The star Regulus leads the Moon across the sky tonight. The bright heart of the lion is close to the upper right of the Moon at nightfall, with the gap increasing as the hours roll by.

Regulus is about 79 light-years away. That means the light you see from Regulus tonight actually left the star about 79 years ago. So when a particle of light from Regulus hits your eye, it’s ending a journey of 79 years.

As with many things astronomical, though, it’s all relative. For the particle of light itself – a photon – the trip took literally no time at all.

That’s because the photon was traveling at the speed of light – 670 million miles per hour. Nothing can travel faster than that. And only photons can travel at that speed. That’s because photons have no mass – they weigh nothing at all. If anything else were to travel at lightspeed, it would become infinitely massive. So physical objects are limited to just below lightspeed.

As an object moves faster, time appears to slow down for it as viewed by an outside observer – its clock would tick more slowly. So if you could accelerate a starship to just a fraction below lightspeed, it could travel for thousands of years as measured by a clock back on Earth – but just a few years or even less as measured by its own clock.

So as you look at Regulus tonight, remember that the photons are completing a journey of both 79 years – and no time at all.

Script by Damond Benningfield