“Is a day shorter than a year?”
Asking this question would strangely generate different replies. A rushing answer will be “Yes”. But, a smart person must ask for a further clarification. Does the question refer to the day on Earth or on another celestial body?
Colloquially, a day is the time that elapses between sunrise today and sunrise tomorrow. In Celestial Mechanics, a ‘day’ is the span of time of the rotation of a celestial object.
Astronomy refers to the civil day as the Solar or Synodic Day. We measure it according to the position of the sun in the sky. We define it as the time interval required for the sun to go from crossing the meridian (the line dividing the sky into an eastern and western half) directly opposite an observer to returning to that meridian.
Another type of day is the Sidereal Day (from Latin word ‘sidus’ meaning star), the time elapsed between two successive meridian crossings of a fixed star, and is about four minutes shorter than a solar day.
This means that a star will return to its original place in the sky four minutes earlier everyday.
A sidereal day is the actual time it takes the earth to complete one spin around itself. The earth must then turn for four additional minutes. That’s in order to bring the sun to its original point in the sky.
We calculate a month by the movement of the moon, which orbits +the earth, which orbits the sun.
The Synodic Month that we generally know is the time between two successive full moons. It’s also the period of time it takes the sun to return to its position in the sky with respect to the moon. This process takes 29.5 days. In other words, a day on the moon lasts 29.5 Earth days.
On the other hand, a Sidereal Month is about 27.3 Earth days; the time by which the moon returns to the same position with respect to the stars. Or the time it takes the moon to move completely around the Earth. Because a full moon requires a sun-earth-moon alignment, the moon must travel over two more days to get back to its original position [1].
A year for a layman lasts 365 days. But, for an astronomer it depends on whether it’s a sidereal, Julian, tropical/solar, or any other type of a year.
A dictionary definition of a year is the 365.25 solar days by which a planet revolves around the sun. It’s also the time when the sun returns back to a specific reference position in the sky.
While a Sidereal Year is the time it takes the Earth to rotate around the sun with respect to fixed stars, and is about 20 minutes longer than the solar year [2].
Time in Celestial Objects: Table of Planets’ days and years [3]
| Planet | Orbital Period | Rotation Period |
| Mercury | 87.96 Earth days | 58.7 Earth days |
| Venus | 224.68 Earth days | 243 Earth days |
| Earth | 365.26 days | 24 hours |
| Mars | 686.98 Earth days | 24.6 Earth hours |
| Jupiter | 11.862 Earth years | 9.84 Earth hours |
| Saturn | 29.456 Earth years | 10.2 Earth hours |
| Uranus | 84.07 Earth years | 17.9 Earth hours |
| Neptune | 164.81 Earth years | 19.1 Earth hours |
The relationship between days and years isn’t straightforward. They are both distinct measures, and it just happens that on Earth a year is equal to about 365 days.
Yet in fact, different celestial bodies have different shapes and orbits, and this results in each body having distinct definitions and measurements of time.
For example, Jupiter takes about 10 Earth hours to make a full spin, but takes 12 Earth years to make one revolution around the sun.
Saturn takes an almost equal time as Jupiter to make a spin, but takes more than twice as long to rotate around the sun.
Outer planets – Jupiter, Saturn, Uranus and Neptune – called “Jovian Planets”, are larger, more massive, and composed mostly of gas. They generally spin faster than the inner, rocky, denser planets – Mercury, Venus, Earth, and Mars – also called terrestrial planets. Jovian planets thus have shorter ‘Earth’ days than terrestrial planets.
Inversely, gaseous planets have longer planetary years than terrestrial planets, particularly due to their greater distance from the sun.
This greater distance also results in a weaker gravitational pull from the sun on the planet, causing the planet to rotate at a relatively slower speed.
The different spinning/rotational speeds of celestial bodies are mostly attributed to how the body was initially formed and the gravitational forces that surround it.
Similarly interesting is the phenomenon of the continuous movement of the body, which is explained with the concept of angular momentum.
Angular Momentum
Angular momentum is a conserved property of objects rotating on an axis, meaning that rotation remains constant in both direction and magnitude.This phenomenon was explained in the 17th century by German mathematician and astronomer Johannes Kepler.
Kepler’s law of planetary motion explains that during a planet’s elliptical orbit around the sun, the planet moves faster when it is closer to the sun (perihelion point), and slower when it is away from the sun (aphelion point).
This is because the closer the planet is to the sun, the stronger is the sun’s gravitational pull on the planet, and the faster the planet moves – and the opposite is true.
Kepler’s law also states that a line that connects a planet to the sun sweeps out equal areas in equal times. It is due to these variations that the angular momentum is conserved, and planet remains in constant orbit [4].
A Day in the Holy Qur’an
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Colloquially, a day is the time that elapses between sunrise today and sunrise tomorrow. In Celestial Mechanics, however, the exact definition of a ‘day’ depends on the span of time considered when discussing the rotation of a celestial object.
We live in a universe where the notion of time is very versatile. In the early 20thcentury, renowned German scientist Albert Einstein developed the theory of relativity, explaining this flexibility of time and space and relating them to gravitational fields.
Much earlier in the 7th century, the Noble Qur’an was revealed with indications to this concept. In several verses in the Qur’an, Allah SWT emphasizes that the duration of a day in His measures is different from its duration in the human measures.
In the following verses, Allah SWT states that a day in His judgment is equivalent to a thousand Earth years.
“They ask you to hasten the punishment. God will not break His promise. A day with your Lord is equivalent to a thousand years in the way you count.” (Surat Al-Haj 22:47).
“He directs the whole affair from heaven to earth. Then it will again ascend to Him on a Day whose length is a thousand years by the way you measure.” (Surat As-Sajdah 32:5).
In another verse, which denotes another context, a day in Allah’s measures is equal to fifty-thousand years: “The angels and the Spirit ascend to Him in a day whose length is fifty thousand years.” (Surat Al-Ma’arij 70:4).
Other verses explain that people in the Hereafter will perceive time to be much shorter than Earth time; “He will say, ‘How many years did you tarry on the earth?’ They will say, ‘We tarried there for a day or part of a day. Ask those able to count!’ He will say, ‘You only tarried there for a little while if you did but know!” (Surat Al-Mu’minun 23:112-114).
This verse, while carrying deep semantics in signifying that our time for Earth is too short to be wasted, also shows that the duration of a day on Earth is not the same as its duration in Allah’s SWT specific time-keeping measures that only He Almighty knows of.
Atheists claim that there is a contradiction between verses 32:5 and 70:4, because the first states that the day in Allah’s measures is equivalent to a thousand years and the latter states that it is equivalent to fifty thousand years.
This claim, besides being too naïve, is easily shattered by the scientific facts: the verses may very well refer to different things traveling to different places, which explains the different durations. In addition, the latter verse doesn’t state that the fifty thousand years are in terms of ‘Earth’ years.
In another claim they argue that verses don’t differentiate between synodic and sidereal periods. This claim is also simply counter-argued by understanding that the Noble Qur’an is a book of guidance, and not science.
The Noble Qur’an doesn’t contain a simple definition of different notions of time; neither does it literally explain the idea of time relativity.
Initially, the verses can be interpreted according to their context which results in different psychological perception of time. However, as science and research reveal more about this concept, scientific signs included in these verses become evident.
Reading the related verses with the comprehension of the scientific knowledge that time is relative rather than absolute will allow for better understanding of the Qur’an.