To speak of these three movements so essential to life on our planet, we need to frame them not only within the solar system, as we have already done in speaking about the interaction between the sun, the Earth and the moon, but also within our galaxy, the Milky Way, which may well contain over 100 billion solar systems similar to ours. According to the observations made of the outer space by the Hubble telescope, there may be 100 billion galaxies in the universe but this number is likely to increase to 200 billion as telescope technology gets more advanced.
It is providential that Earth is a telluric or terrestrial planet in the context of the solar system where most of the planets are gaseous; it is also providential the position Earth occupies among the four terrestrial planets, third place between Venus and Mars, where it is not too far from the sun like Mars, a too cold planet, nor too close to the sun like Venus, the hottest planet in the solar system.
It is providential the magnitude of our planet. Earth at 6,371 km in diameter is only slightly bigger than Venus at 6,052 km. Venus has an atmosphere of gas composition similar to that of Earth before the emergence of life; but because Venus is too close to the sun, life forms were not able to develop.
Furthermore, if Earth was smaller, like Mercury at 2,440 km or like our moon, it would not have large enough force of gravity to develop an atmosphere. If it was 6 times larger, like Neptune at 24,622 km, it would be gaseous like this planet. On the other hand, even if it was only twice as large, life as we know it would not be possible: animals and plants with the same structure and mass would weigh twice as much and collapse under their own weight.
Finally, it is also providential the position of our solar system in relation to the center of our galaxy. Our solar system finds itself 27,000 light years from the center of our galaxy. The Milky Way has more than 200 billion stars and a diameter of 100,000 light years. In our galaxy, there are zones of habitability and zones of inhabitability. The zones of habitability forms a ring around the center of the galaxy, the inner circle being 13,000 light years from the center and the outer ring at 32,600 light years from the same center.
This is the zone where systems that contain planets capable of harboring some kind of life may appear. On one hand, in the regions beyond those of 32,600 light years from the center of the galaxy, the star metallicity is too low to permit formation of telluric planets like Earth. On the other hand, less than 13,000 light years from the center, the exposure to highly energetic winds, such as supernovas, would be very hostile to life. Within our galaxy, therefore, we are in a very privileged position.
The Orbits of the Planets are Spiral not Elliptical
According to what we were taught in school, stars are motionless, only planets move. For this reason, we have the concept that the sun is a fixed, static, motionless star while the Earth, as well as the other planets in the solar system, revolves around the sun in an elliptical orbit. However, this is not what really happens; the revolution of the planets around the sun is not elliptical but spiral.
The incredible truth is that the sun revolves around the center of the galaxy at a dizzying speed of 828,000 km/hr or 230 km/s, and takes 225 million years to complete a full orbit. It is therefore incorrect to say that the Earth and the other planets move around the sun. The truth is that the sun, in its orbit around the center of the galaxy, drags the planets along with it.
The movement the planets trace in both being dragged by the sun and orbiting around it is, of course, a spiral not elliptical, which would result if the sun was motionless. The spiral is therefore the geometric shape of motion – a combination of a circle and a line, two diametrically opposed ways of understanding time.
The Greeks understood time as a circle – spring, summer, fall, winter – from where the myth of the eternal return was born. The Jews, on the other hand, understood time as a line that comes from the past and passes through the present to head into the future. They have as a paradigm their epic of the exodus from Egypt: the past, represented by the slavery, which passes through the present sacrifice with the crossing of the desert towards the future, the Promised Land of freedom and progress.
The sun moves, and as the consequence the planets follow the sun, or rather, the sun drags with it the planets that revolve around it, tracing out a spiral. This geometric figure is very familiar to us because it is the shape of our galaxy, the Milky Way, the form of hurricanes and tornadoes, the shape of the propellers that move both the airplanes and the ships, the shape of the screws piercing into the wood or other objects, the geometric shape of our genetic code, the DNA, and even the way our hair grows on top of our head, it traces out the shape of the Milky Way.
The spiral is also the geometric figure that water forms on its path down the drain in a sink or toilet bowl; this whirlwind rotates counterclockwise in the northern hemisphere and clockwise in the southern hemisphere, with no whirlwind at the equator. Even hurricanes and typhoons spin in spiral but in opposite directions according to the hemisphere in which they occur.
The Liturgical Year – Christian Concept of Time
The Christian concept of time is also a spiral that revolves around an axis that is Christ. As the Earth completes one revolution around the sun every 365 days, and so we complete a revolution around our Sun that is Christ, the axis of Salvation History: for “Jesus Christ is the same yesterday and today and forever.” (Hebrews 13:8)
I am the vine, you are the branches. Those who abide in me and I in them bear much fruit, because apart from me you can do nothing. (John 15:5)
As the Earth cannot live without the sun, we also cannot live without Christ, from him we receive life like branches from the vine stock. “Put on the Lord Jesus Christ, and make no provision for the flesh, to gratify its desires.” (Romans 13:14)
There is a Portuguese folk song that sings, “Spring goes back and forth, youth goes and does not come back”, which sums up the two ancient concepts of time into one. We live through the seasons again and again, and the seasons of the Church, such as Advent, Lent etc. again and again; but the spring or the Advent of last year is not the same as this year’s nor will it be the same as that of next year.
