Take a journey into the past

October 06, 2017

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

Did you know that when you gaze up at a clear evening sky, you are actually looking into the past?

Even 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 cannot use everyday terms. To write out the distance in kilometres to a nearby galaxy, you would need 19 zeros, so astronomers use the term light year — the distance light travels in one year.

It’s 300000km a second, or almost 10 trillion kilometres a year. See what we mean?

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

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

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

Stars though are a different matter. 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 900years.

I would have to wait here until the 30th century to see it. The light would take that long to reach me. True.

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 have got the best skies in the world. Why not go out and do a bit of time-travelling tonight?

David Reneke is an Australian astronomer and writer for Australian Science magazine. His free astronomy newsletter can be found at www.davidreneke.com

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