Beauty of sky

Total solar eclipse, 17.08.2017, Riverton, WY (United States)

Sky brightness measurements

Illumination of sky       Sky surface brightness

The illuminance level and sky brightness are very important elements of solar eclipse phenomena, because both are clearly noticeable by observer along with his eye adaptation. When the eclipse phase is deep and turning into totality the light level changes drastically, much quicker than in any other ocassion. It makes the solar eclipse one of the most notable phenomena exhibited on the sky.
The illuminance level during the solar eclipse can be measured with Lux units. For this purpose an observer can use a Lux meter . This typical equipment allow us to record the illuminance changes in Lux units. The illuminance level measurement can be done alternatively by mobile phones with light measurement Android apps installed. This is a good amateur way to make these measurements, however this solution will not bring a true result due to some deviations from the reference luminance, which occur and are depend from the hardware and software of your phone. This option is good only for orientation with light level changes then.
I carried out the illumination level changes measurement with use the Samsung Galaxy S5 device. Despite worse measurement accuracy I was able to distinguish the most important moments of sky darkening and next, brightening after the totality. My device led me to record the illuminance changes also within the seconds of totality.
For the purpose of light level measurements I used the Eclipse Droid - the Wolfgang's Strickling android app, which perfectly fit to these purposes. The interface of this astronomic app is very friendly. It computes your local circumstances for solar eclipses in wide time range throughout XXI st century. The app itself is multifunctial. The details of the Eclipse Droid you can find here. For my purposes I used the eclipse real-time simulation and the Lux Meter built-in, which in conjunction with MS Word for Android could record detailed log regardless of the time frame. Eventually I have got a several hundreds pages of light level log from the Great American Eclipse.

There is at least a few publications, where the problem of illumination changes during the solar eclipse has been raised (Lee, 2012)(Shettle, 1989)(Mollman, Vollmer, 2006). Moreover some people, such Wolfgang Strickling or Carsten Jonas, who do this kind of measurements regularly. Nearly all of these observations has their own common denominator - they are taken in solar direction only. Almost nobody took a measurements in other direction than solar. Only in Wolfgang Strickling light measurement chart from Indonesia I can guess that he set his instrument towards shadow-in direction. Making a observation of illuminance level outside the solar direction was my basic goal during the Great American Eclipse. I set my smartphones in 2 directions bearing them at around 45 deg angle towards: - WNW, from where umbra was approaching (shadow-in). It was a view nearly onto antisolar point with initially one of the darkest section of sky. - ESE, oposite direction, where umbra receded (shadow-out). This direction was in less than 90 deg angular distance to the Sun with one of the brightest section of sky. My measurements was carried out from 1st contact to around 60% obscuration during 2nd partial phase.

One of the Samsung Galaxy S5 phones, set towards the shadow-in sky (WNW direction) .
I am prepared for eclipse observation.

Shadow-in direction - the WNW section of sky was initially one of the darkest part of the sky due to long angular distance to Sun (read more here). Despite a high haze level on the eclipse day the north-western sky was navy blue with around 9500 Lx initial value given. There were patchy cirrostratus clouds in this section of sky lifting up the iluminance level locally. This is normal, when a non-cohesive light scattering plays a role in the atmosphere. Anyway these clouds interfered into around 1/3 of partial phase, what shows a jagged line on the chart. As the Moon started to cover more and more of the solar disk these clouds moved southwards giving completely clear blue sky (at least within the measurement frame) around 45 min after 1st contact. The sky remained clear over the rest of the observation. See the described observation sections below:

See the photo and measurements below. I put a results both in normal and logarytrhmic scale. The most important moments listed above has been marked on the main chart. The period of totality has been detailed.

Cirrostratus cloudiness seen about 10:43 local time (UTC-6).
About 11:10 (UTC-6) clouds moved southward, giving a way to clear, blue sky.
Clear shadow-in sky under darkening conditions at over 0.9 eclipse magnitude (look on the left, where sky appears to be darker).
Luminosity changes towards the shadow-in direction (WNW).
Luminosity changes towards the shadow-in direction (WNW) - explanation.
Luminosity changes towards the shadow-in direction (WNW) - logarithmic scale.
Light level changes towards shadow-in (WNW) direction - comparison before and after totality - logarithmic scale.
Illumination changes throughout the totality period towards WNW (shadow-in) direction.
Illumination changes throughout the totality period towards WNW (shadow-in) direction - logarithmic scale.
Illumination changes throughout the totality period (+- 2 minutes) towards WNW (shadow-in) direction.
Illumination changes throughout the totality period towards WNW (shadow-in) direction - logarithmic scale.
Illumination changes throughout the totality period (+- 2 minutes) towards WNW (shadow-in) direction - logarithmic scale.

Overall light level difference towards WNW (shadow-in) direction.
Luminosity difference towards WNW (shadow-in) direction from obscuration 66% upwards.

Shadow-out direction - was the section of sky with ESE direction, where umbra was receding after the totality. Similarly to shadow-in section here also cirrostratus clouds were presented for around first part of the partial phase. These clouds moved southward around 10-15 min earlier than in opposite section of sky. Initial sky illumination level for the shadow-out direction was around 60000 Lx according to my smartphone. Due to cloud presence this value was lifted up to around 61500 Lx . Here I also divided this measurement for shorter periods in order to highlight the most important things, which I spotted.

Cirrostratus cloudiness seen about 10:43 local time (UTC-6) towards ESE direction.
The ESE sky around 11:30 (UTC-6), when clouds was seen far above southern horizon.
Luminosity changes towards the ESE (shadow-out) direction.
Overall light level difference towards ESE (shadow-out) direction.
Luminosity changes towards the shadow-out direction (ESE) - logarithmic scale.
Light level changes towards shadow-out (ESE) direction - comparison before and after totality.
Light level changes towards shadow-out (ESE) direction - comparison before and after totality - logarythmic scale.
Illumination changes throughout the totality period towards ESE (shadow-out) direction.
Illumination changes throughout the totality period (+- 25 seconds) towards ESE (shadow-out) direction.

Illumination change throughout the totality period towards ESE (shadow-out) direction - logarythmic scale.
Illumination change throughout the totality period (+- 25 seconds) towards ESE (shadow-out) direction - logarythmic scale.
Light level changes towards shadow-out (ESE) direction - comparison before and after totality - logarythmic scale.
Luminosity difference towards ESE (shadow-in) direction from obscuration 85% upwards.
Luminosity difference towards ESE (shadow-in) direction during a whole observation period.

Next >>>


Back to main menu