The Nature of Time in the Quran

Summary

The Quran presents a multifaceted and sophisticated understanding of time that is neither abstract nor arbitrary but is woven into the fabric of creation as a divinely measured and controlled phenomenon. Time is not addressed in a single, dedicated chapter but permeates the entirety of the text through intricate linguistic structures, including verbal tenses, specific nouns, and rhetorical devices like antithesis and paranomasia. The Quran establishes that celestial bodies, specifically the sun and moon, function as precise instruments for timekeeping, making the measurement of time a manifestation of divine order essential for both practical life and religious observance.

A central theme is the relativity of time, which manifests in two distinct forms. First is objective relativity, a concept analogous to modern physics, where the duration of an event is dependent on the observational domain (e.g., human vs. divine). Verses describe a single divine day as equivalent to one thousand or even fifty thousand human years, illustrating that temporal intervals are not absolute across different frames of reference. Second is psychological relativity, which concerns the subjective, internal human experience of time's passage. This inner perception is fluid, influenced by emotional and spiritual states, causing time to feel prolonged during suffering and fleeting during joy, as exemplified by the disbelievers' perception of their entire lives as lasting a mere "hour" on the Day of Judgment.

Ultimately, the Quran asserts that while humanity can and should strive to measure time, absolute and perfect knowledge of its nature and precise enumeration belongs exclusively to Allah. Time is a creation, and its ultimate master is the Creator, who is not constrained by it.

1. The Conceptual and Linguistic Framework of Time

The Quranic conception of time diverges from purely philosophical or scientific views by integrating it into a divine framework. While thinkers have often linked time to motion, change, or mental constructs, the Quran presents time as a fundamental component of creation, expressed with meticulous linguistic precision throughout its verses.

1.1. Quranic References to Time

• Direct Reference: The most direct reference is Surah Al-Asr, which opens with an oath, "By time."
• Indirect References: Time is indirectly signaled in every verse through two primary linguistic mechanisms:
  • Near Indication (Verbs): Verbs inherently convey temporal information by shifting between past, present, and future tenses, capturing motion, sequence, and the unfolding of events. For example, "Musa said" (past), "He knows" (present continuity), and "The foolish will say" (future).
  • Distant Indication (Nouns and Particles): Nouns and particles that do not inherently signify time can become temporal markers in specific contexts.
    • Al-Sa'ah (The Hour): Refers to the monumental future moment of the Day of Judgment.
    • Thumma (Then): A particle indicating sequence with a delay, marking distinct stages (e.g., "...then He causes you to die, then He brings you to life...").
    • Fa (So/And): A particle signaling immediate succession, showing an instant occurrence (e.g., "...'Be,' and it is" (fa-yakun)).

1.2. Literary Devices Conveying Temporal Concepts

The Quran employs sophisticated rhetorical techniques to express temporal ideas, often through the use of opposites and wordplay.

Literary Device	Description	Quranic Example
Antithesis	Presenting paired ideas that correspond in agreement or opposition to enhance depth and beauty.	"And out of His mercy He made for you the night and the day, so that you may rest therein [night] and seek of His bounty [day]..."
Tbaq	Pairing a word with its direct opposite within the same sentence to create harmony and precision.	Night and Day (paired nearly 100 times), The First and The Last.
Paranomasia	A form of wordplay where words with similar or identical sounds have different meanings.	"And the day the Hour is established, the criminals will swear that they did not remain but an hour." The first Hour is the Day of Resurrection; the second hour is a short span of time.

1.3. The Absolute Nature of Divine Time: Al-Awwal and Al-Akhir

The names of Allah, Al-Awwal (The First) and Al-Akhir (The Last), are used to denote pre-eternity and eternity, respectively.

• Al-Awwal: Signifies the one before whom there is nothing. The definite, masculine, singular form of the word appears only once in this absolute sense, corresponding perfectly to the unique, unambiguous, and powerful attributes of Allah.
• Al-Akhir: Signifies the one after whom there is nothing. When used absolutely, without qualification, it refers exclusively to Allah as the one without end.

