Click here if you want to read this page in Spanish

The Universe is much larger than we can imagine. It is enormously extensive, it has billions of galaxies.

In every galaxy there are billions of stars.

The Sun is a star in one of the galaxies.

The planet Earth is one of the components of the solar system.

How big and extensive is the Universe?

In the 6th century BC the sky was thought to be a rigid canopy into which brilliant celestial bodies were embedded like diamonds.

Therefore, the starry sky was called firmament.

The sun, the moon and the planets could not be in the same awning as the fixed stars. Each was supposed to be in its own vault.

First astronomical measurements

The first known scientific measurement of a cosmic distance was made in 240 BC by the director of the Library of Alexandria, the Greek Eratosthenes.

His calculations led him to deduce that the diameter of the Earth was 12,000 km.

The Greek sage Aristarchus wanted to calculate the distance between the Sun and the Earth, but lacking appropriate instruments, he deduced that the Sun was 40 times farther than the Moon. Now it is known that it is 400 times further away.

Based on this diameter of the Earth, in 150 BC another Greek sage, named Hipparchus, calculated that the distance between the Earth and the Moon was 384,000 km.

Moon earth
Orbit of the Moon. Credit: web “researchgate.net”

Ptolomy stated that the Earth remained fixed, that the Moon was about 384,000 km away and that the rest of the stars were beyond, at an indeterminate distance in the sky.

Measurements made with the help of telescopes

In 1650, the Belgian astronomer Godofrey Wendolin repeated the measurements of the Greek Aristarchus, but with a more sophisticated instrument: a telescope.

Sun Earth
Earth revolution.  Credit: web “exampariksha.com”

He concluded that the Sun was 240 times farther away than the Moon.

In other words, the distance between the Earth and the Sun was 97 million kilometers.

Until 1673 the technique of “parallax” had been applied to measure the distance to the Moon.

In that year, the French astronomers Dominique Cassini and Jean Richter, very ingeniously, managed to determine the parallax of Mars.

From there, Cassini calculated that the distance between Earth and the Sun was 136 million km.

This is a very approximate value at the real distance, which is 150 million km.

It was an extraordinary achievement, because it served to know the dimensions of the solar system: Venus, Mars, Jupiter, Saturn.

Orbits of the sun’s Planets.  Credit: web “nationalgeographic.org”

The distance to a small planetoid called Eros was even calculated.

But the stars were still inaccessible to any method of measuring distances.

The stars continued to be beyond, in the “firmament”, at “enormous” distances, billions of kilometers away.

It was deduced that when shining so far from the Earth, the stars must be bodies in flames and as big as the Sun.

Measuring distances from the stars

By the 1830s, telescopes and parallax techniques had been greatly perfected.

Astronomers turned to calculating the distances of the brightest stars.

In 1838, the Swan star 61 was reported to be 103 billion km away (11 light years). Soon after, the star Alpha Centaurus was calculated to be 4.3 light years away.

By 1900, the distances of 70 stars had already been determined, out of the nearly 6,000 that can be distinguished with the naked eye.

Ever since Galileo pointed his rudimentary telescope skyward, it was known that there are hundreds of thousands of stars.

And so it happened, until Henrietta Leavitt, one of the “Harvard calculators” hired by the astronomer Edward Pickering, arrived.

Henrietta Leavitt, brilliant astronomer who found a ruler to measure the size of the Universe. Credit: Wikipedia.

Henrietta Leavitt had to write down the data for each star, including its size (related to brightness) and compare them with those obtained in the same space sector but at different times of the year.

Magallanes. The Universe
This image from NASA’s Spitzer Space Telescope shows the Large Magellanic Cloud. Credit: web “nasa.gov/multimedia”

One day in 1904 when she was cataloging a crystallographic plate of the “Small Magellanic Cloud“, she observed a certain pattern in the behavior of the variable stars found in the constellation Cepheus.

Excited by the discovery, Pickering ordered more plates, and for four years Henrietta Leavitt kept an eye on the characteristics of these stars.

How to measure the universe was discovered

In 1912, Henrietta Leavitt wrote a three-page paper that she titled “Periods of 25 Variable Stars in the Small Magellanic Cloud“.

This claim that the brightest stars have the longest periods, ended up totally revolutionizing astronomical science.

This short article, which has been fundamental in astronomy, established the great rule that allowed us to begin to measure the size of the Universe.

A year later, many more relative and absolute distances between one star and another could be determined thanks to the patterns discovered by Leavitt.

She had discovered a way to measure the distance between very distant stars quite accurately.

Milky Way. The Universe
Actually astronomers are mapping the arms of the Milky Way. Credit: web “sciencemag.org/news

Thanks to this new method, in 1918 the size of the Milky Way was calculated.