that is designed to measure the NSB in units of magnitudes per square arcsecond (mag_SQM/arcsec²). It is sensitive to wavelength range 320 - 700 nm and has a field of view of 20°. The SQM-LU-DL device provides data-logging that can be retrieved using a USB connection to the computer. Each SQM-LU-DL device is factory-calibrated and the absolute precision of each device is believed to be ±0.10 mag_SQM/arcsec². The operating temperature range is 40 to 85^oC (Cinzano 2007).

The block diagram of the device and a data-logging system inside are shown in Fig. 1 (as taken from Kolgushkina 2018).

According to the calculations made by Cinzano (2005) to evaluate the relationship between the SQM photometric band and the Commission Internationale de l’Elcairage (CIE) photometric system. The CIE photometric system is represented by three colors (blue, green, red) and hence it can be used to represent the human’s eye color perception. The CIE system of physical photometry specifies procedures for the quantitative evaluation of optical radiation in terms of the spectral luminous efficiency functions of two such standard observers, the SQM is properly calibrated over Illuminant A, which represents typical, domestic, and tungsten-filament lighting.

2.2 Observations of the ZSB at Kottamia Observatory

The KAO, in Egypt, is located at (29° 55´ 35.24" N, 31° 49´ 45.85" E), 470m above the sea-level. We observed two lunar eclipses; the total in 2018 (a duration of 6 hrs) and the partial in 2019 (for 5 hrs and 40 mins). During both eclipses we measured the zenith sky brightness (ZSB) using a SQM (SQM-LU-DL-V model) photometer which recorded our measurements every 2 minutes. The observations of the total lunar eclipse have been started from the beginning of the first penumbra until the end of the second partial eclipse. We stopped our observations at the beginning of the second penumbra due to atmospheric conditions. The observations of the partial lunar eclipse have been started from the beginning of the first penumbra until the end of the second penumbra.

3.1 The total lunar eclipse of July 27^th, 2018

The values of the ZSB (expressed in mag_SQM/arcsec²) obtainedduring the total lunar eclipse of July, 27^th 2018, are listed in Table 1, at the corresponding times of observations (in UT).

Fig. 2 represents the change in the ZSB during four time intervals (a, b, c and d) that represent the first penumbra, first partial eclipse, the totality, and second partial eclipse, respectively.

From the figure, it is clear that the first region (a) shows the variation of the zenith brightness during the first penumbra from 17:20 UT to 18:26 UT. The first penumbra started with the moon at an altitude of about 7° and a zenith brightness of 14.17 mag_SQM /arcsec². The ZSB gradually is decreased until it reaches 18.81 mag_SQM /arcsec² at the beginning of the first partial eclipse.

The second region (b) characterizes the zenith brightness variation during the first partial eclipse which started at 18:26 UT. During this phase, the ZSB decreased gradually to reach 20.49 mag_SQM /arcsec² by 19:28 UT.

During the totality which region (c) signifies, the ZSB remained almost constant at an average value of 20.32 mag_SQM /arcsec² with a variation of less than 0.48 mag_SQM /arsec² during the totality.

Region (d) demonstrates the zenith brightness during the second partial eclipse. At 21:14 UT, the ZSB was 20.01 mag_SQM/arcsec² and it increased gradually reaching 16.71 mag_SQM/arcsec² by the end of this phase with the Moon at an altitude of 40°.

Table 1. Measurements of the time variation of ZSB (in mag_SQM/arcsec²) as a function of lunar altitude during the different phases of the total lunar eclipse of July 27^th, 2018. The time is the universal time (UT)

