Celestial dynamics

The Universe

More than 4.000 years ago, the humans started to think in the Universe and how did it affected their life. The sun and the moon coming in and out, the change of the seasons, the movement of the stars, all these were things that people already though about at that point.

For years, the Universe and our relationship with it was of deep interest of ancient populations. From then until the beginning of the 20th century (including ancient Greeks, Ptolemy, Copernicus, Kepler, Galilee, Newton and even Einstein) it was though that the known Universe was limited to our Galaxy, the Milky Way. Then, Edwin Hubble observed that the Andromeda Galaxy was a lot farther (2.5 million light years) than the limits of the Milky Way (150,000 light years). Since then it is known that the Universe is much bigger than we though and that is still growing.

It’s been calculated that the observable Universe is approximately 93 billion light years and includes more than 125,000 million galaxies. Our galaxy, the Milky Way, has approximately 6.000 million planetary systems. Our planetary system is the Solar System which travels in circles around the Milky Way at a speed of 215 kilometres per second.

Solar system

When the ancient Greeks started to get interested in the Universe and its functioning, they realized there were immobile stars and others that were mobile (which they called planets). For years it was thought that the Universe moved around the Earth (Geocentric Theory), until Copernicus and Kepler suggested that, against what was traditionally though, the Sun was the centre of our Solar System and that the planets moved around it (Heliocentric Theory). Galilee, already in the 17th century, proved right this theory by using for the first time a telescope.

The orbits of the planets occurred because of the gravitational force among them and the Sun (as bigger the mass or closer the objects, bigger is the gravity between them). The mass of the Sun is so big that whole matter in the Solar System moved around in elliptical orbits, such movement is known as Translation. The planets can be classified in:

• Inner planets: Mercury, Venus, Earth, Mars.
• Asteroid Belt: million of rocky fragments smaller than 1,5Km.
• Outer planets: Jupiter, Saturn, Uranus, Neptune.
• Dwarf planets: Ceres, Pluto, Haumea, Makemake, Eris.

Earth

Our planet describes and elliptic (translation) trajectory of 930 million kilometres, at a average distance of 150 million kilometres and lasts 365 days, 5 hours, 48 minutes and 45.22 seconds to close it. Since 1 year has 365 days, almost 6 hours annually are left. To fix this error, the leap-years were proposed, adding 1 day every 4 years (6 hours per year x 4 years= 24 hours) in the calendar (29th February).

During the formation of the planets, pieces of matter of all sixes collided giving movement to the big planets (Rotation movement) and providing them with natural satellites. The rotation movement is that with which the planets and satellites spin on a rotation axis. This movement in the planets determines the day and night and in Earth has a total duration of 23 hours, 56 minutes and 4 seconds.

The terrestrial rotation axis passes through the geographic North Pole (NP) and the geographic South Pole (SP) and has an inclination of 23o 5' over the perpendicular of the Solar System. The rotation movement allows any point on earth to daily go from darkness to sunlight. At the equator, the days and nights are of 12 hours of light and 12 hours of darkness. On the contrary in as we move towards the poles, the difference between day and night become bigger; in the north hemisphere the days are longer during the summer and the nights are longer during the winter (conversely to what happen in the south hemisphere); days and nights can be of up to 6 months due to the inclination of the Earth axis and the relative position of the Earth with the Sun.

Additionally, the inclination of the rotation axis drives the seasons. This axis has a permanent direction; which makes that the Earth is illuminated with different intensity (the rays of the Sun need to go through a wider atmosphere) and quantity (due to the duration of number of light hours) of light.

In winter at the northern hemisphere, the distance Earth-Sun is minimal but the relative position of the Earth with the Sun does shorter days and lower temperatures.

In summer at the northern hemisphere, the distance Earth-Sun is maximal but the relative position of the Earth with the Sun does longer days and higher temperatures.

Summer Winter

Summer Winter 24h light 24h darkness

Did you know?

The ancient Greek designated the 5 mobile stars or planets with the name of their Gods: Apollo/Hermes, Aphrodite, Ares, Zeus and Cronus. Later on, the Romans changed those names by the ones of their own gods and those are the one we know nowadays: Mercury, Venus, Mars, Jupiter and Saturn.

The elliptic orbits of the Solar System during the translation of the planets make them go faster when they are closer to the Sun and slower when they are farther from the Sun.

The movement of the planets is not actually elliptic, it is indeed helicoidally. The movement of the Solar System through the Galaxy together with the movement of the planets around the Sun makes them to move helicoidally in their movement through the space.

More information

If the Earth axis would not be inclined from the vertical, the Sun would be all year long on the equator, describing every day the same trajectory in the sky, reaching the same height, there will always be the same number of hours of light, and there would not be seasons.

Broaden your knowledge

The Big Bang Theory

The Big Bang theory is the most spread and accepted theory in the scientific community regarding the origin of the Universe, in which a primal particle smaller than a fraction of an atom with a huge mass, unimaginable hot and with an enormous quantity of energy in the form of superforce (combination of the 4 forces of the nature: gravity, weak nuclear, strong nuclear and electromagnetic), expanded until the formation of the Universe.

Superforce Gravity Electromagnetic Weak Nuclear Strong Nuclear

Temporal lapse:

• First fraction of a second (10^-18s) The gravity force splits off from the superforce as the Universe expands at a superhigh velocity (faster than the limit cosmic speed), and the Universe starts to cool down. Guth’s Hiperinflation theory.
• Before the first second (< 1s) The 4 forces of the nature splits off from each other loosing temperature.
• After 3 minutes (3 min) The temperature of the Universe has drop down to almost 555 million Celsius, which allowed the formation of atoms of Hydrogen and Helium.
• 380.000 years The light starts to travel through darkness.
• One billion years The stars take shape and with them the atoms of Nitrogen, Carbon and Oxygen. The big nebulae start to spin and flatten forming huge starts with stellar discs around them.
• Nine billion years Matter and gravity combined to form a star, pressure creates heat at its core, which triggers thermonuclear fusion clearing away residual gases, a circumstellar disc of dust remains which creates a gyratory disc of planets, forming the our Solar System.
• Nowadays (13,7 billion years) he Universe continues to expand at a rate of 73,2 kilometers per second per megaparsec (1 megaparsec are around 3,3 million light years) and has more than 125.000 million galaxies.

Same planet facts

• Mercury is the smallest planet in the Solar System. It has extreme temperatures of over 400ºC during the day and -185ºC at night. It has no atmosphere, so if we look up from its surface we will see a black sky and a Sun 2,5 bigger than from Earth.
• Venus is the most similar planet to Earth in terms of size, mass and density. However, its temperature is over 500ºC due to its atmosphere rich in CO2 and sulphuric acid clouds. It rotates in the opposite direction than other planets and its days are longer than its years.
• Mars Its land is red and has big canyons and polar icecaps.
• Jupiter is a giant planet of gas, with a size 1.300 times the Earth. It has a big spot on its side, which is a huge thunderstorm active since at least 150 years ago (since was discovered using a telescope).
• Saturn has massive rings of ice dust and small particles covered in ice.
• Uranus also has rings and rotates laterally.
• Neptune is the most windy planet of the Solar System with speeds of up to 2.000Km/h.

Planetary summary

Inner Planets

Asteroid Belt

Millions of rocky fragments of a diameter smaller than 1,5Km.

Outer Planets

Dwarf Planets