APPARENT SOLAR DAY Meaning and
Definition
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An apparent solar day, also referred to as a true solar day, is a fundamental unit of timekeeping based on the Earth's rotation relative to the position of the Sun. It is a period of time that spans from one instance of the Sun reaching its highest point in the sky (known as local solar noon) to the next. The duration of an apparent solar day is not constant throughout the year due to the Earth's elliptical orbit around the Sun, which results in variations in the Sun's speed across the sky.
The length of an apparent solar day varies due to two phenomena: the tilt of the Earth's axis and the Earth's elliptical orbit. The tilt causes the Sun's path across the sky to change throughout the year, resulting in differences in the length of daylight hours. Additionally, the Earth's orbit is elliptical, meaning the Earth's speed varies as it moves around the Sun. As a result, the position of the Sun in the sky shifts slightly each day, causing apparent solar days to be slightly longer or shorter than 24 hours.
To ensure consistency in timekeeping, an average apparent solar day is established, known as the mean solar day, which serves as the basis for our modern civil time standard. It is important to note that the actual length of an apparent solar day can deviate from the mean solar day by several seconds throughout the year. This deviation is caused by various factors, including the Earth's axial tilt, gravitational interactions with other celestial bodies, and even the Earth's crustal movements.
