Variability in low altitude astronomical refraction as a function of altitude

Area of Science:

• Astronomy
• Atmospheric Physics
• Geophysics

Background:

• Low altitude astronomical refraction (LAAR) is crucial for understanding atmospheric effects on celestial observations.
• Previous studies on LAAR have primarily focused on higher altitudes or different geographical locations.
• Accurate measurement of LAAR over sea horizons is essential for validating atmospheric models.

Purpose of the Study:

• To measure and quantify the variability of low altitude astronomical refraction (LAAR) of the setting Sun.
• To investigate the relationship between LAAR variability and the altitude of the Sun over a sea horizon.
• To assess the accuracy and limitations of digital imaging systems for astronomical refraction measurements.

Main Methods:

• Conducted 14 sunset observations over a sea horizon from Barbados between 2005 and 2007.
• Utilized a digital video camera (Canon XL2) and a digital SLR camera (Canon EOS 5D) to record the Sun's upper limb and apparent horizon.
• Calculated LAAR variability at 14 standard altitudes ranging from 0.01 to 4.5 degrees.

Main Results:

• Observed a decreasing trend in relative LAAR variability with increasing altitude, from +/- 0.0195 at 0.01 degrees to +/- 0.0142 at 4.5 degrees.
• Extrapolation suggested a relative variability of +/- 0.0038 at 15 degrees.
• Error propagation analysis indicated that measurement accuracy might be compromised at altitudes above 2 degrees.

Conclusions:

• The study provides empirical data supporting the decrease in LAAR variability with altitude over a sea horizon.
• Digital camera systems offer a viable method for LAAR measurements, but limitations exist at higher altitudes.
• Further research is needed to refine measurement techniques and extend observations to higher altitudes for improved atmospheric modeling.