2. Divine Measurement and Human Limitation

The Quran establishes that time is precisely measured and regulated by divine decree through natural cycles, while simultaneously highlighting humanity's inherent inability to grasp its full reality.

2.1. Celestial Bodies as Timekeepers

The movements of the sun and the moon are described as operating by "precise calculation" (bi-husban). These celestial bodies are the foundation for determining the cycle of day and night, lifespans, and appointed terms. Without them, humans would lack a basis for measuring time's passage.

The concept of Al-Misan (the balance) extends beyond justice or physical scales to encompass all instruments of measure, including clocks and calendars. This links the precise measurement of time to divine order and its necessity for religious obligations like daily prayers and fasting.

2.2. The Inability to "Enumerate" Time

The verse, "Allah determines the night and the day; He knew that you would never be able to enumerate them," underscores a key principle: humanity's limited capacity to comprehend time.

• Scholarly Interpretations:
  • The term "enumerate" (tuhsuhu) is metaphorical, derived from using pebbles (hassa) for counting. It suggests that humans cannot perfectly count or measure their acts of worship or the exact divisions of time.
  • Al-Hassan al-Basri interpreted this as a call to strive for the best possible performance of duties while acknowledging that complete perfection in measurement is unattainable.
  • The structure of the verse, placing "Allah" at the beginning, is seen by some scholars as emphasizing exclusivity, meaning only God possesses full and absolute knowledge of temporal quantities.

The conclusion is that while humans must diligently determine times and measures, they must also recognize that complete, absolute knowledge remains with Allah alone.

3. Categories and Expressions of Time

The Quran uses specific terms for different temporal experiences and distinguishes between times of special spiritual significance.

3.1. Specific Temporal Units

The Quran does not use modern units like seconds or minutes, as its language reflects its era. Instead, it employs terms with rich contextual meanings.

• Al-An (Now): Appears eight times in its definite form. Its meaning varies with context:
  • This immediate moment: As in Pharaoh's last-minute repentance while drowning.
  • A boundary between past and future: As when the Children of Israel tell Moses, "Now you have come with the truth."
  • The present extending indefinitely: As in the verse, "So now have relations with them..."
• Anafen (Recently/Just Now): Indicates a very short time in the past, used to describe the hypocrites' inattentiveness to the Prophet's message.
• Baghtah (Suddenly): Appears 13 times, always in connection with punishment or the Day of Judgment, signifying an unexpected and unanticipated arrival.

3.2. Blessed Time vs. Sacred Time

A crucial distinction is made between sacred and blessed time.

• Sacred Time: Refers to the four sacred months (Dhul-Hijjah, Muharram, Rajab, and one other) wherein specific legal rulings, such as regulations on warfare, apply.
• Blessed Time (Baraka): Refers to moments endowed with divine blessing, growth, and abundance for reasons known to Allah. This is not tied to legal rulings. A time becomes blessed by the significance of the event occurring within it (e.g., the "blessed night" in which the Quran was revealed). Its superiority is determined by spiritual value, not physical length, as exemplified by "The night of decree is better than a thousand months."

4. The Relativity of Time in the Quran

The Quranic framework presents time as relative, anticipating concepts later formalized in modern physics. This relativity appears in both objective, domain-dependent forms and subjective, psychological forms.

4.1. Objective Relativity: Time Across Different Domains

Similar to how time is relative in Einsteinian physics (depending on velocity and gravity), the Quran describes temporal intervals that differ based on the frame of reference.

