Category Archives: Astronomy

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

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 →

What is a solstice?

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You need to know that the Earth’s axis is not nicely aligned with the Earth’s orbit – it’s tilted from the vertical by about 23 degrees.  So as the Earth moves around the Sun, one of the poles is leaning towards the Sun.  For six months it’s the north pole, and for six months it’s the south pole (This affects how much sunlight each side of the world gets, and causes the seasons).  The day of the year when each … Continue reading →

How far away is the Sun?

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Earth orbits the Sun at an average distance of 149,598,000 kilometers.  Astronomers refer to this distance as 1 Astronomical Unit (AU), which is a handy unit for measuring distances within the Solar System.  But the actual distance changes every day, because the Earth’s orbit is elliptical and not perfectly circular.  On the 3rd of January each year, Earth is at its closest point to the Sun, with a distance of 147,098,074 km.  It is at its furthest point (152,097,701 km) on … Continue reading →

Why is the Sun so hot?

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The Sun is a gigantic naturally occurring hydrogen bomb..  It is composed mostly of hydrogen, with a small percentage of helium and trace amounts of other elements, but is so huge that its own weight compressed the hydrogen at the core to the point where nuclear fusion can take place.  Hydrogen is consumed at a rate of billions of tonnes every second.  By human standards this is such a ridiculously fierce explosion that if we could set off our entire … Continue reading →

Who discovered Pluto?

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Clyde Tombaugh discovered Pluto in 1930, at the Lowell Observatory where he worked as a junior astronomer.  Tombaugh was a keen amateur astronomer who, having reached the limits of the telescope his father bought him as a child, built his own.  He took his astronomy very seriously.  Professional astronomers at Lowell Observatory were so impressed by the detailed sketches he sent them of Jupiter and Mars that they offered him a job as a junior astronomer.  His job was to … Continue reading →