Compiled by A. Cengia - Australia

In the period of June and early July 1996 while on a quick excursion in mainland Australia the opportunity was used to make observations on the onset of twilight and dawn. For all the localities that were to be visited, the expected times of twilight for 18 degrees were worked out before hand so that it was only a matter of being prepared a few minutes early to observe and write in a note book at what time the horizon lighting conditions were changing. The particular reason for the observations was to pursue the time of '' the white thread of dawn ''. One is told that traditionally the event takes place at a specific degree before sunrise where as in some countries or continents different degrees are used. The morning observations were made at different days while in transit from one locality to an other and recorded without any concern as to what it may represent or mean in terms of personal expectations. In some localities the visibility of the morning sky was not always ideal particularly for Sydney, Melbourne and Hobart but despite any local or beyond the horizon cloud cover, reasonably accurate observations were and are still possible. Observations which carry the same time mark were taken on different days and are a useful coincidence. At the end of the excursion the data was processed as follow. :
All the data from each locality was sorted in the order of the time it was taken. Next, the twilight times for the locality at different degrees was worked out and matched with the times of the observations corresponding with the degree time. Even with the limited number of observations, the result gives an indication of what light change takes place at what degree.
Question mark.
A - Is twilight the same thing as dawn.
B - Is dawn preceded by twilight, the scattered light from below the horizon including moon light, seen faintly on the horizon even before the 18 deg. mark
C - Is '' the white thread of dawn '' the point in time after twilight when the white light from the sun despite any clouds or moon lighting is seen to begin expanding at the horizon and climbing up the sky, when at this beginning the light contrast between night and dawn is equal to the contrast of a black and white thread.
---------------------------------------------
An observation totally unrelated to the twilight which was found interesting. While flying near Alice Springs, in the middle of Australia at about 9000 metre in a cloud free sky, the earth below could be seen through what appeared to be a grey bubble. The horizon could not be defined except that it was somewhere over there. After descending below 9000 metre the ground and the horizon in particular became very well defined. During the twilight observations, a note was made of seeing a light path in the sky, light reflected from something which could not be seen except by the result, the same way moon light can be seen reflected on still water as a light path.

---------------------------------------------
The raw information below was transferred from a notebook as it was entered.
---------------------------------------------

Location = Darwin TZ = 9.5  Lat. = -12.3333  Long. = 130.8333 
        observations carried out from the city limit.    11,12,15 / 6 / 1996 
 
