The official definition of the speed of light in a vacuum is c = 299 792 458 meters per second. To help you make sense of this, here are some other ways of describing this: Light travels at just over a billion kilometers (or 600 million miles) per hour. If you could make a beam of light curve, and sent it on a trip around the Earth, it would make the journey 7 times in under a second. It takes … Continue reading →
Dear Urban Astronomer, Is it true that space junk is constantly falling down to earth? Should I be worried that I’ll get hit? Regards Cathrine Dear Cathrine Your letter included a link to this Washington Post article, which talks about UARS (Upper Atmosphere Research Satellite) which will be making an uncontrolled re-entry. At the time I write this, NASA’s best estimate is that it will re-enter the Earth’s atmosphere “Sometime in the afternoon” on Friday 23 September, New … Continue reading →
That’s a dangerous question! Last time somebody asked it seriously, they demoted Pluto! A planet is any object orbiting the Sun which is heavy enough to both squeeze itself into a spherical shape and clear the space around its own orbit (in other words, its gravity must be powerful enough that anything nearby gets drawn in and either smashes into the planet or gets catapulted away into space). By this definition, we have eight planets in our Solar System. It’s … Continue reading →
In 1930, Clyde W Tombaugh completed the search for Planet X, the outermost planet at the far reaches of the solar system. This object was named Pluto. Then, on 24 August 2006, the IAU (International Astronomy Union) formally voted on a new definition of the word ‘Planet’, which excluded Pluto. For seventy-six years, our solar system had been understood to have nine planets, then suddenly it went back to eight. The response was furious. Both the astronomical community and the … Continue reading →
Mercury is the smallest planet known, with a diameter of just under 4900km. Until it was disqualified as a planet in 2006, though, Pluto held the title at somewhere over 2300km – less than half the size! Thanks to the formal definition of a planet, Mercury is not likely to lose its place at the bottom of the size ranks for a while: we can see quite clearly that there is nothing smaller in our Solar System which meets all … Continue reading →
