How to take sextant altitude of stars, moon? is it possibe when its very dark?

No, we cannot take the sextant altitude when it is dark. That is because if it is dark we would not be able to see the horizon.

But if it isn't dark we cannot see the stars.

Then how can we take the sextant altitude of the stars for the star sight?

We can. We just need to know the right time. The time when there is enough sunlight to clearly see the horizon and enough darkness to clearly see the stars in the sky.

What time is this?

This time is the time for "Nautical twilight". 

Twilight is the light received from the sun when the sun is below the horizon. That is before sunrise in the morning and after sunset in the evening.

In the morning, the twilight starts when the sun is 18 degrees below the horizon and it ceases with the sunrise.

In the evening, the twilight starts with the sunset and ceases when the sun is 18 degrees below the horizon.

This twilight time is divided into three phases.

  • Astronomical Twilight
  • Nautical Twilight
  • Civil Twilight

It is the Nautical twilight that we are interested in.

But we do not need to wait on the bridge wings and wonder when will be the nautical twilight. We can get the time for nautical twilight from the Almanac.

Now from this time, we want to get the time when nautical twilight starts.

The time given in the almanac are

For the evening: Twilight times are the end of twilight times

For the morning: Twilight times are the beginning of twilight times. 

So if you want to take the star sight in the evening, to get the start of nautical twilight time, you need to look under the "civil twilight" column and not under "Nautical twilight" column.

This is because in the evening time are the end of twilight times and end of civil twilight means the start of nautical twilight.

Now let us say your ship's position is at 10 N / 76 E and the ship's time is 5 hours ahead of GMT.

Let us calculate the time for the start of Nautical twilight in the evening.

LMT for end of Civil Twilight: (18h  13m)

LIT (Longitude 76E): (5h  4m)

GMT for end of Civil Twilight: (13h  09m)

SMT for end of Civil Twilight: 18h 09m

So, in this case, you should be ready at sharp 1809 Hrs on bridge wing with your sextant for the star sight.