18d     5.48    Black every where. One bird is heard once 
        5.50    A second bird is heard. As black shadows two large bats seen flying 
17d     5.52    As a contrast in shadows the horizon is faintly visible without 
                -- being defined. 
        5.52    There is no moon.  A second bird is heard. 
        5.53    Faint stars are visible where the horizon is supposed to be. All is black 
        5.56    A faint contrast line is visible at the horizon. No change in the stars 
16d     5.57    Darkness, no horizon 
        5.57    The sky is lighter  1/4 of the way up. One bird is noisy 
        6.00    A faint brown light as a contrast, determines the horizon. 
                -- One bird is heard. 
15d     6.02    There are two horizons. One is about 3 centimetre higher. 
                -- There are fires somewhere 
        6.04    The sky is lighter above the first horizon which is a cloud's edge. 
                -- Birds are heard 
        6.04    More bird calls are heard 
 14d*   *6.05   The brown contrast is more pronounced. A second bird is heard 
        6.07    In contrast a faint shadow is seen above the horizon. 
        6.07    The outline of objects 100 to 150 metre away are shadows in the sky 
        6.08    The horizon is visible as black below the brown. 
                -- Objects seen against the horizon are becoming defined shadows. 
                -- The stars close to the horizon are still visible 
        6.08    The outline of the cloud is wider. 
        6.08    The outline of objects 100 to 150 metre away are strong shadows but 
                -- not identifiable 
        6.09    The faint shadow is a cloud on the horizon with a light behind it 
                -- becoming wider. 
13d     6.10    The horizon sky is lighter and wider above the faint brown cloud which 
                -- has lighter holes. 
        6.10    The sky is lighter to nearly overhead and the outline of objects 
                -- are more defined 
        6.12    The outline of objects 100 to 150 metre away are well defined 
                -- against the horizon 
        6.12    The outline of objects 100 to 150 metre away are well defined. 
        6.12    The light is widening and the brown is more intense 
12.5d   6.13    The sky above the cloud is whitish. The stars are still visible 
        6.14    The horizon is widening and the sky past overhead is lighter. 
        6.14    Against the horizon, the outline of objects are identifiable 
12d     6.15    The horizon sky is brown and becoming wider 
        6.16    The light is widening and getting higher, the stars are still visible. 
        6.17    The sky is light all over and only some stars are visible 
        6.17    The sky is lighter to 1/4 of the way up the western horizon. 
                -- Most eastern stars are gone 
11d     6.19    Very few eastern stars left 
        6.20    The sky above the brown horizon is becoming whitish 
        6.21    The sky to past overhead is grey white 
        6.22    Whitish clouds are visible a 1/4 of the way up the sky 
9d      6.28    The stars above the horizon are gone 
        6.28    Only half a dozen stars are visible any where 
        6.29    The sky is white in the east, bluish overhead and blue grey to the west 
8d      6.31    All the sky is now grey blue and only a few stars are to be seen anywhere 
        6.35    All the stars are gone except for very few 
7d      6.37    no change 
6d      6.40    Only one star remains to be seen. 
        18.29   SUNSET in the park overlooking the sea near the city 
7d      18.57   The first stars seen east of overhead 
10d     19.11   The last west star in the Ursa Minor I think is faintly visible. 
11d     19.15   Some stars just past overhead 
14d     19.26   The west horizon is black 
        19.28   Western sky faintly lighter then eastern side 
15.5d   19.34   In the milky way east of overhead some patches of faint white near 
                -- Ursa Minor are visible 
16d     19.39   All the sky is black to the horizon 
17d     19.41   One bird is heard 
18d     19.44   All black, the same bird is heard 
--------------------------------------------- 
 
                N. T. Kakadu National Park.  ~ Lat -13.954 Long = 132.592  Tz = 9.5 
                Cooinda lodge, Yellow River in the middle of the bush.  14 / 6 / 1996 
 
19d     5.36    From the bush the horizon itself is not visible 
        5.36     the moon is a fine slice just obscured by the tree tops about 1 
                -- 50 metres away 
        5.36    The sky is clear with a faint light path extending west from the moon 
        5.53    The moon is clear of the trees and the horizon sky is lighter with 
                -- no change in star visibility. 
16d     5.53    Some cars are driving about to the west and the first bird is heard. 
15d     5.57    The horizon is lighter and the trees are visible as shadows 
        5.59    In the milky way just past overhead, the star clusters seen as 
                -- a whitish cloud are disappearing 
        5.59    The other stars are visible 
 14d*   *6.01   The eastern sky is lighter then the western but there is 
                -- no change in the shadows. 
        6.03    Some eastern stars are gone and tree branches against the eastern sky 
                --are strong shadows. 
        6.03    The bush and the ground is all black 
13d     6.06    The main stars in the milky way are visible and the horizon is lighter. 
        6.06    The bush is black and a second bird is heard 
        6.08    A third bird is heard and there are only a few stars in the east 
12d     6.09    The horizon is whitish and wider, the tree branches are identifiable 
                -- against the sky. 
        6.09    A third bird is heard and only the major stars are visible anywhere 
11d     6.15    The ground with scattered leaves is visible, the wider horizon 
                -- is brown with white above it 
        6.15    A few stars are still visible. Crows and other birds are heard 
10d     6.19    The horizon is orange with strong white light above it, 
                --grey blue overhead & grey to the west 
        6.19    A few stars are faintly visible and different birds are heard. 
        6.22    To the east the trees are easily seen but in the west the leaves 
                -- are not easily identifiable 
9d      6.25    The hand writing in the note book can be seen but not read. 
        6.27    The hand writing can be read 
8d      6.29    Three or four stars are left in the sky. The bush can be seen east to west 
        6.29    One of the many friendly mosquitos is seen flying across the note book. 
--------------------------------------------- 
 
