Times: AST, MST and LMT

Definition and Explanation

AST (Apparent Solar Time) is a timekeeping method that uses the position of the Sun in the sky.
The Sun reaches its highest point (the zenith) at ‘noon’. Time is measured from its transit across the meridian.
AST varies throughout the year due to the Earth’s elliptical orbit and tilt.
MST (Mean Solar Time) is an averaged version of AST, maintaining a uniform day length.
MST creates a ‘fictional’ Sun, or mean Sun, that moves along the celestial equator at a uniform speed.
MST is the time we see on our clocks and used in our daily activities and schedules.
LMT (Local Mean Time) is MST calculated for a specific longitude.
As the Earth is divided into 360 degrees of longitude, each degree has a 4-minute difference in LMT.

Comparison of Times

AST is ‘real’ time, based on the actual position of the Sun, whereas MST is a convenience that averages the time.
AST varies through the year, with apparent solar days getting longer or shorter by up to 16 minutes.
MST creates uniform 24-hour days, but isn’t always aligned with the Sun’s zenith at noon.
LMT varies as you move east or west because of the Earth’s rotation.

How to Determine Times

To determine AST, one would measure the Sun’s zenith, which varies throughout the year.
MST is determined by averaging out AST over the year, leading to consistent 24-hour days.
LMT can be calculated by knowing your longitude and adjusting GMT (Greenwich Mean Time) by 4 minutes per degree.

Importance in Astronomy

AST provides more accurate solar measurements, while MST provides consistent and practical timekeeping.
Astronomers and navigators often use LMT to adjust their observations to a standard time reference.
These time concepts are essential to understanding the observations and movements of celestial bodies.

Study and understand these methods of time-keeping and their applications to grasp astronomical phenomena and navigation techniques better.