It takes the sun somewhere between 225 million and 250 million years to make a full orbit around the center of the galaxy.
Earth is approximately 4.5 billion years old, which means it has lived through about 20 complete loops around the galaxy.
If the sun travels in an approximate circle around the center of the galaxy, then that circle can be divided up into degrees — every circle is 360 degrees. Every degree can be divided up into 60 pieces, each called an arc minute. Every arc minute can be divided into 60 arc seconds. (These units can also be used to measure the distance between objects on the sky). The creators of Galactic Tick Day then divided each arc second by 100 (a “centi-arc second”).
Sneider said they call each of these centi-arc seconds a “tick,” sort of like the tick of a second hand on a clock. Every time the sun completes one centi-arc second in its orbit around the galactic center, they will celebrate Galactic Tick Day.
With that in mind, the sun moves through one centi-arc second of its orbit around the galaxy every 633.7 days, or 1.73 years, and Sept. 29, 2016, will be the 235th Galactic Tick Day.