                Location = Alice springs TZ = 9.5  Lat. = -23.666  Long. = 133.8333 
                Alice springs  17,18,19 / 6 / 1996 
 
22d     5.34    All is black. Past overhead the faint star clusters are visible 
                -- in the milk way 
        5.35    It is black and overcast and only a few stars are visible 
21d     5.41    No horizon and all is black except for a faint hazy light path 
                -- in the sky from the East 
        5.42    The clouds are easier to see going towards the horizon like a pathway 
20d     5.45    A very faint horizon as a contrast in shadows 
        5.48    A faintly illuminated cloud 1/4 of the way up the horizon. 
        5.48    The overhead clouds are easier to see. A bird is heard 
19d     5.50    The hazy light path is more pronounced 
        5.52    The pathway in the  clouds is whitish up to 1/4 of the way. 
                -- The horizon is a faint shadow 
        5.53    A faint horizon is seen as a change in shadow contrast 
17.5d   5.57    The pathway is wider. No other changes 
        5.58    The horizon is still a shadow but wider 
17d     6.01    The horizon is a black shadow against a lighter sky 
        6.03    The faint hazy path is wider at the horizon which is lighter 
16d     6.04    To the right the horizon is wider 
        6.05    The horizon is a well defined black shadow. 
                -- There are no objects to compare against 
        6.06    Crows are heard once 
15d     6.07    The sky is lighter and the clusters in the milky way are less dense 
                -- The visibility of the other stars is unchanged 
        6.10    There are two black shadows about 2.5 cm above the horizon 
        6.11    Crows are herd again, there is car and bus traffic 
        6.12    The horizon is a faint blue and becoming wider 
 14d*   *6.13   Stars have become visible in the pathway which has not changed. 
                -- More birds are heard 
        6.14    The milky way star clusters are no longer visible. 
                -- Crows are heard, there is vehicle traffic 
        6.16    The horizon is whitish blue extending upwards 3/8 of the way 
        6.17    The horizon is easier to see as two contrasts widening north and south 
13d     6.18    The horizon is a faint yellow white and blue above it 
        6.21    The horizon is widening but still as two shadows. More crows are heard 
        6.22    The milky way is beginning to disappear 
12d     6.23    The clouds are lighter to 1/4 past overhead 
        6.24    The horizon can be seen at the north, in the south there are hills very close 
        6.25    The horizon is a faint yellow then white then blue and grey to the west 
        6.25    There is more definition at the horizon sun rise 
11d     6.28    Most stars are gone except for overhead 
        6.28    Faint yellow light is expanding north to south 
        6.30    Only the brightest stars are left, the sky is dark grey in the west 
        6.30    The grey clouds at the horizon are a faint red underneath 
        6.34    More birds are heard 
        6.36    The eastern horizon is yellow and the veil seems to be overhead 
9d      6.36    The west horizon is visible but there is not much light about. 
                -- The crows are noisy 
6d      18.22   The veil is 1/4 of the way above the west horizon. 
                -- Some stars inside the veil. 
        18.22   The crescent is 1/8 of the way above the west horizon 
7d      18.25   Ursa Minor is visible. The sky at the horizon is a strong yellow, 
                -- above it is blue white and then blue with the rest a dark ash grey 
--------------------------------------------- 
 
                Location = Sydney TZ = 10  Lat. = -33.8833  Long. = 151.1667 
                Observation from the city with the true horizon obstructed by buildings 
                Sydney 25,127 / 6 / 1996 
 
17d     5.35    All is black in the sky, stars are visible 
14.5d   5.50    All is black with an overcast sky. The clouds are seen from the city lights 
14d     5.53    Stars are seen through the cloud. All is black 
        5.53    There is a whitish transparent broken cloud extending past overhead 
                -- like a wide pathway. 
 13.5*  *5.55   The horizon is not well defined and clouds above it are seen as dark 
                -- shadows in the lighter sky. 
        5.55    The horizon sky seem to be cloud free. 
                -- There is a faint light extending  1/4 way up the sky. 
13d     5.58    The horizon is beginning to widen, it is faint yellowish in the centre and 
                -- whitish blue higher up. 
        5.58    The outline of objects against the horizon are more defined 
        5.59    The sky in the east is faintly lighter and one star is becoming harder to see 
        6.01    The horizon and objects against the horizon are well defined 
12d     6.02    Still overcast with no stars, the horizon is widening quickly 
        6.02    The sky a 1/4 of the way up is lighter and the horizon is well defined 
                -- with some clouds above it 
        6.06    The horizon is becoming wider and the sky is lighter up to overhead 
11d     6.08    The horizon is lighter and wider, the sky light is higher 
10.5d   6.10    The main stars are still visible 
--------------------------------------------- 
 
