Cycles of Darkness and Light
In this chapter we will discuss what the word "day" means. How do we define the word "day"? How do we know when a "day" has passed?
A Criterion for Observing the Passing of a Day
Science requires operational criteria for observing facts. Let’s examine different situations that people call “day” or “night” and see what is common about them.
On Mars a day lasts 24 hours, 39 minutes, 35.24409 seconds, because Mars rotates more slowly that the Earth. The Moon turns so slowly that days last about four weeks or about a month.
On the Earth, darkness and light regularly alternate in cycles. Except on the equator the durations of the dark and bright phases of the cycle vary throughout the year. The sum of the two also varies from 24 hours throughout the year, because the Earth’s orbit is not circular but slightly elliptical. In certain extreme places on Earth the phases of the cycles have other durations. When we go north of the Arctic Circle or south of the Antarctic Circle, nights and days suddenly last for months instead of hours. The arctic regions are alternately the land of darkness at noon and the land of the midnight Sun. At the poles of the Earth there are six months of darkness and six months of daylight. There a day lasts a year. The Sun provides the daylight at the poles of the Earth just as it does everywhere else on the Earth, but it produces days of markedly different durations.
What is common about days on Mars, on the Moon, on Earth at the poles, and on Earth in the equatorial or temperate zones? When all of nature is dark we say it is “night,” but when nature is bright we call it “day.” The light we see depends on our location or point of view. In different situations the duration of a day may vary considerably, but the definition remains the same throughout the universe and at all times. A day is one cycle of alternation between darkness and light.
In outer space, it is always day when the observer is close to the Sun. The Sun’s daylight phase has lasted almost 5 000 million years. The Earth as a whole is always in daylight.
How far do we have to go from the Sun to find night? The Moon gives us a criterion for finding where and when night begins and ends.
Darkness does not have to be complete to be the darkness of night. The most intense nighttime light is that of a full Moon at midnight. We may therefore define the boundary between day and night as the place where the intensity of sunlight diminishes to the brightness of the full Moon at midnight. The Moon and Sun as seen from the Earth appear to have the same size. The intensity of moonlight is 2.09 parts per million of the intensity of sunlight. Therefore the boundary between day and night lies where sunlight drops to the intensity of moonlight on Earth. Put the other way around, noonday sunlight is 478 000 times as intense as the light of the full Moon at midnight. Light intensity decreases in proportion to the square of the distance from the source. The square root of 478 000 is 692. Day ends and night begins when we go out 692 times the distance of the Earth from the Sun. The average distance to Pluto is 39.5 times the distance of the Earth from the Sun, so the boundary of night is 17.5 times farther away than Pluto. Therefore the entire solar system is always in daylight. At any point close to the Sun a planet or asteroid is in perpetual day. Far out in the darkness is perpetual night, until we come close to another star.
From all the foregoing, we can see that the duration of a day is not an essential part of a scientific criterion for observing the passing of a day. The scientific criterion is as follows: A day is a natural cycle of darkness and light.
On Mars a day lasts 24 hours, 39 minutes, 35.24409 seconds, because Mars rotates more slowly that the Earth. The Moon turns so slowly that days last about four weeks or about a month.
On the Earth, darkness and light regularly alternate in cycles. Except on the equator the durations of the dark and bright phases of the cycle vary throughout the year. The sum of the two also varies from 24 hours throughout the year, because the Earth’s orbit is not circular but slightly elliptical. In certain extreme places on Earth the phases of the cycles have other durations. When we go north of the Arctic Circle or south of the Antarctic Circle, nights and days suddenly last for months instead of hours. The arctic regions are alternately the land of darkness at noon and the land of the midnight Sun. At the poles of the Earth there are six months of darkness and six months of daylight. There a day lasts a year. The Sun provides the daylight at the poles of the Earth just as it does everywhere else on the Earth, but it produces days of markedly different durations.
What is common about days on Mars, on the Moon, on Earth at the poles, and on Earth in the equatorial or temperate zones? When all of nature is dark we say it is “night,” but when nature is bright we call it “day.” The light we see depends on our location or point of view. In different situations the duration of a day may vary considerably, but the definition remains the same throughout the universe and at all times. A day is one cycle of alternation between darkness and light.
In outer space, it is always day when the observer is close to the Sun. The Sun’s daylight phase has lasted almost 5 000 million years. The Earth as a whole is always in daylight.
How far do we have to go from the Sun to find night? The Moon gives us a criterion for finding where and when night begins and ends.
Darkness does not have to be complete to be the darkness of night. The most intense nighttime light is that of a full Moon at midnight. We may therefore define the boundary between day and night as the place where the intensity of sunlight diminishes to the brightness of the full Moon at midnight. The Moon and Sun as seen from the Earth appear to have the same size. The intensity of moonlight is 2.09 parts per million of the intensity of sunlight. Therefore the boundary between day and night lies where sunlight drops to the intensity of moonlight on Earth. Put the other way around, noonday sunlight is 478 000 times as intense as the light of the full Moon at midnight. Light intensity decreases in proportion to the square of the distance from the source. The square root of 478 000 is 692. Day ends and night begins when we go out 692 times the distance of the Earth from the Sun. The average distance to Pluto is 39.5 times the distance of the Earth from the Sun, so the boundary of night is 17.5 times farther away than Pluto. Therefore the entire solar system is always in daylight. At any point close to the Sun a planet or asteroid is in perpetual day. Far out in the darkness is perpetual night, until we come close to another star.
From all the foregoing, we can see that the duration of a day is not an essential part of a scientific criterion for observing the passing of a day. The scientific criterion is as follows: A day is a natural cycle of darkness and light.
An Ancient Criterion
Some people are surprised to discover that the scientific criterion above appeared very early in ancient times, in the Bible. Its creation narrative organizes the stages of creation in days. God created the heavens and the Earth at the beginning of the first evening, and ended by creating the first human couple on the sixth morning.
In our modern way of thinking a day is 24 consecutive hours. On first reading the Genesis narrative, many people think that Moses was wrong about the timing of creation. To them Moses seems to say that the creation of the universe took just 6 x 24 = 144 consecutive hours. That contradicts what science now tells us, that the first stars are thousands of millions of years older than the Earth and Sun, and the first light is older still. But the discrepancy disappears when we ask who was first to use the word day in the creation narrative.
In our modern way of thinking a day is 24 consecutive hours. On first reading the Genesis narrative, many people think that Moses was wrong about the timing of creation. To them Moses seems to say that the creation of the universe took just 6 x 24 = 144 consecutive hours. That contradicts what science now tells us, that the first stars are thousands of millions of years older than the Earth and Sun, and the first light is older still. But the discrepancy disappears when we ask who was first to use the word day in the creation narrative.