The duration of the pre-dawn period, also known as twilight, can vary depending on several factors such as geographical location, time of year, and atmospheric conditions. During this period, the sky gradually starts to lighten before the actual sunrise. The two main phases of twilight are called civil twilight and nautical twilight, which are defined based on the Sun's position below the horizon.
Civil twilight begins when the Sun is 6 degrees below the horizon and ends when it reaches 0 degrees (the horizon itself). This is the period when there is enough light for most outdoor activities to be conducted without artificial lighting. Civil twilight generally lasts for about 30 to 40 minutes before sunrise.
Nautical twilight begins when the Sun is 12 degrees below the horizon and ends when it reaches 6 degrees. During nautical twilight, there is still sufficient light for sailors to take reliable measurements at sea using the horizon as a reference point. Nautical twilight typically lasts for about 40 to 50 minutes before sunrise.
It's important to note that the exact duration of twilight can vary based on factors such as latitude, the time of year (with greater variations near the poles), and local atmospheric conditions, including the presence of clouds, pollution, or other factors that can scatter or absorb light.
If you want to determine the specific duration of twilight for a particular date and location, you can refer to astronomical tables or consult online resources or smartphone apps that provide sunrise and sunset times, as they often include information about twilight phases as well.