What is our place in the Milky Way? And our place in the Universe? In ancient times, many people had the idea our planet Earth to be at the centre of the Universe, as stated by Aristotle and Ptolomeus in their ptolemaic – aristotelic concept of universe: according to this model, Earth is at the center of the universe and all the other celestial bodies orbit around it. Today lots of people think the same. But is this really the case? To answer this question, let’s try to to a travel in the universe, through space and time; we will start our travel from our planet to reach, in the end, the extreme boundaries of the universe.
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During the 1600s, Galileo Galilei, the famous Italian astronomer, was one of the first people, during modern age, to have some doubts about the geocentric model of universe: thanks to telescopic observations, he was able to demonstrate our Earth is not at the rotation centre of planets and the Sun, but really it is the Sun itself. Moreover, observing planet Jupiter, he discovered that the giant planet is the rotation center for its moons. So, Galileo became aware that the center of the Solar System was the Sun, not the Earth!
The Solar System is made by a star, the Sun, eight planets and different types of minor celestial bodies, like comets, asteroids and dwarf planets.
Well, the Earth isn’t at the center of the Solar System, maybe is the closest planet to our Sun? No it isn’t, because it is only the third planet from the Sun: the closest planet to our star is Mercury, followed by Venus and then Earth. The Earth moves around the Sun, our star, just like all the other celestial bodies in the Solar System do: this implies that the Sun, and not our planet, is the center of rotation of the Solar System! The Earth takes a year, 365 days, to travel its orbit, and its average distance from the Sun is 150 million kilometers, which is the measure unit of distances in the Solar System known as the astronomical unit and abbreviated AU. Why do we talk about average distance? Because the orbit traveled by the Earth around the Sun is not circular but elliptical, and this means that there will be an aphelion (i.e. the point of the Earth’s orbit farthest from the Sun, just over 1 AU away from it) and a perihelion (the point of Earth’s orbit closest to the Sun, just under 1 AU). An alternative way to define the astronomical unit passes through the light time, in particular we can say that the average distance Earth – Sun is equal to about 8 light minutes: this means that sunlight takes 8 minutes to arrive on Earth, so that the sunlight we see at a certain moment is not that of that moment but it is the sunlight which left from the Sun 8 minutes earlier! In other words: if the sun went out for example at 2.30 pm, we would only notice it at 2.38 pm! Or again: if you could travel aboard the Star Wars Millennium Falcon it would take you only 8 minutes to travel from the Sun to the Earth (when in reality it takes a few years). To give a more concrete idea of the dimensions of the Solar System: if the Sun were a sphere with a diameter of 14 cm, Pluto would be at 700 m from the Sun, like seven regular soccer fields!
The nearest celestial body to Earth is the Moon, our satellite: to reach it you should take three days off! It’s the same time taken by Apollo astronauts to cover the distance of nearly 400 thousand kilometers that separate Moon and Earth. But if you had Star Trek Enterprise, and travel at maximum curvature, you would only take less than 2 seconds to reach the Moon!
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