                Location = Melbourne TZ = 10  Lat. = -37.8333  Long. = 145.01 
                Observation from the city with the true horizon obstructed by buildings 
                Melbourne 30 / 6 / 1996  and  2 / 7 / 1996 
                When the sky is not overcast only a few brightest stars are visible 
 
16d     6.13    Many stars are visible but only the brighter ones 
15d     6.18    A faint horizon can be seen as a contrast below a hazy light 
14.5d   6.21    The horizon contrast is more pronounced but not any wider 
14d     6.23    In the clouds the horizon can be seen as a contrast below a hazy light 
        6.23    The clouds seem to have a light source behind them 
        6.23    The sky is 100% overcast. The tallest buildings are in the clouds seen 
                -- by the city lights 
        6.24    The contrast is becoming more pronounced, the sky light is 
                -- wider and higher 
13.5d   6.26    Faint whitish clouds are visible a 1/4 of the way up the sky 
 13d*   *6.30   The clouds at the horizon have more light in them 
        6.31    A wider horizon is visible at the same brightness, the stars are still there 
        6.32    The sky is still 100% overcast, the horizon is a contrast in a cloud 
                -- slowly becoming wider 
12d     6.35    The horizon light is much brighter, wider and higher. 
                -- The stars are still there 
11d     6.39    The light in the east is whitish up to overhead and black past it, 
                -- some clouds overhead 
6d      17.40   The veil is just past overhead. The first two stars are visible inside the veil 
--------------------------------------------- 
 
                Location = Hobart TZ = 10  Lat. = -42.8839  Long. = 147.3289 
        observations carried out from the city boundary about 100 metre above sea level 
                Hobart  17 / 7 / 1996 
 
16d     6.06    A clear sky. Heavy fog is moving down the river like flowing water, 
                -- it is less dense higher up. Arooster is heard. All is black. 
        6.08    Very faint horizon is visible as a contrast between two shadows. 
                -- Rooster heard again. 
15d     6.12    The horizon is a more defined contrast, identifiable with faint 
                -- whitish clouds 1/8 way up the sky. 
14.5d   6.16    The horizon is becoming more identifiable but only as a contrast, 
                --the light intensity is not great. The river fog is braking up. 
                -- The stars visibility is unchanged. 
13.5d   6.20    The horizon line is more defined below the white clouds. 
                -- More faint clouds higher up the sky. 
 13d*   *6.24   The white clouds at the horizon is clear sky in between the clouds. 
                -- The sky is wider and lighter 1/4 up. 
        6.27    Very dense fog is developing and the horizon cannot be seen. 
                -- No change in the stars. The city street lights below are beginning 
                -- to disappear in the fog. 
12d     6.30    The grey fog is now quite dense and getting higher, the visibility 
                -- is about 100 metre and getting shorter. One star is visible overhead. 
 
From the data above it is suggested that the white thread of dawn 
for the above latitudes is either as follows or a few minutes closer to sunrise:. 
 
14 degree for > Lat. = -12.3333     Long. = 130.8333 Location = Darwin TZ = 9.5 
14 degree for > ~ Lat = -13.954     Long = 132.592 N.T. Kakadu National Park Tz = 9.5 
14 degree for > Lat. = -23.666     Long. = 133.8333 Loc. = Alice springs TZ = 9.5 
14 degree for > Lat. = -33.8833    Long. = 151.1667 Location = Sydney TZ = 10 
13 degree for > Lat. = -37.8333    Long. = 145.01 Location = Melbourne TZ = 10 
13 degree for > Lat. = -42.8839    Long. = 147.3289 Location = Hobart TZ = 10