• Key Verses:
  • "A day with your Lord is like a thousand years of what you count."
  • "The angels and the spirit ascend to Him in a day whose measure is 50,000 years."
• Interpretation: These verses show that a single event can span vastly different durations when measured in the human domain versus the divine domain. The measurement is domain-dependent rather than entity-dependent. Al-Qurtubi concluded that in the hereafter, one divine day equals 1,000 Earth years, serving as a comprehensible conversion factor.
• Example of Divine Manipulation: The story of the man who passed a ruined town and was caused to die for 100 years. Upon revival, he felt he had remained for "a day or part of a day." In this event, Allah suspended time for the man and his food while allowing it to proceed for his donkey (which decayed). This demonstrates Allah's absolute dominion over time, compressing or extending it in different domains simultaneously, independent of physical laws.

4.2. Psychological Relativity: The Subjective Experience of Time

This refers to the inner, subjective perception of time's passage, which is unique to each individual's consciousness (nafs).

• Definition: The human perception of time, the sense of its passing slowly or quickly, and the estimation of its duration based on inner experience. Modern psychology confirms that time seems to slow during anxiety and speed up during joy.
• Quranic Illustrations:
  • The "Heavy Day": The Day of Judgment is described as "heavy." For the disbeliever, its 50,000-year duration is felt as unbearably long and burdensome. For the believer, it is perceived as near and passes with serenity.
  • Underestimation of Earthly Life: On the Day of Judgment, disbelievers will perceive their entire worldly existence—even if it lasted a century—as fleeting. They will swear they remained "but an hour" or "only a little." This reflects how inner perception, especially in moments of reckoning, can radically distort the memory of objective time.

Psychological time is distinct from blessed time; the former is a subjective estimation that can be incorrect, while the latter is a real, fixed spiritual value determined by God.

5. Foundational Principles of Time

The analysis of time in the Quran rests on a set of core principles and can be broken down into essential components for a systematic understanding.

5.1. A Framework for Analysis

Any temporal phenomenon in the Quran can be analyzed through five essential components:

1. The object experiencing time.
2. The temporal measure.
3. The unit of time.
4. The temporal domain (e.g., human, divine).
5. Motion.

5.2. Core Principles of Temporal Relativity

• God is not constrained by time; He encompasses all of creation across all temporal frameworks.
• All created beings are subject to the flow of time.
• For Allah, concepts like little and much, or one and many, are the same; His command is simply "Be, and it is."
• Human understanding of the reality of time is inherently limited.

Time perception is a field of study within psychology and neuroscience that refers to the subjective experience of time, which is measured by someone's own perception of the duration of the indefinite and continuous unfolding of events. The perceived time interval between two successive events is referred to as perceived duration. Another person's perception of time cannot be directly experienced or understood, but it can be objectively studied and inferred through a number of scientific experiments. Time perception is a construction of the brain that is manipulable and distortable under certain circumstances. These temporal illusions help to expose the underlying neural mechanisms of time perception.

Pioneering work, emphasizing species-specific differences, was conducted by Karl Ernst von Baer.^{[citation needed]} Experimental work began under the influence of the psycho-physical notions ofGustav Theodor Fechner with studies of the relationship between perceived and measured time.

William J. Friedman (1993) also contrasted two theories for a sense of time:^[1][2][3]

• The strength model of time memory. This posits a memory trace that persists over time, by which one might judge the age of a memory (and therefore how long ago the event remembered occurred) from the strength of the trace. This conflicts with the fact that memories of recent events may fade more quickly than more distant memories.

• The inference model suggests the time of an event is inferred from information about relations between the event in question and other events whose date or time is known.

