Bellatrix, Spica, and Omega Herculis have something in common. They’re all good “semiquincentennial” stars. That has nothing to do with the stars themselves – only their distance. All three of them are roughly 250 light-years away. In other words, we see them as they looked roughly 250 years ago – around the time of the American Declaration of Independence.

One light-year is the distance light travels in a year – almost six trillion miles. So each of the anniversary stars is about 1.5 quadrillion miles away – the number 15 followed by 14 zeroes.

The distances were measured with a technique known as parallax. Astronomers look at a star when Earth is on opposite sides of the Sun. That causes a tiny shift in the star’s position against the background of more-distant objects. The size of that shift reveals the star’s distance. But there’s a slight margin of error, so the distances might not be exact.

The best measurements say that Bellatrix is a little more than 250 light-years away. Omega Herc is a little less. Spica is in the middle – almost exactly 250 light-years from Earth. So we see it as it looked around the year 1776.

Spica is the brightest member of that trio. It’s low in the southwest as night falls. Earth won’t see the light it emits tonight until the American quincentennial – 250 years from now.

Script by Damond Benningfield