Region	Eclipse Phase	UT (hr:min)	Brightness (mag_sqm/arcsec²)	Lunar altitude (Degree)
a	First Penumbra	17:20	14.17	7
		17:30	16.35	8
		17:42	17.9	11
		17:54	18.43	13
		18:02	18.52	14
		18:10	18.61	16
		18:16	18.67	17
b	First Partial eclipse	18:26	18.81	18
		18:32	18.93	20
		18:40	19.17	21
		18:44	19.34	22
		19:00	19.98	24
		19:14	20.41	26
		19:22	20.47	27
c	Umbra	19:28	20.49	28
		19:44	20.44	30
		19:54	20.44	31
		20:04	20.44	33
		20:14	20.39	34
		20:24	20.32	35
		20:34	20.07	36
		20:44	20.14	37
		20:54	20.19	38
		21:04	20.34	38
d	Second Partial eclipse	21:14	20.01	39
		21:24	20.14	40
		21:34	19.82	40
		21:46	18.55	40
		21:54	18.18	40
		22:04	17.33	40
		22:14	17.01	40
		22:20	16.71	40

3.2 The partial lunar eclipse of July 16^th and 17^th, 2019

Measurements of the ZSB values during the partial lunar eclipse of 2019 are listed in Table 2, and visualized in Fig. 3 during the eclipse intervals which are indicated by the letters a, b and c, respectively.

In Fig. 3, the first region (a) represents the variation of the zenith brightness during the first penumbra from 18:43 UT to 20:01 UT which started with a lunar altitude of about 19° and a zenith brightness of 17.70 mag_SQM/arcsec². The ZSB gradually increased until 19:27 UT and started to decrease reaching 17.84 mag_SQM/arcsec² at the beginning of the partial eclipse.

During the partial eclipse, region (b), the ZSB remained almost constant with an average value of 19.44 mag_SQM/arcsec² for about 16 minutes. The minimum measured brightness was 19.44 mag_SQM/arcsec² at the middle of the duration of the partial eclipse when the moon was at an altitude of about 37° at 21:31 UT. The ZSB increased gradually till the end of the partial eclipse reaching 17.85 mag_SQM/arcsec² at 22:59 UT.

The third region (c) shows the variation of the zenith brightness during the second penumbra that started at 22:59 UT when the lunar altitude was about 36°. The sky brightness decreased until the end of the second penumbra to about 17.32 mag_SQM/arcsec² at 00:21 UT.

Table 2. Measurements of the time variation of ZSB (in mag_SQM/arcsec²) as a function of lunar altitude during the different phases of the partial lunar eclipse of July 16^th and 17^th, 2019. The time is the universal time (UT)

Region	Eclipse Phase	UT (hr:min)	Brightness (mag_sqm/arcsec²)	Lunar altitude (Degree)
a	First Penumbra	18:43	17.7	19
		18:51	17.66	21
		18:59	17.6	22
		19:13	17.57	24
		19:25	17.55	26
		19:37	17.59	27
		19:49	17.69	29
b	Partial eclipse	20:01	17.84	30
		20:13	18.09	31
		20:25	18.38	32
		20:37	18.69	34
		20:49	18.98	35
		21:01	19.2	35
		21:19	19.4	36
		21:31	19.44	37
		21:43	19.41	37
		21:55	19.29	37
		22:09	19.06	37
		22:23	18.73	37
		22:33	18.49	37
		22:45	18.17	36
c	Second Penumbra	22:59	17.85	36
		23:13	17.6	35
		23:25	17.47	34
		23:37	17.37	33
		23:49	17.36	32
		0:01	17.39	30
		0:13	17.39	29
		0:21	17.32	28

It is well known that maps of naked eye star visibility and telescopic limiting magnitudes are useful to quantify the capability of the population to perceive the Universe, to evaluate its evolution. Cinzano et al. (2001) extended the method introduced by Cinzano et al. (2000) to map the artificial sky brightness in large territories from DMSP satellite data, in order to map the naked eye star visibility and telescopic limiting magnitudes. Cinzano et al. (2001) considered the altitude of each land area, natural sky brightness in the chosen sky direction, stellar extinction in the chosen photometric band and eye capability as well as mountain screening for near zenith sky directions. The construction of the maps of naked eye star visibility and total sky brightness in V band in Europe at zenith was the ultimate outcome of that study. Following the strategy of Cinzano et al. (2001), we are going to consider the stellar and planet’s visibility during the lunar eclipses of 2018 and 2019.