Another theory involves the brain's subconscious tallying of "pulses" during a specific interval, forming a biological stopwatch. This theory alleges that the brain can run multiple biological stopwatches at one time depending on the type of task one is involved in. The location of these pulses and what these pulses actually consist of is unclear.^[4] This model is only a metaphor and does not stand up in terms of brain physiology or anatomy.^[5]

Changes with aging[edit]

Psychologists have found that the subjective perception of the passing of time tends to speed up with increasing age in humans. This often causes people to increasingly underestimate a given interval of time as they age. This fact can likely be attributed to a variety of age-related changes in the aging brain, such as the lowering in dopaminergic levels with older age; however, the details are still being debated.^[38][39][40]In an experimental study involving a group of subjects aged between 19 and 24 and a group between 60 and 80, the participants' abilities to estimate 3 minutes of time were compared. The study found that an average of 3 minutes and 3 seconds passed when participants in the younger group estimated that 3 minutes had passed, whereas the older group's estimate for when 3 minutes had passed came after an average of 3 minutes and 40 seconds.^[41][42]

Very young children literally "live in time" before gaining an awareness of its passing. A child will first experience the passing of time when he or she can subjectively perceive and reflect on the unfolding of a collection of events. A child's awareness of time develops during childhood when the child's attention and short-term memory capacities form—this developmental process is thought to be dependent on the slow maturation of the prefrontal cortex and hippocampus.^[5][43]

In Vedic timekeeping, a tithi (also spelled thithi) is a lunar day, or the time it takes for the longitudinal angle between the Moon and the Sun to increase by 12°. Tithis begin at varying times of day and vary in duration from approximately 19 to approximately 26 hours.^[1]

Panchanga[edit]

A Hindu muhurta (moment) can be represented in five attributes of Hindu astronomy namely, vara the weekday, tithi, nakshatra the Moon's asterism,yoga the angular relationship between Sun and Moon and karana half of tithi.

Tithi plays an important role along with nakshatra in Hindu's daily as well as special activities in selecting the muhurta. There are good tithis as well as bad tithis.

There are 30 tithis in each lunar month, named as :

Sl.No	Krishnapaksha (dark fortnight)	Shukla paksha (bright fortnight)	Deity and properties[citation needed]
1	Prathama	Prathama	The presiding deity of the first lunar day is Agni and it is good for all types of auspicious and religious ceremonies
2	Dwitiya	Dwitiya	Vidhatr or Bramha rules this lunar day and is good for the laying of foundations for buildings and other things of a permanent nature.
3	Tritiya	Tritiya	Gauri is the lord of this day and is good for the cuttings of one's hair and nails and shaving.
4	Chaturthi	Chaturthi	Yama/Ganapati is lord of the 4th lunar day, which is good for the destruction of one's enemies, the removal of obstacles, and acts of combat.
5	Panchami	Panchami	The Naaga or Serpents rules this day, which is favourable for administering medicine, the purging of poisons, and surgery.
6	Shashti	Shashti	Karttikeya presides over this day and is favourable for coronations, meeting new friends, festivities, and enjoyment.
7	Saptami	Saptami	The 7th lunar day is ruled by Surya; one may begin a journey, buy conveyances, and deal with other such things as a movable nature.
8	Ashtami	Ashtami	The Rudra rule this day, which is good for taking up arms, building of one's defenses, and fortification.
9	Navami	Navami	The Ambikaa rules this day, with is suitable for killing enemies, acts of destruction, and violence.
10	Dasami	Dashami	The day is ruled by Dharmaraja and is auspicious for acts of virtue, religious functions, spiritual practices, and other pious activities.
11	Ekadasi	Ekadashi	Rudra rule this day; fasting, devotional activities, and remembrance of the Supreme Lord are very favourable.This day has special religious significance inHinduism and Jainism—usually observed by fasting
12	Dvadasi	Dwadashi	The Vishnu or Aditya rules this day, which is auspicious for religious ceremonies the lighting of the sacred fire, and the performance of one's duties.
13	Trayodasi	Thrayodashi	The day is ruled by Cupid and is good for forming friendships, sensual pleasures, and festivities.
14	Chaturdashi	Chaturdashi	Kali rules this day suitable for administering poison and calling of elementals and spirits.
15	Amavasya (new moon)	Purnima or Paurnami (full moon)	The Pitru-devas rule the New Moon suitable for the propitiation of the Manes and performance of austerities.Purnima is ruled by Moon and suitable for merry making,fire sacrifice.

