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How to Infer Scales of the Solar System, Galaxy & Universe

Instructor: Katie Fullerton

Katie is an experienced high school physics and chemistry teacher with masters degrees in engineering and physics.

The physical scales used to measure time and space in the solar system, galaxy, and universe vary widely. Inferences can simplify comparisons and give scientists an idea of the size of objects.

Comprehending the Universe

Our everyday experiences with the physical world define our sense of space and time. Unfortunately, those scales are not sufficient when we consider objects in our solar system and beyond. Their size is so vast that it's mind-boggling!

Luckily, scientists have defined larger units that allow the human brain to better comprehend distances and times relevant to these larger systems. These units can be separated into two general categories: spatial (space-based) and temporal (time-based). Let's take a closer look.

Spatial

Distances in the solar system are so large that most humans cannot comprehend the scale of it, much less galactic or universal scales. For each level (solar, galactic, and universal), scientists have defined different units.

Units on the Scale of the Solar System

You can probably imagine a 5 km run. What about 10 km? That's a bit longer, but it's still a distance that most people can comprehend. 100 km can take an hour or two to drive. The United States is about 4,000 km wide. Beyond that, most people cannot picture such a distance.

The distance between the sun and the Earth averages 150 million kilometers (km), or 93 million miles.

Astronomical Unit

In order to simplify the distance between the sun and various planets, scientists have defined the astronomical unit (AU) . 1 AU is defined as 1.496 x 108 km. In other words, over 149 million kilometers.

The AU is used to simplify the distances between planets and the sun
Solar System Orbits

Using astronomical units allows scientists to compare the relative distances of the planets much more easily than comparing with kilometers. The table below lists the average orbital distances from the sun of all the planets in both kilometers and astronomical units.

Planet Orbital Distance
in km
Orbital Distance
in AU
Mercury 57.9 million 0.39
Venus 108.2 million 0.723
Earth 150 million 1
Mars 227.9 million 1.524
Jupiter 778.3 million 5.203
Saturn 1,427 million 9.539
Uranus 2,871 million 19.18
Neptune 4,497.1 million 30.06

Parsec

Another unit used is the parsec. Most people are familiar with the parsec from science fiction, but it is an actual scientific unit. Its definition is based on the concept of parallax, a tool used to measure the distance to various stars in our galaxy. It is used to measure distances outside our solar system, but still relatively close within our galaxy.

One parsec is defined as 3.086 x 1013 km (over 30 trillion), or 206,264 AU. During the visual observation era of astronomy, the parsec was preferred. After the advent of radio astronomy in the 1960s, the light year became the preferred unit in astronomy. However, parsecs are still used by astronomers who are researching our local galactic neighborhood.

Units on the Scale of our Galaxy and Universe

The nearest star to our sun, Alpha Centauri, is 4.13 x 1013 km, or 276,363.5 AU. Neither of these measurements is particularly useful when trying to get a sense of scale. Instead, scientists use the light year to reference distances on galactic and universal scales.

A light year is the distance that light travels through a vacuum in one Earth year, or 9.46 x 1012 km (over 63 thousand AU). For reference, light takes 8 minutes to travel the 93 million miles between the sun and the Earth, so one light year is a significant distance.

Galaxies are typically between 50,000 and 200,000 light years across. That means that light from the farthest away edge of our galaxy has been traveling for hundreds of thousands of years. When we look at stars across the galaxy, we are seeing them not as they are at this very moment, but as they were thousands of years ago.

Galaxies are arranged in clusters. Overtime, they tend to move closer to each other due to gravity. The distances between galaxy clusters can range from 1 to 2 million light years. The known universe itself is 14.3 billion light years in any given direction, though it may be physically larger. Light from beyond that distance simply hasn't had time to travel to Earth.

Temporal

Temporal, or time-based, scales are less astronomy-specific, so you'll recognize many of the units from day to day life. Astronomers still utilize seconds, minutes, hours, and years.

Years are defined based on an Earth year, or the time required for Earth to make one full orbit of the sun, or 365.25 days. Seconds are defined based on the vibration of electrons in a Cesium-133 atom. Despite these consistent definitions, events throughout the universe occur on a number of different scales.

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