4.1 Stellar visibility during different phases of the eclipse

During the different phases of the lunar eclipse, the sky brightness changes depending on the phase of the eclipse. The number of the visible stars depends on the total sky brightness (Cinzano et al. 2001 b). As a first step, aiming to find out the number of visible stars around the zenith during the eclipse, we used eq. (1) (Carlin 1998) to convert the ZSB values to zenithal limiting magnitude (ZLM).

Limiting mag = 7.93 - 5 log (10^{(4.316 - (V/5))} +1) (1)

where V is the sky brightness in mag/arcsec².

The number of visible stars around the zenith was calculated during the total and the partial lunar eclipses. Using Eq. 2 (National Solar Observatory. “Magnitude”, 2008)¹.

log (N) = 0.754 + 0.4896 V + 0.001159 V² - 0.000235 V³ (2)

The number of the visible stars during the period of the total and partial lunar eclipses is represented graphically in Fig. 4 and 5, respectively.

From Fig. 4 and 5, one can conclude that the number of the visible stars increases from the first penumbra until the mid of the eclipses then starts to decrease until the end of each of the eclipses. Variations in the number of visible stars in Fig. 4 during the interval of 20:30 UT to 21:24 UT could be attributed to seeing conditions.

4.2 Naked Eye Limiting Magnitude and The visibility of planets during Lunar Eclipses

Nawar (1983) deduced the visibility of stars and planets during twilight, using the relation between the sky brightness I expressed in S10 (v) (number of stars of tenth magnitude per square degree), to find the relation between the threshold stellar magnitudes and Log (sky brightness). Stars and planets visibility charts during high solar activity at some solar depressions below horizon were drawn by Nawar (1983). Nawar et al. (2007) studied the visibility of stars and planets during the total solar eclipse of 2006. They converted the sky brightness I from S10 (v) to mag/arcsec² and the corresponding threshold magnitude (m₀) were drawn.

In the present work, to study the visibility of planets during the total lunar eclipse of 2018 and the partial lunar eclipse of 2019, the sky brightness was converted to Naked eye limiting magnitude (NELM) using Eq. (1).

Table 3. The computed NELM for sky brightness during the total and the partial lunar eclipses

Total lunar eclipse			Partial lunar eclipse
UT	Brightness	NELM	UT	Brightness	NELM
(hr:min)	(mag_sqm/arcsec²)	mag	(hr:min)	(mag_sqm/arcsec²)	mag
17:20	14.17	0.45	18:43	17.7	3.71
17:30	16.35	2.51	18:51	17.66	3.68
17:42	17.9	3.88	18:59	17.6	3.63
17:54	18.43	4.32	19:13	17.57	3.60
18:02	18.52	4.40	19:25	17.55	3.58
20:10	18.61	4.47	19:37	17.59	3.62
20:16	18.67	4.52	19:49	17.69	3.71
18:26	18.81	4.63	20:01	17.84	3.83
18:32	18.93	4.72	20:13	18.09	4.04
18:40	19.17	4.90	20:25	18.38	4.28
18:44	19.34	5.03	20:37	18.69	4.53
19:00	19.98	5.48	20:49	18.98	4.76
19:14	20.41	5.76	21:01	19.2	4.92
19:22	20.47	5.80	21:19	19.4	5.07
19:28	20.49	5.81	21:31	19.44	5.10
19:44	20.44	5.78	21:43	19.41	5.08
19:54	20.44	5.78	21:55	19.29	4.99
20:04	20.44	5.78	22:09	19.06	4.82
20:14	20.39	5.75	22:23	18.73	4.56
20:24	20.32	5.70	22:33	18.49	4.37
20:34	20.07	5.54	22:45	18.17	4.11
20:44	20.14	5.59	22:59	17.85	3.84
20:54	20.19	5.62	23:13	17.6	3.63
21:04	20.34	5.72	23:25	17.47	3.52
21:14	20.01	5.50	23:37	17.37	3.43
21:24	20.14	5.59	23:49	17.36	3.42
21:34	19.82	5.37	0:01	17.39	3.45
21:46	18.55	4.42	0:13	17.39	3.45
21:54	18.18	4.12	0:21	17.32	3.38
22:04	17.33	3.39	̅	̅	̅
22:14	17.01	3.11	̅	̅	̅
22:20	16.71	2.84	̅	̅	̅

To study the visibility of planets during the total and partial lunar eclipses, the rising and setting times of planets must be considered besides the NELM of the planets. Table 4 lists the rising and setting times of planets and their naked eye visibility during the total and partial lunar eclipses. We used Eq. (3) (Schaefer 1990) to convert the NELM values to sky brightness in mag/arcsec² (Bmpsas).

