Salaamun Aleikom Brothers and sisters in faith,
There seems to many enterpretations of the word night. If you ask one person when night is they might say; when it is dark outside. Then you may proceed to ask another person and they may say; night is at sunset, however in order for us to know what Allah means by night as we all know the answer is in his scripture.... lets take a look to see how he uses this word. There are two imoprtant verses concering this issue.
2187 - It has been made lawful for you during the night of fasting to approach your women sexually. They are a garment for you and you are a garment for them. God knows that you used to betray your souls so He has accepted your repentance, and forgiven you; now you may approach them and seek what God has written for you. And you may eat and drink until the white thread is distinct from the black thread of dawn then you shall complete the fast until night and do not approach them while you are devoted in the temples. These are God's boundaries, so do not transgress them. It is thus that God makes His revelations clear to the people that they may be righteous.
Clearly it says to fast until night not sunset, eventhough some have translated it and used the word sunset for the arabic word LAYL.
The following verse gives us a clear visual description of the sky at the time of NIGHT/LAYL.
676 When the night covered him, he saw a planet, and he said "This is my Lord." But when it disappeared he said "I do not like those that disappear."
This indicates that night is when you can see planets this occurs about 30 - 40 minutes after sunset
"...30 - 40 after sunset the sky becomes dark enough that the planets and bright stars become visible. The planets can be recognized by their brightness and position in the sky. Venus is the brightest planet and will be easily seen in the west as the sky becomes dark. Jupiter is the second brightest planet and will become visible in the eastern sky" http//www.mcpstars.org/node/13
The sky is still too bright to see any planets with the bare eye at sunset.
My conclusion is as follows ; When we can see planets/stars in the sky, it is a sign that we can break fast.