Category Archives: Astronomy

All about astronomy: What it is, how the Solar System works, famous astronomers and more

Are the universal constants changing?

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The Fine-Structure Constant The two fundamental assumptions about modern cosmology (the study of the origins of the universe) and astrophysics are homogeneity and universality.  The first means that matter is evenly spread out through the universe (if you average out over large enough scales), while the second means that the laws of physics are the same throughout the universe.  The principle of homogeneity has always seemed arbitrary to me, and unjustified; at the local level we find matter is quite clumpy, from … Continue reading →

What is dark matter?

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  Dark matter is the name used for the material out in the universe which we cannot see, but can detect through it’s gravitational influence.  When we look at galaxies, we can see a lot of matter in the form of stars, gas and dust.  We can also see how the stars within those galaxies are moving in their orbits around the galactic core.  The laws of physics tell us that these orbits should be at certain speeds, depending on … Continue reading →

How fast is the Earth travelling through space?

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Brace yourself, because this question is a lot more complex than it sounds.  We’re asking what the Earth’s speed is, but speed is always measured in relation to something and we usually use the Earth itself as the reference point.  When we say a car is traveling at 120 km/h, we mean that it travels 120 km across the Earth in one hour and ignore the fact that the Earth itself is moving.  In ordinary everyday situations, we simply assume … Continue reading →

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 →