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.

What is the distance from Earth to the Moon?

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384,400km.  However, since the Moon’s orbit is not perfectly circular (because orbits like that only exist in theory), this distance is only an average.  At its closest point (known as perigee), the Moon is 363,104 km from Earth and at its furthest point (known as apogee)it is 405,696 km away.  Incidentally, these distances are from the centre of the Earth to the centre of the Moon.  If you want the distance from surface to surface, you’ll have to take into … Continue reading →

How long is a day on Saturn?

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10 hours and 40 minutes.  Saturn spins around almost as fast as Jupiter, and shows the same flattening at the poles.  Both planets generate such huge centrifugal forces that they are not round, but visibly elliptical.  People don’t usually notice this about Saturn, though, because the beautiful system of rings masks the effect.     Comments?  Questions?  Why not mail me at uastronomer@gmail.com

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What is a Lunar Eclipse?

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The spectacle of the Moon passing through the Earth’s shadow is not as simple as you might expect. Rather than simply passing into the darkness to vanish for a while, before re-emerging into the light, we see quite a complex sequence of events. This is because the Earth’s shadow is composed of two regions: The penumbra and the umbra. The umbra is the fully dark part of the shadow, and is shown in the diagram below as the dark grey … Continue reading →

What is the difference between a lunar eclipse and a solar eclipse?

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Lunar eclipses are rather more common than Solar Eclipses, even though they have the same cause.  When the Earth, Moon and Sun are all positioned in a straight line, the Sun casts a shadow from one of the remaining two objects onto the other.  When the Moon is in the middle of the alignment, the Moon’s shadow is cast upon the surface of the Earth, and this gives us a Solar Eclipse.  When the Earth is in the middle, it’s … Continue reading →

How many moons does Mars have?

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Mars has two moons:  Phobos and Deimos.  They’re unusual, compared to other planet’s satellites, because they’re so tiny and orbit so close to the planet.  The larger of the two, Phobos, is shaped something like a potato and has a diameter of anything from 20 km to 28 km, depending on how you measure it.  It orbits at an altitude of only 9,270 km (compared to Earth’s moon, at over 350,000 km!).  Although Deimos orbits at a more reasonable distance … Continue reading →