Bmpsas = 21.58 - 5 log (10^{(1.586 - NELM/5)}-1) (3)

Table 4. Planetary rising and setting times, and their naked eyes visibility during the total and partial lunar eclipses at the specified dates

Total lunar eclipse: Friday, July, 27^th 2018
Planet	Rise (hr:min)	Set (hr:min)	Meridian (hr:min)	NELM (mag)	Brightness (mag/arcsec²)	Visibility
Mercury	Fri 06:47	Fri 19:37	Fri 13:12	1.9	15.69	Visible with difficulties
Venus	Fri 08:41	Fri 21:08	Fri 14:54	-4.27	9.39	Visible
Mars	Fri 19:07	Sat 05:02	Sat 00:04	-2.78	10.89	Visible
Jupiter	Fri 12:54	Fri 23:46	Fri 18:20	-2.13	11.54	Visible
Saturn	Fri 16:43	Sat 02:56	Fri 21:49	0.17	13.88	Visible
Uranus	Fri 23:04	Sat 12:03	Sat 05:34	5.79	20.45	Invisible
Neptune	Fri 20:56	Sat 08:29	Sat 02:42	7.84	28.45	Invisible
Partial lunar eclipse: Tuesday, July, 16^th-17^th 2019
Planet	Rise	Set	Meridian	NELM	Brightness	Visibility
Mercury	Tue 05:53	Tue 19:08	Tue 12:31	4.31	18.41	Visible with difficulties
Venus	Tue 04:27	Tue 18:27	Tue 11:27	-3.89	9.77	Visible with slight difficulties
Mars	Tue 06:18	Tue 19:55	Tue 13:06	1.82	15.6	Visible with difficulties
Jupiter	Tue 16:07	Wed 02:22	Tue 21:15	-2.51	11.16	Visible
Saturn	Tue 18:21	Wed 04:37	Tue 23:29	0.08	13.79	Visible
Uranus	Wed 00:00	Wed 13:05	Wed 06:32	5.81	20.49	Invisible
Neptune	Tue 21:47	Wed 09:25	Wed 03:36	7.85	28.71	Invisible

From Table 4, we conclude that all planets are visible except Uranus and Neptune due to their rising and setting times and the eclipses do not play any rule in their visibility.

We compare our results for the variation of ZSB with observations of the total lunar eclipse of 2014 (Theran and Dominguez 2016), the total solar eclipse of 2016 (Pramudya and Arkanuddin 2016), and the total lunar eclipse of 2019, (Birriel and Adkins 2019).

5.1 A comparison with observations during the total lunar eclipse April 14^th, 2014

Theran and Dominguez (2016) studied the variation in the ZSB during the total lunar eclipse of April 14^th and 15^th, 2014 at La Tatacoa Observatory in Colombia using SQM-LE. They noticed a gradual decrease in the brightness from the beginning of the penumbra U1 until the beginning of the totality U2. During the totality, they found that the brightness was almost constant, at 21.5 mag/arcsec² then its brightness gradually increases by the beginning of the second partial eclipse U3 until the end of the second penumbra U4 (see Fig. 5 in Theran and Dominguez 2016). It can be seen that the brightness of the zenith sky during the total lunar eclipse of 2014 is lower than that obtained in the present work by about 1.06 mag_SQM/arcsec² during the totality.

