It’s in the stars, October 9

STELLAR HISTORY: When we look at the stars we are seeing them as they were hundreds, even millions, of years ago.
STELLAR HISTORY: When we look at the stars we are seeing them as they were hundreds, even millions, of years ago.

DID you know that when you gaze up at a clear evening sky, you're actually looking into the past? No really … it’s true. Every time you look at the sky you are seeing history happening.

Just with the naked eye, you can see starlight that was emitted years, or even centuries ago. And if you know where to look, you can see galaxies so far away that their light has been travelling since before humans walked the Earth.

Add the magnifying power of a telescope, and you'll journey to the time when dinosaurs lived.

So, when you use a telescope you are in fact using a time machine.

“Space is so vast we can’t use everyday terms,” Dave Reneke, from Australasian Science magazine said.  

“To write out the distance in kilometres to a nearby galaxy you’d need 19 zeros, so astronomers use the term ‘light year’ - the distance light travels in one year.

“It’s 300,000 kilometres a second, or almost 10 trillion kilometres a year.”

Planets are closer than stars so we talk about them in light minutes or light hours.

Moonlight is the exception; it takes 1.3 seconds to reach us on the ground.

The light from the sun takes a bit over eight minutes, 20 seconds to travel to Earth, so we can say that it's 8.3 light minutes away.

Stars though are a different matter.

Space is so vast we can’t use everyday terms.

David Reneke

“To really put things in perspective, consider this. If one of the stars in the familiar ‘Saucepan’ constellation (Orion) exploded tonight I wouldn’t know about it for 900 years,” Dave said.  

“I’d have to wait here until the 30th century to see it.

“The light would take that long to reach me."

Right now, Venus and Mars are about 10 light minutes away from Earth.

The resulting 20-odd minute round trip for radio signals presents serious challenges for future Mars explorers. 

We typically see Jupiter and Saturn as they were more than an hour ago.

As a rule, using a telescope makes an object's light brighter and its image larger, but it doesn't shorten the light's travel time at all.

This month, in Australia, we’ve got the best skies in the world.

Why not go out and do a bit of ‘time travelling’ tonight?

Get David Reneke’s free astronomy newsletter by visiting www.davidreneke.com. You can also get your free copy of the 353-page ‘Complete Idiots Guide to Astronomy’ at the same time.