Civil Twilight and Daylight Calculations

Determining Civil Twilight Times

Understanding and determining the beginning and end of civil twilight is essential for pilots operating under Visual Flight Rules (VFR) in Australia. This section will guide you on extracting civil twilight times and highlight factors that can cause variances in perceived daylight.

Learning Outcome 1: Extracting Civil Twilight Times from AIP

Civil twilight is defined as the time when the sun is 6° below the horizon. It is important for pilots as operations should not commence or conclude outside these times under VFR.

Steps to Determine Civil Twilight Time:

1. Identify Location and Date:

   • Use either a specific search tool like Geoscience Australia’s Geodetic Calculators or the AIP GEN 2.7 daylight and darkness graphs.
   • Enter the latitude and longitude for the intended location.
   • Select the correct time zone (AEST, ACST, AWST, AEDT, ACDT, UTC).

2. Select Event and Timeframe:

   • Choose “Civil Twilight” as the event.
   • Pick the date or period for which you desire twilight times.

3. Retrieve and Verify Results:

   • For precise planning, confirm the times with supplementary sources if necessary.
   • Note that technological tools calculate these times based on astronomical parameters and geographical location.

Learning Outcome 2: Factors Causing Daylight to End Earlier

While the AIP provides official twilight times, several factors can lead to earlier perceived daylight end. Pilots must be aware of these to ensure safety and regulatory compliance.

Factors Influencing Daylight Perception:

• Geographical Factors:

  • Latitude and Elevation: Higher altitudes and latitudes experience shifts in daylight duration due to the sun’s path and angle of elevation How Long Does a Sunrise and Sunset Last? - Consensus.

• Terrain:

  • Mountainous regions can obstruct sunlight, causing earlier dusk that isn’t reflected in official times Factors that affect the time of last light and DR - Bob Tait’s Aviation Theory School.

• Meteorological Factors:

  • Atmospheric Refraction: Can cause sunrises and sunsets to appear delayed; however, official times account for average refraction Effect of atmospheric refraction - Tong Yu-fai.
  • Cloud Cover and Visibility: Conditions such as overcast skies, fog, and haze can create the illusion of an early end to daylight Factors that affect the time of last light and DR - Bob Tait’s Aviation Theory School.

• Astronomical Considerations:

  • Seasonal Variations: The Earth’s tilt and orbital changes affect the sun’s position and daylight hours throughout the year.

Conclusion

Determining civil twilight times is an essential skill for pilots to ensure compliance with VFR regulations. While official resources provide a robust framework, pilots must also consider external factors affecting daylight perception. Utilizing tools such as Geoscience Australia’s calculators and comprehending environmental influences enable effective and safe flight planning.