History[edit]

Main article: History of timekeeping devices

Ancient Egyptians used sundials that "divided a sunlit day into 10 parts plus two "twilight hours" in the morning and evening."^[3] The Greek astronomer,Andronicus of Cyrrhus, oversaw the construction of a horologion called the Tower of the Winds in Athens during the first century BCE. This structure tracked a 24-hour day using both sundials and mechanical hour indicators.^[3]

Ancient Sumer and India also divided days into either one twelfth of the time between sunrise and sunset or one twenty-fourth of a full day. In either case the division reflected the widespread use of a duodecimal numbering system. The importance of 12 has been attributed to the number of lunar cycles in a year. In China, the whole day was divided into twelve parts.

Astronomers in Egypt's Middle Kingdom (9th and 10th Dynasties) observed a set of 36 decan stars throughout the year. These star tables have been found on the lids of coffins of the period. The heliacal rising of the next decan star marked the start of a new civil week, which was then ten days. The period from sunset to sunrise was marked by 18 decan stars. Three of these were assigned to each of the two twilight periods, so the period of total darkness was marked by the remaining 12 decan stars, resulting in the 12 divisions of the night. The time between the appearance of each of these decan stars over the horizon during the night would have been about 40 modern minutes. During the New Kingdom, the system was simplified, using a set of 24 stars, 12 of which marked the passage of the night.

Ancient Sinhalese in Sri Lanka divided a solar day into 60 Peya (now called Sinhala Peya). One Sinhala Peya was divided into 24 Vinadi. Since 60 (peya) x 24 (vinadi) = 24 (hours) x 60 (minutes), oneVinadi is equal to one present-day standard minute.

Earlier definitions of the hour varied within these parameters:

• One twelfth of the time from sunrise to sunset. As a consequence, hours on summer days were longer than on winter days, their length varying with latitude and even, to a small extent, with the local weather (since it affects the atmosphere's index of refraction). For this reason, these hours are sometimes called temporal, seasonal, or unequal hours. Romans, Greeks and Jews of the ancient world used this definition; as did the ancient Chinese and Japanese. The Romans and Greeks also divided the night into three or four night watches, but later the night (the time between sunset and sunrise) was also divided into twelve hours. When, in post-classical times, a clock showed these hours, its period had to be changed every morning and evening (for example by changing the length of its pendulum), or it had to keep to the position of the Sun on the ecliptic (see Prague Astronomical Clock).
• One twenty-fourth of the apparent solar day (between one noon and the next, or between one sunset and the next). As a consequence hours varied a little, as the length of an apparent solar day varies throughout the year. When a clock showed these hours it had to be adjusted a few times in a month. These hours were sometimes referred to as equal or equinoctial hours.
• One twenty-fourth of the mean solar day. See solar time for more information on the difference to the apparent solar day. When an accurate clock showed these hours it virtually never had to be adjusted. However, as the Earth's rotation slows down, this definition has been abandoned. See UTC.
• Julius Caesar's astronomers explained the need for 12 months in a year and the addition of a leap year to synchronize with the seasons. At the time, there were only ten months in the calendar while there are just over 12 lunar cycles in a year.
  The months of January and February were added to the calendar and the original fifth and sixth months were renamed July and August in honour of Julius Caesar and his successor Augustus.
  These months were both given 31 days to reflect their importance, having been named after Roman leaders.

