What is Twilight?

Calgary does not have true night between the third week of May and the third week of July.  The best we can manage between those times is astronomical twilight.  What are the different definitions of twilight?

There are three types of twilight defined below.

Civil Twilight
The Sun is located less than 6° below the horizon.  Civil Dawn/Dusk happens when the geometric center of the Sun is exactly at 6° below the horizon.

Nautical Twilight
The Sun is located between 12° and 6° below the horizon.  Nautical Dawn/Dusk happens when the geometric center of the Sun is exactly at 12° below the horizon.

Astronomical Twilight
The Sun is located between 18° and 12° below the horizon.  Astronomical Dawk/Dusk happens when the geometric center of the Sun is exactly 18° below the horizon.

When the Sun is 18° below the horizon it is officially Night.

It is well known that locations near the poles may experience days where there is no twilight and days that are completely night.

Many laws are incorporated in relation to civil twilight regarding driving, flying and even burglary.  Venus is typically visible during this time.  Nautical twilight is generally the time when sailors may use the stars to take measurements for manual navigation.  Once astronomical twilight is over and night arrives you will be able to see as many objects in the sky as your eyes and light pollution permits.  Deep-sky objects are able to be resolved and astronomers are happy.

To determine when these specific types of twilight occur in Calgary the following website is useful:

https://www.timeanddate.com/sun/canada/calgary

In the dead of Winter we get a whopping 12 hours of night.  However, the temperature and snow generally limit the joy of astronomy.  My favourite months for astronomy are March, April, September and October.  They offer a good balance of darkness and tolerable weather.

The image of a waxing crescent Moon and Venus below was taken Fall 2016 during nautical twilight.

Moon and Venus (Nautical Twilight)