# How is the distance to the Moon and other celestial bodies measured?

Planets and stars look like small dots of lights from Earth. Humans cannot determine how far celestial bodies are just by looking up the sky with naked eyes. The good thing, astronomers have come with some ways to measure their distances from their observation stations.

Back then, people thought that Earth was the center of the universe where everything around it would revolve. Ptolemy of Alexandria developed this model, which he called the geocentric theory. In the late 1600s, an idea opposing the geocentric theory was born. It was Nicolaus Copernicus’s heliocentric model. It states that the Sun lies at a central point wherein the Earth and other celestial bodies revolve around it. Other famous scientists such as Galileo Galilee and Johannes Kepler backed up Copernicus’ idea with thorough observation and mathematical formulae, making it the most acceptable space theory until today.

The heliocentric theory led to the advancement and accuracy in inventing and using astronomical tools. A further discovery was made possible and modern scientists realized that the Sun is one of the universe’s stars. The world now has a greater understanding of how vast the outer space and its unexplored region is. One thing is for sure, our planet and everything around it is continuously moving. Johannes Kepler first observed this and made a deduction about how far are the planets is from another.

## Johannes Kepler

Johannes Kepler is a German astronomer and mathematician best known for developing three laws regarding planetary motion. These laws are: 1.) Planets move in elliptical orbits 2.) Planets move at different speed while orbiting the Sun 3.) Planets complete a revolution around the Sun, depending on their distances and orbital radius.

Kepler discovered that the closer is a planet from the Sun, the faster it could complete its orbit. This allowed him to estimate the planets’ distances. For instance, he knew that Maris s closer to the Sun than Saturn because Mars can complete its solar orbit in less than two years while the latter takes about 29 years. Back then, Kepler could not calculate their actual distances but understood that Mars and Saturn were 1.5 times and ten times farther from the Sun than Earth, respectively.

## Parallax Measurement

The first astronomer to make a close measurement of the celestial bodies’ distances was Gian Domenico Cassini. Cassino used the parallax technique to measure how far Mars was from the Earth in 1672.

To understand parallax, try holding your thumbs up at an arm’s length and look at it with only your right eye. Do the same with the other one. Did you notice that your thumb seems to have different distances when viewed from each eye? That is because our eyes were separated by a few inches. This caused our thumb to look like shifting back and forth in position. The distance of the thumb appearing to move is its parallax.

By definition, parallax is the difference in direction a celestial object has when an observer views it from two separate points. Parallax measurement directly finds the distance of a celestial body from the Earth (geocentric parallax) and the Sun (heliocentric parallax).

The observer’s two different points and the objects’ actual position form a triangular shape. When the baseline of this imaginary triangle is calculated, the celestial distance can be solved. For example, a team of astronomers would view the Moon from California, and the other team would observe it from Sydney. Using observations from the Earth separated by thousands of miles can reveal the distances of nearby celestial objects.

To find the distances of an object much farther than the planets, astronomers observe it from different places of the Earth’s solar orbit. They measure the position of a celestial body from the Earth several months apart. This separates the scientist’s “two eyes” into a hundred million miles apart. The more separation there is, the more accurate will the parallax measurement be towards much farther objects.

## That’s great! But how does measuring faraway astronomical distances help scientists?

Now that we have the technology to measure astronomical distance, future space expeditions will be more planned. If we sent a spacecraft to another planet, we could estimate the time it would take for the radio signal to travel to and from observatory stations.

Knowing these distances also helps develop our knowledge about the world outside of us. Astronomers can now establish the relationship between a star’s color, brightness, distance, and age. Through it, we knew that our Sun is about 4.6 billion years old and will die five billion years later, swallowing Earth and the rest of the solar system.