For thousands of years, devices have been used to measure and keep track of time. The current sexagesimal system of time measurement dates to approximately 2000 BC, in Sumer. The Ancient Egyptians divided the day into two 12-hour periods, and used large obelisks to track the movement of the Sun. They also developed water clocks, which were probably first used in the Precinct of Amun-Re, and later outside Egypt as well; they were employed frequently by the Ancient Greeks, who called them clepsydrae. The Shang Dynasty is believed to have used the outflow water clock around the same time, devices which were introduced from Mesopotamiaas early as 2000 BC. Other ancient timekeeping devices include the candle clock, used in China, Japan, England and Iraq; the timestick, widely used in India and Tibet, as well as some parts of Europe; and the hourglass, which functioned similarly to a water clock. The sundial, an early clock, relies on shadows to provide a good estimate of the hour on a sunny day. It is not so useful in cloudy weather or at night and requires recalibration as the seasons change (if the gnomon was not aligned with the Earth's axis). The earliest known clock with a water-powered escapement mechanism, which transferred rotational energy into intermittent motions,^[1] dates back to 3rd century BC ancient Greece;^[2] Chinese engineers later invented clocks incorporating mercury-powered escapement mechanisms in the 10th century,^[3]followed by Arabic engineers inventing water clocks driven by gears and weights in the 11th century.^[4]

The first mechanical clocks, employing the verge escapement mechanism with a foliot or balance wheel timekeeper, were invented in Europe at around the start of the 14th century, and became the standard timekeeping device until the pendulum clock was invented in 1656. The invention of the mainspring in the early 15th century allowed portable clocks to be built, evolving into the first pocketwatches by the 17th century, but these were not very accurate until the balance spring was added to the balance wheel in the mid 17th century. The pendulum clock remained the most accurate timekeeper until the 1930s, when quartz oscillators were invented, followed byatomic clocks after World War 2. Although initially limited to laboratories, the development of microelectronics in the 1960s made quartz clocks both compact and cheap to produce, and by the 1980s they became the world's dominant timekeeping technology in both clocks and wristwatches. Atomic clocks are far more accurate than any previous timekeeping device, and are used to calibrate other clocks and to calculate the proper time on Earth; a standardized civil system, Coordinated Universal Time, is based on atomic time.

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The heliacal rising (/hɪˈlaɪəkəl/, hi-ly-ə-kəl) of a star occurs when it first becomes visible above the eastern horizon for a brief moment just before sunrise, after a period of time when it had not been visible.^[1]

Each day after the heliacal rising, the star will rise slightly earlier and remain visible for longer before the light from the rising sun makes it disappear (the sun appears to drift eastward relative to the stars by about one degree a day along a path called the ecliptic). Over the following days the star will move further and further westward (about one degree per day) over the dome of the pre-dawn sky, until eventually it is no longer visible in the sky at dawn because it has already set below the western horizon. This is called the cosmical setting.^[2] The same star will reappear in the eastern sky at dawn approximately one year after its previous heliacal rising. Because the heliacal rising depends on the observation of the object, its exact timing can be dependent on weather conditions.^[3]

Some stars, when viewed from a particular latitude on Earth, will not have a heliacal rising or setting. These are Circumpolar stars, which are either always in the sky, or never. For example, theNorth Star is not visible in Australia and the Southern Cross is not seen in Europe, because they always stay below the respective horizons.

Constellations containing stars that rise and set were incorporated into early calendars or zodiacs. The ancient Egyptian calendar was based on the heliacal rising of Sirius. The ancient Egyptians devised a method of telling the time at night based on the heliacal risings of 36 stars called decan stars (one for each 10° segment of the 360° circle of the zodiac/calendar). TheSumerians, the Babylonians, and the ancient Greeks also used the heliacal risings of various stars for the timing of agricultural activities.

To the Māori of New Zealand, the Pleiades are called Matariki, and their heliacal rising signifies the beginning of the new year (around June). The Mapuche of South America called the PleiadesNgauponi which in the vicinity of the we tripantu (Mapuche new year) will disappear by the west, lafkenmapu or ngulumapu, appearing at dawn to the East, a few days before the birth of new life in nature. Heliacal rising of Ngauponi, i.e. appearance of the Pleiades by the horizon over an hour before the Sun approximately 12 days before the winter solstice, announced we tripantu.

