# Mean Solar Day: Explanation, History, and Application

## Introduction

The mean solar day is an important concept for tracking interplanetary movements. It is an average of observed length of a day over a given period of time and is used instead of the apparent solar day, as the length of the latter is subject to small variations. To understand the mean solar day, one must first understand the definition of a solar day, the context of its use, the history of its invention and our current understanding and application of it.

## Definition of Mean Solar Day

A solar day is the total amount of time that elapses between two successive returns of the Sun to the observer’s local meridian. The solar day is normally defined in terms of the mean motion of the sun, which is equal to the sidereal day multiplied by the number of mean solar days per sidereal day. This number is 1.00273790935, also known as the mean solar day in seconds.

## Context

In astronomy, the mean solar day is used as the basis for calculating a sidereal day, which is the time taken for the earth to rotate around its own axis with respect to the background stars. This information is important when tracking the planets and other heavenly bodies. The mean solar day is also useful for understanding time dilation effects, which occur when time passes more slowly in certain regions of space due to the presence of a massive object such as a black hole. Understanding the mean solar day is important for accurately navigating space and keeping track of the relative motion between objects in the universe.

## Description of Speed in Seconds

To measure the speed of a mean solar day, one can calculate the number of seconds taken for the planet to complete a rotation around its own axis with respect to the background stars. The mean solar day in seconds is 86,400 seconds, or 1.0027379 sidereal day. Similarly, a sidereal day is the period of rotation of a celestial body with respect to the fixed stars, and is measured in seconds as well. A sidereal day is 23 hours, 56 minutes and 4.0905 seconds, or 86164.0905 seconds. Therefore, the mean solar day is approximately 13.9208 seconds longer than a sidereal day.

## History of Mean Solar Day Invention

The concept of the mean solar day was first proposed by astronomer William Herschel in the 1800s. He proposed that it could be used to calculate the relative motion of the sun and earth more accurately than using the apparent solar day. This was due to the fact that the apparent solar day was subject to small variations in length due to the uneven speed of the planet’s rotation.

In order to create a more accurate means of calculating the relative motion of the sun and earth, Herschel proposed a mean solar day which was equal to the sidereal day multiplied by the number of mean solar days per sidereal day. This number is 1.00273790935, also known as the mean solar day in seconds.

## Our Understanding and Application of Mean Solar Day

The invention of the mean solar day revolutionized the way that the relative motion of the sun and the earth is tracked. By using the mean solar day instead of the apparent solar day, astronomers were better able to predict the trajectory of planets and other celestial bodies. This understanding is used today in navigation and space exploration, as knowledge of the motion of planets and other objects is necessary for accurate navigation and exploration.

In addition to its use in navigation and exploration, the mean solar day has also been used to set clocks and official times. The Greenwich Mean Time (GMT) is based upon the mean solar day and is used as the basis for calculating both local and international times. This helps greatly to ensure that, regardless of where one is in the world, things are running on the same schedule.

## Conclusion

In conclusion, the mean solar day is an important concept for tracking interplanetary movements. It is an average of observed length of a day over a given period of time, and is used in order to smooth the irregularities observed in apparent relative motion of sun and earth. It can be used for tracking the planets and other heavenly bodies, understanding time dilation effects and for accurately navigating space.

The mean solar day is also used for setting clocks and official times, as evident by its use in the Greenwich Mean Time (GMT). Additionally, it is 13.9208 seconds longer than a sidereal day, which is measured in 86164.0905 seconds. In response to the question ‘How much is 1 mean solar day in seconds?’, the answer is 86,400 seconds, or 1.0027379 sidereal day.
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