The rectilinear concept of time reveals that time is a continuous unrepeatable ongoing process – as the Greek philosopher Heraclitus said, we cannot bathe twice in the same river. The circular concept of time calls for constancy, the seasons repeat again and again; the spiral is the combination of the constant and the variable. Christ is the axis of History, He is the constant; the variable is each year around Him for the purpose, as St. Paul says, of putting on Him, that is, of us being more and more like Christ.
The Movement of Revolution of the Planets
Using the Earth’s year of 365 days (time taken for the planet to go one orbit around the sun) and Earth’s day of 24 hours (time taken for the planet to turn once around its own axis) as reference, here are the days and years it takes the other planets in the solar system, including our moon, to do these two types of revolution:
Mercury: a year lasts 88 days, a day lasts 58 days and 16 hours
Venus: a year lasts 225 days, a day lasts 243 days
Earth: a year lasts 365 days, a day lasts 24 hours
Moon: a year equals a day and both last 29.5 days
Mars: a year lasts 687 days, a day lasts 24 hours and 37 minutes
Jupiter: a year lasts 4330 days, a day lasts 9 hours and 56 minutes
Saturn: a year lasts 10,756 days, a day lasts 10 hours and 15 minutes
Uranus: a year lasts 30,687 days, a day lasts 12 hours and 14 minutes
Neptune: a year lasts 60,190 days, a day lasts 16 hours and 7 minutes
The Three Movements of the Earth
In school, we learn that the Earth has two movements, rotation around its axis taking 24 hours for a complete turn, and revolution around the sun taking 365 days which makes up one year with its 4 seasons which are more noticeable north of the Tropic of Cancer and south of the Tropic of Capricorn. However, Earth has yet one other less perceptible movement, but also cyclical that varies in the space of 26,000 years.
The Movement of Revolution
As we have said, the sun together with the Earth and the other planets of the solar system revolve around the center of our galaxy; but this movement does not affect us because it is a joint movement, that is, of the whole solar system in relation to the center of the galaxy.
What is more important is the movement of revolution that the Earth takes around the sun, as it is dragged by the sun’s gravitational force. Earth completes a turn around the sun in 365 days, 5 hours and 57 minutes, that is, 365.2422 days, the duration of one year, at a speed of 106,000 km/hr which is equivalent to travelling through space a distance of 2,544,000 kilometers every day. Why then do we not see the Earth move but rather the sun? For the simple reason that we are following this movement, just like when we are inside a moving car what we see moving is not the car but the trees that are still.
Going back to the traditional elliptical shape, for an easier understanding of this movement and its effects, the equinoxes (spring and autumn) correspond to the center of the ellipse and the solstices (summer and winter) to the poles of the ellipse.
The rotation of the Earth around its own axis alone does not explain the seasons of the year; these occur fundamentally because the Earth’s axis, an imaginary line that runs through the Earth’s north and south poles, is tilted 23.5 degrees relative to our orbital plane, that is, the plane of Earth’s orbit around the sun. If the Earth’s axis was not tilted and hence it travels parallel to the sun’s axis, there would be no seasons and the climate would always be the same.
During the spring equinox (March 21), it is spring in the northern hemisphere and autumn in the southern hemisphere, and during the autumn equinox (September 23), it is autumn in the northern hemisphere and spring in the southern hemisphere. The word equinox is derived from Latin word aequinoctium to mean equal night, or when the day and the night have the same duration, that is, 12 hours of daylight and 12 hours of darkness. This occurs twice a year, on March 21 and September 23, when the sun’s rays fall directly on the Earth’s equator.
In relation to the sun, the tilting of the Earth makes no difference when it is halfway in its orbit around the sun. But during the solstices, when the Earth is at the apex of its elliptical path around the sun, it shows to the sun one hemisphere more than the other. On June 21, it shows the northern hemisphere so that the rays of the sun fall perpendicularly over this hemisphere. On December 22, the sun is on the other apex of the ellipse, so that it shows the southern hemisphere where it is summer and hides from the northern hemisphere where it is winter.
During the winter solstice (December 22), it is winter in the northern hemisphere and the summer in the southern hemisphere; the situation is reversed during the summer solstice (June 21) where it is summer in the northern hemisphere and winter in the southern hemisphere. During the solstices, the hemisphere that experiences summer is met with an increase in hours of daylight, while the hemisphere that is in winter experiences a decrease in hours of daylight. During the winter solstice, the North Pole is blanketed in darkness all day long and the South Pole has 24 hours of daylight; during the summer solstice, the situation is reversed and there is daylight at the North Pole all day long while the South Pole is plunged into complete darkness all day.