When a planet has a heliacal rising, there is a conjunction with the sun beforehand. Depending on the type of conjunction, there may be a syzygy, eclipse, transit, or occultation of the sun. The detailed search for the Moon's heliacal rising (a.k.a. the new moon) often determines the start of a month in a lunar calendar, which may have religious or political significance.

The corresponding rising of a celestial body above the eastern horizon at sunset is called its acronychal rising, which for a planet signifies a solar opposition in astrology, another type of syzygy. If the moon has an acronychal rising, it is usually a full moon or potentially a lunar eclipse.

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Timekeeping devices of early civilizations[edit]

See also: History of calendars

The sun rising over Stonehenge on the June solstice

Many ancient civilizations observed astronomical bodies, often the Sun and Moon, to determine times, dates, and seasons.^[5][6] Methods of sexagesimaltimekeeping, now common in Western society, first originated nearly 4,000 years ago in Mesopotamia and Egypt;^[5][7][8] a similar system was developed later inMesoamerica.^[9] The first calendars may have been created during the last glacial period, by hunter-gatherers who employed tools such as sticks and bones to track the phases of the moon or the seasons.^[6] Stone circles, such as England's Stonehenge, were built in various parts of the world, especially in Prehistoric Europe, and are thought to have been used to time and predict seasonal and annual events such as equinoxes or solstices.^[6][10] As those megalithiccivilizations left no recorded history, little is known of their calendars or timekeeping methods.^[11]

Ancient Egypt[edit]

See also: History of timekeeping devices in Egypt

Ancient Egyptian sundial (c. 1500 BC) from the Valley of the Kings. Daytime divided into 12 parts.

The oldest known sundial is from Egypt, it dates back to around 1500 BC (19th Dynasty), and was discovered in the Valley of the Kings in 2013.^[12] Sundialshave their origin in shadow clocks, which were the first devices used for measuring the parts of a day.^[13] Ancient Egyptian obelisks, constructed about 3500 BC, are also among the earliest shadow clocks.^[6][14][15]

The Luxor Obelisk in Place de la Concorde, Paris, France

Egyptian shadow clocks divided daytime into 12 parts with each part further divided into more precise parts.^[16] One type of shadow clock consisted of a long stem with five variable marks and an elevated crossbar which cast a shadow over those marks. It was positioned eastward in the morning, and was turned west at noon. Obelisks functioned in much the same manner: the shadow cast on the markers around it allowed the Egyptians to calculate the time. The obelisk also indicated whether it was morning or afternoon, as well as the summer and winter solstices.^[6][17] A third shadow clock, developed c. 1500 BC, was similar in shape to a bent T-square. It measured the passage of time by the shadow cast by its crossbar on a non-linear rule. The T was oriented eastward in the mornings, and turned around at noon, so that it could cast its shadow in the opposite direction.^[18]

Although accurate, shadow clocks relied on the sun, and so were useless at night and in cloudy weather.^[17][19] The Egyptians therefore developed a number of alternative timekeeping instruments, including water clocks, and a system for tracking star movements. The oldest description of a water clock is from the tomb inscription of the 16th-century BC Egyptian court official Amenemhet, identifying him as its inventor.^[20] There were several types of water clocks, some more elaborate than others. One type consisted of a bowl with small holes in its bottom, which was floated on water and allowed to fill at a near-constant rate; markings on the side of the bowl indicated elapsed time, as the surface of the water reached them. The oldest-known waterclock was found in the tomb of pharaohAmenhotep I (1525–1504 BC), suggesting that they were first used in ancient Egypt.^[17][21][22] Another Egyptian method of determining the time during the night was using plumb-lines called merkhets. In use since at least 600 BC, two of these instruments were aligned with Polaris, the north pole star, to create a north–southmeridian. The time was accurately measured by observing certain stars as they crossed the line created with the merkhets.^[17][23]