Category Archives: Questions and Answers

Most people have many questions about astronomy, and aren’t happy with the standard answers. Here are some articles trying to change this. If you have any questions you would like to see answered, ask me in an email to the address below, and I will answer your question in an article on this list

What is a star?

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A star is a vast sustained nuclear explosion at the centre of an enormous sphere of superheated gas (or plasma).  The Sun is the nearest star to Earth.  Massive clouds of hydrogen gas (often laced with trace amounts of heavier elements) start condensing through various mechanisms until they’re dense enough for gravity to start drawing the cloud in upon itself.  As it contracts, the temperature and pressure begin to rise.  After a while the cloud will be so dense and … Continue reading →

How many planets are visible tonight?

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There are five planets which are visible to the naked eye.  They move through the sky, from week to week, so you need to know where to look if you want to see them.  The good news is that the movements of all the planets are limited to a narrow strip of sky called the ecliptic, along with the signs of the zodiac, which is halfway between the north and south poles.  This means that, unless you’re in the arctic … Continue reading →

What is the Face on Mars?

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On 19 June 1976, the first of the two Viking probes reached Mars orbit, and began the first phase of its mission:  photograph the surface of Mars so that the engineers back in the JPL lab on Earth could find a suitable landing spot (The entire set of images returned by the probes can be ordered on CDROM from NASA, incidentally).  On 25 July, the image to the left arrived.  It was of an area known as Cydonia, and showed … Continue reading →

How many stars can we see?

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There are about 6000 stars in the night sky that are visible to a healthy adult standing in a properly dark location.  Of course, you can never see more than half the sky at a time, so to see all of them you would need to live in the tropics and spend some time looking upwards regularly — it takes a full year for the entire sky to pass overhead.  So at any one time, there’s something between 2500 and … Continue reading →

What does the Earth weigh?

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Trick question!  The Earth weighs nothing at all, but itdoes have a mass of 5.9×1021 kilograms (or in regular figures:  5 900 000 000 000 000 000 000 tonnes).  Why the difference?  Mass and weight are actually two totally different things.  If you think about astronauts orbiting the Earth in the International Space Station, they float around totally weightless.  Yet they still have mass and inertia.  Weight only exists when something resists the pull of gravity – usually, on earth, … Continue reading →