It is difficult to imagine the Earth without the four seasons, because they define its balance. In agriculture, there would be less variety of plants and animals, and certain viruses and pests would thrive to disastrous consequences, such as mosquitoes and the transmission of diseases they carry. Therefore, it would not only be the agriculture of the planet that would be affected, but also the health of the planet itself. Without the four seasons, the planet would always be cold on one side and hot on the other, not a flourishing scenario.
The Oscillation of the Earth’s Axis
The Earth’s axis is tilted 23.5 degrees; in this way, the Earth’s axis followed out into the space from the North Pole will point to a particular star and this star from the northern hemisphere is known as the North Star or the Pole Star. Presently the North Star is the star Polaris. But since the Earth’s axis also moves, the North Star will not always point to the star Polaris. As the axis moves like a wobbling spinning top, Earth traces out with its axis a circle that is completed every 26,000 years. It takes 72 years for the Earth to move one degree in this imaginary circle traced out by its axis.
The Polaris in the Ursa Minor constellation is today the North Star. In year 7500, it will be Alpha Cephei in the constellation of Cepheus; in year 11500, it will be Delta Cygni in the constellation of Cygnus; in year 14000, it will be the star Vega in the constellation of Lyra; in the year 23000, it will be star Thuban in the constellation of Draco; finally in the year 26000, the Pole Star or the North Star will again be Polaris in the constellation of Ursa Minor.
If the Earth’s axis was not tilted 23.5 degrees as it is, the perpendicular axis of the Earth, known as the north ecliptic pole would be pointed to the constellation of Draco, Latin for Dragon. For this reason, the center of the sphere that the axis of the Earth traces out every 26,000 years has this constellation at the center.
The Rotation of the Earth
The Earth rotates around its own axis (imaginary) and completes a turn every 24 hours. This counterclockwise movement is decelerating due to the influence of the moon on the Earth; that is, in the past, the days were shorter and in the future, the days will be longer. At the time of the dinosaurs, each day lasted 22 hours.
Why does the Earth spin around itself? To answer this question we must go all the way back to the Big Bang itself, the beginning of everything: time, space and movement. Everything that still moves in the universe today is the result of that initial movement, that is, from the big explosion. The Big Bang, as we know it, gave rise to a great cloud of hydrogen and dust.
As this cloud was not a homogeneous mass, as it expanded it provoked asymmetric gravitational forces that following the law of the conservation of angular momentum (it is more difficult to change the axis of rotation of a moving wheel than to change it when it is at rest) resulted in celestial bodies, both stars and planets, that are round.
Therefore, it is still the inertia of the first movement, from the great explosion, that keeps the stars and our Earth in particular rotating uninterrupted, until they are subjected to another movement, like an impact with another celestial body. Therefore, our planet was born already rotating around itself and it is logical that it will continue to do so while it exists.
Factors that Influence the Velocity of the Rotation
There are factors that influence the speed of rotation of the planets around themselves:
- The force of gravity of the sun – The closer a planet is to its star, the more noticeable this influence; while Mercury makes one complete turn around its axis, Earth would have already turned 58 times.
- The duration of the formation of the planet – The faster the gravitational collapse that formed a planet, the greater the conservation of angular momentum, that is, the greater its speed of rotation.
- The impact of meteorites – The impact of comets or meteorites, depending on their magnitude, disturbs the initial inertia of a planet and can decelerate or even project it out of its orbit.
- The influence of the moon – As we have already said, the moon decelerates the Earth’s rotational movement. The Earth loses its speed at a rate of one thousandth of a second per year. On the other hand, and by virtue of the effect the moon has on the tides, the moon is moving away from the Earth at a rate of one millimeter per year. In millions of years, each day will have 25 or 26 hours.
The most noticeable effect is of the day and night. If we embark on a spaceship and set ourselves hundreds of kilometers above the North Pole, we will see the Earth turning counterclockwise.
The rotational motion has a constant effect on the climate of our planet, that is, an effect that does not vary according to the seasons. This is known as the Coriolis effect: in the northern hemisphere the systems of low atmospheric pressure turn to the left and of the high pressure to the right; in the southern hemisphere the opposite is true.
The imperfect spherical shape that the Earth finds itself, somewhat flattened at the poles and bulging at the equator, is also a consequence of its rotational motion. For this reason, the flight paths that go from one side of the planet to the other pass through the poles and not through the equator.
The spinning movement favors or tones the magnetic field that surrounds the planet, and protects it from the sun’s rays and, above all, from the solar winds by diverting them to the poles, as we have already seen, giving rise to the auroras at the poles.
Finally, over time the fact that the Earth rotates around itself causes other changes or variations, such as the depth of the seas and oceans, the height of the mountains, and the movement of the tectonic plates.
We live on a planet that for billions of years has favored life, but is a living planet that is constantly changing, although imperceptible at times. Just as there was a time when there was no life on this planet, there may reach a time when it does not meet the necessary conditions to maintain life in the form we know it today. For now, the fact that the Earth is the third planet in our solar system and has three different movements does not happen by chance, these are the requirements for life as the Trinitarian God of life has ordained it.
Fr. Jorge Amaro, IMC