5.2 A comparison with observations during the total solar eclipse March 9th, 2016

Pramudya and Arkanuddin (2016) measured the ZSB during the total solar eclipse on March 9^th, 2016 using SQM-LU in Ternate, Indonesia. The brightness of the sky decreased with time until the eclipse’s totality, it remained constant then it started to gradually increase during the second partial eclipse until the end of the eclipse; see Fig. 3 in Pramudya and Arkanuddin 2016. A difference of about 8.08±0.09 mag_SQM/arcsec² between the total solar eclipse of 2016 and our observations during the total lunar eclipse 2018 can be seen.

5.3 A comparison with observations during the total lunar eclipse January 20^th-21^st, 2019

Birriel and Adkins (2019) studied the sky brightness during the total lunar eclipse (January 20^th-21^st, 2019) using SQM-LE. Their results are displayed in Fig. 1 (Birriel and Adkins 2019) which displays mag_SQM/arcsec² versus local (Eastern Standard) time. From the figure, it is clear that the maximum ZSB magnitude is 19.1 ± 0.1 during the interval from 23:30 to 1:30 EST.

By comparing both observations, we can say that our results at the maximum partial eclipse of July 2019, is approximately the same as their results, but they are less than our average value by 1.1± 0.1 mag_SQM/arcsec² during the totality.

This study is the first of its kind to study the effect of lunar eclipses on the variation of the sky brightness at the zenith point (ZSB) at the site of Kottamia Astronomical Observatory (KAO), in Egypt, using a SQM photometer. We studied the variation of ZSB during both the total and the partial lunar eclipses occurred in 2018 and 2019, respectively, and the effect of these variations on the visibility of stars and planets above the KAO location. Observational results revealed a similar trend for the variations of ZSB during partial and total eclipses; the sky starts to dim until reaching the mid-period of the eclipse then the brightness starts to increase to reach its normal value by the end of the eclipse. Results also implied that the variations may affect the naked eye observations of stars and planets.

During the total lunar eclipse of 2018, the average recoded values for ZSB were 17.6, 19.74, 20.39 and 18.45 mag_SQM/arcsec² during the first penumbra, the first partial eclipse, the totality and the second partial eclipse, respectively. For the partial lunar eclipse of 2019, the recorded ZSB values were 17.70 mag_SQM/arcsec² at the first penumbra, 19.44 mag_SQM/arcsec² at the middle of the eclipse, and reached a value of 17.32 mag_SQM/arcsec²by the end of the second penumbra. At the middle of the eclipse, the sky brightness during the total lunar eclipse was about 0.95 mag_SQM/arcsec² lower than that observed during the partial lunar eclipse. This value is similar to the difference in the lunar magnitude at the same time interval.

Stellar visibility during the lunar eclipse depends on many factors such as the phase of the eclipse, the altitude of the moon in the sky, artificial lighting and the atmospheric conditions. The relation between the sky brightness and the limiting magnitude assisted in finding the number of visible stars during the lunar eclipses. The maximum numbers of visible stars at the middle of the eclipse events were found to be ~ 3700 and 1780 stars during the total and the partial eclipses, respectively. The visibility of planets has a slightly different concept because their rising and setting times have to be considered with the time interval of the eclipses. Our results showed that most of the planets can be observed except for Uranus and Neptune due to the mismatching of their rising and setting times with those of the eclipses time.

Comparing our results to previous studies showed that variations in the sky brightness have similar trends with comparable values at the middle of the eclipse event up to 1-2 mag_SQM/arcsec² differences (e.g. Theran and Dominguez 2016, Pramudya and Arkanuddin 2016, Birriel and Adkins 2019). We may conclude that these small differences in ZSB are attributed to the geographical differences in observational sites as well as the differences in the atmospheric conditions at these sites during which observations were carried on.

Data availability

Authors can confirm that all relevant data are included in this article.

Declarations

The authors have no conflicts of interest to declare that are relevant to the content of this article.

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No competing interests reported.

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Zenith sky brightness, stellar and planets visibility during lunar eclipses at Kottamia Observatory

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Abstract

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1 Introduction

2 Observations

3 Results and discussion

4 Naked eye limiting magnitudes, and the visibility of stars and planets during lunar eclipses

5 Comparison with previously published observations

6 Summary and conclusion

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References

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