The vastness of the universe we are part of could not be measured with the units of measurement we usually use here on Earth. Of course, we are talking about Space par excellence, which hosts an indescribable quantity, and practically impossible to quantify with scientific precision, of celestial bodies. From this need, therefore, was born the light year, a measure of distance commonly used in astronomy to express distances to (and between) celestial objects located outside the solar system.
This means that it is particularly suitable for distances on an interstellar scale, unimaginable for those who, like us, has a perception of space and time limited to our planet. That's why we're now going to discover together the exact definition of a light year, and especially how much this corresponds to if we wanted to translate it into kilometers. Not only that, we will also explore other units of measurement commonly used by astronomers, so as to have a complete picture of the most valuable tools in the study of the Universe and its extraordinary secrets.
What is a light year
The light year, abbreviated as ly or al, is a measure of distance that is used in astronomy. It corresponds to the distance light travels in a vacuum over the course of an Earth year. Alternatively, we can say that it is equivalent to the distance an object would travel if it traveled at the same speed as light in vacuum, that is almost 300,000 kilometers per second, for a tropical year, that is 365 days, 5 hours, 48 minutes and 46 seconds.
The determination of the speed of light, as many of you already know, is due to the Danish astronomer Ole Rømer, who in the seventeenth century assumed that light had a huge speed but not infinite, as was mistakenly thought until then. It was him to define precisely the speed of light in 1676, while working at the Royal Observatory of Paris directed at the time by Giovanni Domenico Cassini.
Returning to the light year, there is a precise definition given by UAI, the International Astronomical Union, which states: "The light year is the distance that a photon travels in empty space in the absence of gravitational or magnetic field in a Julian year". For completeness, it should be clarified that a Julian year has a duration of 365.25 days, on average composed of 86,400 seconds each, equal in total to 31,557,600 seconds.
There are still other units of measurement of lengths united with the light year, real submultiples such as light month, light week, light day, light hour, light minute and light second. Although not very common, these are obtained by considering the distance traveled by light in a certain unit of time. To give some examples, the distance of the Earth from the Moon is equal to about 1.282 light seconds, while the distance of our planet from the Sun is equal to 8 light minutes.
How long is a light year
The light year, contrary to what you might think at this point and in relation to its name, which can be misleading! is not a unit of measurement of time nor of the "amount" of light, but only of the distance traveled by electromagnetic radiation, light itself, in a vacuum in the interval of one year. In any case, it is absolutely correct to say that the direct observation of a celestial body distant a certain number of light years can show us that same celestial body as it was the same number of years ago and not at the precise moment of its observation.
Exceeding the technicalities, one light year is equivalent to approximately 9.460.730.472.581 kilometers. 9,460 billion km, if that very long number should "scare" us, or about 63,241 times the distance between the Earth and the Sun, known as astronomical unit. You will understand then that we are faced with an enormous distance on a human scale, equal to over 236 million times the circumference of the Earth and calculated from the speed of light in vacuum (c).
Considering that this is equal to 299,792.458 kilometers per second, the value of the light year is given by the following formula: 299,792.458 km/s - 355.25 d - 86,400 s/d ≃ 9,461 - 10^12 km. To get an even more concrete idea, just think that the distance between the Sun and the Earth is equivalent to about 8.3 light minutes, where one light minute is the distance traveled by light traveling in a vacuum for one minute.
So we should ask ourselves how long it takes us to travel the distance of 9,460 billion km today. To answer, we need to know that the escape velocity from Earth is about 40,000 km/h. Exploiting, thanks to our probes, the slingshot effect of fly-by around various planets, we are able to launch a spacecraft up to 60.000 km/h, and even exceed the roof of 80.000 km/h as in the famous case of New Horizons. Doing a quick calculation, to travel a light year at 80,000 kilometers per hour would take about 13,500 Earth years.
Concrete examples of light year
Now that we have given a definition of light year and we have clarified how much corresponds to a light year in kilometers, 9,460 billion km, in fact, it is appropriate to make some concrete examples to quantify even more this unique and useful unit of measurement. As a first thing, we can think that a light year takes about 1.28 seconds to cover the distance between the Earth and the Moon. In addition, on a scale where the Earth had a diameter of 1 cm, a light year would correspond to a distance of 7,423.80 km.
Not only that, we know that it takes about 8.33 minutes, or 8 minutes and 20 seconds, for light to travel from the Sun to our planet. And again, one light hour corresponds to about 1.08 billion kilometers, which is roughly equivalent to the distance between the Sun and Saturn, while the closest star to Earth other than the Sun is Proxima Centauri, which is 4.23 light years away from us.
The disk of our galaxy, the Milky Way, then has a diameter of about 100,000 light years. The closest large one to ours is the Andromeda galaxy, which can be found at a distance of 2.5 million light years. In addition, scientists have observed that the Local Group has a diameter of about 10 million light-years, and that the closest quasar to Earth, known as 3C 273, is located about 3 billion light-years away.
Since it is commonly assumed that the Big Bang occurred about 14 billion years ago, the observable universe, assumed to be spherical in shape and in case it was not expanding, would present a radius of about 13,820,000,000 light-years. Finally, in the Universe the most distant observable objects are located at 13.2 billion light years.
All these numbers translate into staggering considerations for humans. Not surprisingly, the moment we observe a star that is 20 light years away from Earth, it means that we at that moment are seeing the light emitted by that star exactly 20 years earlier. If it were to extinguish at this precise moment, we would see it disappear only in 20 years.
In the same way, since the Sun is 8 million light from Earth, the sunlight that we see and perceive is the one that was emitted by the Sun just 8 minutes before. A further example is that offered by the Andromeda galaxy, which appears to us as it was 2.5 million years ago because its distance from us is about 2.5 million light years.
Alternative units of measurement
In addition to the more widely used light-year, astronomers use two other units of measurement, namely astronomical units (AU) and parsecs. Astronomical units, in particular, are used primarily to measure distances in the solar system. An astronomical unit, according to scientific literature, corresponds to the average distance that separates our Earth from its nearest star, the Sun. We are talking about almost 150 million kilometers, 149,597,870 to be picky. Turning to parsecs, these are used by scholars to express the distances between stars.
A parsec, whose name is an abbreviation of "parallax second", is equivalent to 3.26 light years. This indicates that it corresponds to the distance from which a hypothetical observer would see the mean radius of the Earth's orbit, thus one astronomical unit under an angle of one arc second. Concepts certainly not easy, but that define the observation of stars, planets and celestial bodies in general.
Summarizing, then, we can say that one astronomical unit is equivalent to about 150 million kilometers, which on the factual level translate into the measurable distance between the Earth and the Sun. One light year corresponds, for logical consequence, to 63.067 UA, that is 9.460.000.000 kilometers. One parsec, on the other hand, corresponds as mentioned above to 3.26 light years.
In UA, the figure settles on 205,597 UA, which in turn are 30,840 billion kilometers. That said, the al remains the most popular unit of measurement and probably easiest to explain and understand. And one that fits most of the space objects that "populate" the sky.