Category Archives: Astronomy 101

One of my main goals is to try and share a bit of knowledge and understanding about the incredible universe. I have been steadily writing and posting various articles to explain and educate the masses (that’s you, dear reader!) a bit more about some of the science behind the hobby. An urban astronomer might never see most of the things we talk about here, but it’s still good to know about them.

How do you find the mass of planets?

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Dear Urban Astronomer   At the risk of opening myself to ridiculously complicated maths, how do you actually calculate the mass of a planet, or a sun, or, for that matter, any astronomical body. Thanks,Andrew   Dear Andrew   It’s quite clever, really. You can’t measure the mass of a planet directly, so you have to instead measure it’s effect on other orbiting bodies. Kepler’s laws of planetary motion give us a nice simple equation that relate two object’s masses … Continue reading →

How big is the Milky Way Galaxy?

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Our galaxy contains over 200 billion stars, most of which are arranged in a disc containing a core and spiral arms.  This disc is about 100 thousand light years across, but is only about 1000 light years thick.  A small percentage of the stars are distributed around the Halo, which is a spherical cloud marking the boundaries of the galaxy.  The halo completely envelops the disc, and is also home to many globular clusters.     Comments?  Questions?  Why not mail … Continue reading →

What is a galaxy?

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As it stands, there is no formal definition for what makes a galaxy because, up till recently, astronomers have simply known them when they see them.  This will probably be changing soon, as some astronomers have proposed a formal definition which will then be voted upon at a congress of the International Astronomical Union.  Until then, we’ll stick with the traditional “Island Universe” definition: A galaxy is a vast structure composed of hundreds of billions of stars and enormous clouds … Continue reading →

Which telescope?

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Newcomers to astronomy commonly ask, “What telescope should I buy?”.  First, the easy answer:  Buy a six inch newtonian reflector, with two or three eyepieces, and be sure to not buy it from any place that doesn’t specialize in telescopes (although you might be safe with a large photographic shop, if they have staff who have actually used telescopes to look at the stars).  The problem is that telescopes are currently available in a mind boggling range of prices, designs … Continue reading →

If a spaceship shoots its guns

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Dear Urban Astronomer I am busy designing a science fiction computer game and am trying to figure out the physics of combat.  If a spaceship is flying through space and shoots backwards, what exactly does the bullet do?   Regards KJM Confession to the reader:   Unlike all the other questions I answer in this section, this was not posed directly to me in an email.  Instead, I happened to catch a friend from Twitter posing the question to the … Continue reading →