The Celestial Sphere as a Natural Clock

By Jonathan Ring

Due to the interactions of bodies in the solar system, the skies can be used as a natural clock. The most obvious example is the sun. Even without recorded history, it is not an unwarranted guess that humans have long used the sun to keep track of time. Although dependent on latitude and time of year, the sun appears to rise and set each day in a simple pattern. However, through observing other bodies, larger amounts of time can be distinguished by simple observance.

The moon is the second most luminous object in the sky and easily observable from the Earth. Due to the movement of the moon around the Earth, an effect occurs in which the moon appears to go through phases. These phases are easily distinguishable and are periodic. Therefore, the moon makes an excellent natural clock.

The time that it takes to go from one phase all the way through the other phases and back to the original phase is approximately 29.5 days. Therefore, based on a period of 29.5 days, the moon can be used to keep track of days. A new moon represents the first day of a lunar month. The moon continuously becomes more illuminated until it becomes a full moon. The period of time when the moon is becoming more illuminated is called waxing. After it has reached the full moon stage half way through the cycle, it starts becoming less illuminated every day and is said to be waning. Other important phases of the moon include quarter (either waxing or waning) in which exactly one half of the moon is illuminated.

Due to the physical properties of motion of the Earth, the celestial sphere makes observable patterns of movement across the sky as viewed from Earth. Various lengths of time can be recorded by the observance of these movements. The sun and moon have already been discussed, but the apparent motions of the stars also make patterns that are useful in telling time.

The utility of this method of time telling may seem pointless in the age of technology, where advanced methods for keeping time, including the movement of atoms, are now readily available. However, thousands of years ago, the sun, moon, and stars were the best method for keeping track of time. Due to the agricultural practices, a need to accurately keep track of seasons led civilizations to discover methods of keeping time.

Ancient Egypt provides an excellent example of the utility of the stars in keeping time for agricultural purposes. Original calendar systems were based on the moon, but proved to be too inaccurate. This is unsurprising considering a lunar month is only 29.5 days and a year is not evenly divided by such a month. An important discovery was noted by the Egyptians that led to the formation of a year-long calendar. A star called Sirius rose next to the sun every 365 days, and allowed the Egyptians to then accurately predict seasons. Later improvements included the addition of stars so that time was kept using 36 stars, not to mention the immediate value of the sun and moon.

The planets Venus, Mars, Jupiter, and Saturn are easily visible with the naked eye. They, too, make paths across the nighttime sky. However, the planets' apparent motion generates problems in keeping time. Because of their proximity to the Earth compared to stars, they do not "move" in the same type of observable patterns. All of the planets have different orbital periods, causing an effect known as retrograde motion. Because the Earth orbits the Sun more quickly, it "catches up" and surpasses Mars, Jupiter, and Saturn, causing an observed effect of apparent backwards movement by the planets.

Based on the criteria of natural clocks, other extraterrestrial bodies make excellent clocks. In a given period, they exhibit definable initial and final conditions (these of course being the same position and/or phase). Some celestial bodies go through irreversible processes on a short term basis, for example the moon will never skip phases or go back to others without completing a full cycle.

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