Friday April 19 2019




Village Sky At Night with Brian Watkiss

When a year is a distance

Posted on February 22 2010 at 9:30:28 0 comments

Amateur astronomer Brian Watkiss peers into the Village night sky.

The night sky in March is about the best it gets. But, how far away are all those magnificent objects? Well, the nearest celestial events, shooting stars or meteors, with a few rare exceptions, generally burn up in the atmosphere at about 60 miles high.

Further out is the largest satellite ever (and still growing), the International Space Station, orbiting at a height of around 200 miles. Even here, though, it still loses height at the rate of about a mile and a half every month due to atmospheric drag and has to be boosted back up on a regular basis. See for details of when it is visible from Alvechurch and look out for a shuttle launch due on the 18th.

Other satellites, visible all night, generally orbit at around 400 miles. Communication satellites are not visible, orbiting much further out at 22,000 miles, at which distance a radio signal takes over a quarter of a second to get there and back.

This, together with the delay in the equipment needed, is why news reporters seem to take so long to reply to a question put by the presenter in the studio. The delay to the Moon, at around 250,000 miles away, is over a second.

Mars is visible all this month, high in the south at around 8pm, looking distinctly orange and very bright just below the pair of stars, Castor and Pollux. It is more than 70 million miles away and this distance is getting bigger day by day as we “overtake” it on the inside.

At the moment, it takes nearly 10 minutes for radio waves to cross this gap so the two Mars rovers, Spirit and Opportunity, have to be able to negotiate the Martian surface without assistance from Earth-bound operators. They are, by the way, still operating, although Spirit is bogged down in sand and will never move again.

The next nearest object is the planet Saturn. It is visible all month although very low in the south east at the start of the month, getting better towards the end. Look for it close to the full Moon on the 28th or 29th. It is much further away than Mars, at nearly 800 million miles, so the signals from the Saturn-orbiting Cassini probe take well over an hour to get here.

Now, without wishing to start any heated Friday night debates, light can be considered to be a radio wave travelling at a constant speed, so it suffers from the same delay and the distance to even the nearest stars is so great that it takes years for light to cross the gap. So we use the distance light travels in one year as the basic unit for measuring how far away stars are.

For example, looking south around 8pm, Mars is high in the sky but low down, and very slightly right is the unmistakable bright blue star, Sirius. This is the brightest star you can see and also the closest. Even so, it has taken the light you are seeing just over eight and a half years to get here; that is to say, Sirius is eight and a half light years away.

Looking up and to the right, the huge constellation of Orion dominates this portion of the sky, with the red super giant star, Betelgeuse, at its top left corner. It is just over 400 light years away and if you look at Orion’s belt, the middle star of the three is Alnilam. It has taken that light over 1,300 years to get here.

And you thought it was a long way to the chemist‘s!

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