What Is A Galactic Year?

The Sun in space.
The Sun in space.


A galactic year, which is also called a cosmic year, is the amount of time it takes the Sun to orbit around the center of the Milky Way Galaxy, which is roughly between 225 and 250 million years. The Solar System moves around the center of the galaxy at 514, 000 miles per hour (mph), while the Earth orbits the Sun at a speed of 67,756 mph. A galactic year is used to as a more simple unit to describe the otherwise large number of years associated with cosmic time. For example, instead of describing the Milky Way as 13.4 billion years old, it is listed as 54 galactic years.

Galactic Tick Day

When Earth makes a complete revolution around the Sun, the world celebrates the beginning of a new year. Science enthusiasts around the world established "Galactic Tick Day" to raise awareness of how the Solar System orbits the Milky Way galaxy. However, since the full orbit takes approximately 225 million years, researchers divided the Sun's orbital journey into smaller chunks, such that the holiday is marked every 1.7361 Earth years or every 633.7 days. The first Galactic Tick Day occurred on October 2, 1608, which was one Galactic tick after German spectacle-maker Hans Lippershey filed his patent for the telescope. However, the holiday was first observed on September 29, 2016, which marked the 235th Galactic Tick Day. The next Galactic Tick Day will occur on March 21, 2020.

Determining a Galactic Tick

Despite moving at speeds of 514,0000 mph, it takes the Sun between 225 and 250 million years to complete one orbit around the galactic center. Estimates vary due to certain unknowns, including the exact distance between the Sun and the galactic center, and whether or not this distance has changed over time. Researchers settled on 225 million years as a reasonable estimate because it is the shortest possible time. Earth is about 4.5 billion years old, which means that it has completed 20 orbits around the galactic center. It is believed that the Sun’s path around the galactic center is a perfect circle, meaning that it can be divided into 360 degrees. Each degree is further divided into 60 parts, known as arc minutes. Each arc minute is then further divided 60 times to create 60 arc seconds, and each arc second is further divided by 100, and each is called a "centi-arc second," which creates a "tick." Each tick lasts 1.73 years or 633.7 days.


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