The Race Against Time: Understanding How Long it Takes for Light to Reach Earth

If you hold out your thumb at arm’s length (ideally covering something in front of you, but not necessary), close one eye, and then switch which eye is open, you will see your thumb move slightly. This is a very simple, Earth-based example of parallax, when an object’s location is displaced based on point of view.   By measuring this small displacement and the distance between your eyes, you can calculate the distance to your thumb using trigonometry to find the top of the cone that engulfs the space in between.

While it may look like a two-dimensional projection above our heads, we must remember that space is three-dimensional. Stars may seem like they are right next to each other, but in reality, not only are they far apart from each other, but one is likely closer than the other and it may be the one you don’t expect. In addition, we are on a planet orbiting the Sun, meaning our location in our orbit impacts how we see stars in the night sky. Every six months, the Earth moves about 186 million miles (300 million kilometers) to the other side of the Sun, displacing its view of the universe around it. This is a relatively small amount based on the size of the universe, but enough to cause stars to perform tiny circles throughout the year, so tiny that detecting and measuring them is difficult. 

The ancient Greek astronomer Hipparchus was the first to use parallax to determine the location of a celestial object: the Moon. He had a solar eclipse observed from two separate locations to calculate how far away the moon was. Mars’ distance from Earth was determined using parallax in 1672 by Giovanni Cassini and his colleague Jean Richer during simultaneous observations in Paris and French Guiana. To measure the exact location of small objects in the sky, we break up the sky into degrees. From one horizon to the opposite is 180 degrees, half of a full circle which would go around the Earth. From one horizon to the point straight above your head (the zenith) is 90 degrees. Special tools on telescopes help to measure these degrees similar to a large protractor from your school days. You can also estimate using a closed fist held out at arm’s length being about 10 degrees. A degree is about the width of your pinky held at arm’s length. A degree is then broken into 60 arcminutes and each arc minute is broken into 60 arcseconds. Arcminutes and arcseconds definitely need special tools to measure accurately. Even the closest stars have a shift of less than an arcsecond when analyzing parallax. 

Simultaneous observations from 2 locations on opposite sides of the world worked for objects within our solar system, but for objects outside of the solar system, like stars, we needed to factor in the orbit of Earth, taking measurements from opposite ends. Friedrich Bessel in 1838 then used the method to determine the distance from star 61 Cygni, in the Cygnus constellation, from Earth, springboarding the long process of creating a three-dimensional map of the night sky which took decades and improvements in the telescope to make the process easier. 

Today, we also use parallax by comparing earth observations with observations from probes and missions out in the solar system such as New Horizons, providing even more distance to calculate even more specific measurements. Even using parallax and our updated technology, we eventually find that stars are too far away to calculate the minute changes. Instead, we build on the parallax method using stars that are known as “standard candles” for comparison in intrinsic brightness to determine distance. Using parallax and similar methods we can determine how far away a star is. A light-year is the distance light can travel in 1 year or about 6 trillion miles (10 trillion kilometers) and is the main measurement we use for calculating the distance to other stars. If we took the circumference of the Earth, laid it out on a straight line, and multiplied it by 7.5, we would get one light second. We would then need 31.6 million of those light-second lines to create the distance of a light-year.

Light from the sun takes about 8 minutes to reach Earth meaning our Sun is 8 light-minutes away from us. The closest stars to us are the Alpha Centauri triple-star system which is about 4.37 light-years away, meaning if we could travel at the speed of light (which again is impossible) it would take us just under 4 years and 4.5 months to travel the distance. The closest of these stars is Proxima Centauri which is about 4.24 lightyears away from us, meaning just under 4 years and three months at the speed of light. Remember that a light-year is about 6 trillion miles meaning they are over 26 trillion miles or 43.7 trillion km away. Voyager 1 would take over 73,000 years to arrive at Proxima Centauri.

Another important factor in this section before we move on is redshift. The frequency of light or sound emitted by a moving object changes based on if it is moving closer to the observer or farther away. Objects moving away from the observer shift towards “redder”, longer wavelengths, which is where we get the term redshift. If they are moving toward us, the frequencies shift to shorter, bluer frequencies. This is also known as the Doppler Effect, especially with sound waves, which you can hear as the change in tone when a siren passes by you. Understanding redshift helps us to better calculate the distances and locations of celestial objects, which are always moving. This was first discovered when Ole Rømer was studying the eclipses of Io and fundamentally changed our understanding of astronomy.